Spring Quarter 2021 (current)
Ultracool Subdwarfs, objects that have metallicities significantly lower than that of the Sun and ages > 5Gyr, provide insight into understanding how metallicity affects observable features of low-mass stars and brown dwarfs. Currently, substellar subdwarfs are thought to have cloudless atmospheres due to reduced condensate opacities from their low-metallicities. In this talk, I aim to explore the nature of clouds in subdwarfs using both observational and theoretical approaches I ask: (1) Are subdwarfs cloudless? and (2) How does their thermal profile compare to objects of similar effective temperature (Teff) or spectral type? By creating distance-calibrated spectral energy distributions (SEDs), I will compare one of the bluest known subdwarfs, SDSS J1256, to various aged sources of similar Teff and/or bolometric luminosity to examine the overall SED shape and features in the NIR bands, as well as compare fundamental parameters and place SDSS J1256 in context with the larger subdwarf population. To explore the nature of clouds in subdwarfs, I use the Brewster retrieval framework to examine a sample of subdwarfs and comparative sources to explore what may be causing the differences seen in the SEDs of these sources. In this talk, I will discuss the results for the widely separated co-moving low-mass d/sdL7+T7.5p pair SDSS J1416+1348AB to determine if the pair formed and evolved together and their cloud properties. Additionally, I will discuss preliminary results from a comparative sample of field sources of similar temperature or spectral type to SDSS J1416A to determine how the PT profile of these objects compares and what may drive the differences we see in their spectra.
Our understanding of the motions of stars within our Milky Way and of the many small galaxies that orbit around it has changed dramatically over the past few years owing to new observational surveys and significant advancements in our understanding of galaxy structure. New surveys now enable us to precisely measure the motions of objects that orbit our Galaxy, like clusters of stars, satellite galaxies and stellar streams. The motions of these objects trace the so-called “dark matter” distribution, the unseen material that is expected to exist within and around our Galaxy, making up the bulk of its mass. However, connecting these data to theoretical models requires understanding the influence of the Milky Way’s largest satellite galaxy, the Large Magellanic Cloud (LMC). I will provide an overview of this evolving picture and how we can test the cold dark matter paradigm in the near future using next-generation surveys and simulations that include the LMC.
Please contact the department office for Zoom information.
Galaxy halos are fantastic laboratories for the physics of diffuse plasmas, where physical ingredients such as turbulence, magnetic fields, cosmic rays, thermal instability, and many more play out on grand scales. The results have important implications for gas inflow which fuels star formation, as well as associated feedback and recycling processes. I describe recent progress on understanding the microphysics of the circumgalactic medium via high-resolution, idealized simulations which are complementary to cosmological simulations of galaxy formation. I describe numerical experiments on the interaction between turbulence and cooling, which bears on the survival of cold clouds in galactic winds, with special attention to the resolution required for key physical processes. I also describe cosmic-ray transport effects which have important implications for cosmic-ray driven winds.
Please contact the department office for Zoom information.
Please contact the department office for Zoom information.
Please contact the department office for Zoom information.
The cosmological model based on cold dark matter (CDM) and dark energy has been hugely successful in describing the observed evolution and large scale structure of our Universe. However, at small scales (in the smallest galaxies and at the centers of larger galaxies), a number of observations seem to conflict with the predictions CDM cosmology. Immense progress has been made in the last decade in simulating and understanding the role that baryons have in influencing the evolution and structure of dark matter on small scales and in dwarf galaxies. In this talk I will briefly review the key physical processes introduced by baryons, and discuss the astrophysical implications for interpreting the dark matter model. I will also highlight some remaining challenges and open questions.
Please contact the department office for Zoom information.
Winter Quarter 2021
When the first galaxies formed, their starlight likely reionized and heated the intergalactic hydrogen that had existed since recombination. I will discuss some of the rich physics of the intergalactic medium during this reionization process. In the past few years, there has been significant progress towards understanding reionization. I will highlight this progress, including the possibility that we have already observed reionization’s tail end in the Lyman-alpha forest of high-redshift quasar spectra. I will also discuss future prospects for probing reionization with the 21cm radiation background and with high-redshift Lyman-alpha emitting galaxies.
Please contact the department office for Zoom information.
The ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) isa $40M Centre of Excellence, which is producing a comprehensive picture of the build up of mass, angular momentum, and the chemical elements from the first stars, to (and including) the Milky Way. Our surveys include the measurement of the power spectrum at the Epoch of Reionization with the Murchison Widefield Array (MWA), large HI surveys with the Square Kilometre Array Pathfinder (ASKAP), the ongoing Australian optical integral field surveys of 10^5 galaxies (SAMI, followed by HECTOR), a large galaxy evolution program combining HST, Keck, and ESO spectroscopy of galaxies from z=6 to z=0.5, and the major Australian Galactic Archaeology program (GALAH) to track the chemical history and accretion history of our Milky Way through GAIA and the HERMES instrument on the Anglo Australian Telescope. I will describe the recent discoveries made in ASTRO 3D, our preparation for JWST spectral analysis, as well as providing an update on our ambitious equity and diversity programs, and our nationwide education and public outreach programs.
Please contact the department office for Zoom information.
[Virtual Colloquium] Equity in Science: Representation, Culture, and the Dynamics of Change in Graduate Education
STEM disciplines are believed to be founded on the idea of meritocracy; recognition earned by the value of the data, which is objective. Such disciplinary cultures resist concerns about implicit or structural biases, and yet, year after year, scientists observe persistent gender and racial inequalities in their labs, departments, and programs. In this colloquium Julie Posselt makes the case that understanding field-specific cultures is a crucial step for bringing about real change, and that we can learn as much from quantum as classical dynamics about the dynamics of scientific organizations. She will share research that compares equity, diversity, and inclusion efforts in astronomy and physics, and the subtle ways that exclusion and power operate in scientific organizations and, sometimes, within change efforts themselves.
Please contact the department office for Zoom information.
MOSEL and ZFIRE are deep near-IR spectroscopic surveys that track how galaxies assemble at 1.5<z<3.5. With MOSEL, we confirm a population of Extreme Emission Line Galaxies at z~3 with Oxygen equivalent widths >250 Angstroms and hypothesize that most galaxies at z>3 go through a strong starburst phase. We compare galaxy kinematics to IllustrisTNG to determine how massive galaxies at z~3 build their stellar mass (mostly by accreting other galaxies) and end their star formation (puffy galaxies quench slower). I also summarize results from our ZFIRE survey where we find faint imprints of environmental effects on the Inter-Stellar Medium of cluster galaxies at z~2.
Please contact the department office for Zoom information.
There are observational and theoretical reasons to suspect that up to 10s of percent of massive stars that undergo core-collapse at the end of their lives fail to explode in a canonical energetic supernova explosion. In this talk I will describe what transpires in such nominally failed supernovae and its importance for understanding the masses and spins of black holes (e.g., detected by LIGO). I will also describe how ‘failed’ supernovae may manifest themselves observationally in time-domain surveys.
Please contact the department office for Zoom information.
There’s something odd going on in our solar system. While the planets move on nearly-circular orbits in a well-defined plane, the minor planets beyond Neptune appear to cluster together in a highly-inclined, eccentric, and tilted structure. Astronomers have invoked the presence of an additional planet (Planet 9) or even a primordial black hole in explanation. In this talk I will show that these theories may be unnecessary. In analogy with spiral arms and bars in galaxies, the collective gravity of individually small but collectively massive bodies can create such structures in the outer solar system.
Please contact the department office for Zoom information.
Fall Quarter 2020
Captioned recording available here.
Stars are fundamental to astronomy, and how they form influences everything from exoplanet studies to cosmology. Stars form in heavily obscured molecular clouds, and understanding the initial conditions of star formation persists as one of the leading problems of modern astrophysics. Beyond the Milky Way—especially in high-redshift galaxies—the physical processes driving star formation become even more challenging to decipher. In this talk, I will show how we can advance the field by making vital connections between local, extragalactic, and early universe star formation. I will discuss theoretical models that explore the cosmological evolution of interstellar dust. I will then present state-of-the-art ALMA observations, which are opening new windows onto star formation in a wide variety of environments beyond the Milky Way. And I will introduce innovative programs—in observation and theory—designed to give us fresh perspectives on the intricate structure of the interstellar medium, the precursor to stars.
No recording available.
Neutron star mergers serve as unique laboratories to study the launching of relativistic jets, the production of heavy elements, the emission of gravitational waves, and importantly, the unknown. This field has been in rapid transformation, driven by the 2017 discovery of the first neutron star merger by gravitational waves, GW170817. Short gamma-ray bursts are energetic, jetted explosions which originate from neutron star mergers at cosmological distances. They represent a large and diverse population of mergers, and provide a complementary way to probe merger formation, evolution, and outcomes. In this talk, I present recent results from our multi-wavelength observational campaigns of short gamma-ray bursts, which are yielding new insights into the lives of neutron star mergers. These results include key constraints on the maximum mass of neutron stars, the discovery of a luminous kilonova which may signal the birth of a magnetar, and constraints on formation channels from a growing population of distant events.
Captioned recording available here.
The discovery of numerous small exoplanets has brought the search for life beyond the Solar System into sharp focus on many potentially habitable worlds where life may exist. However, many factors and processes can affect planetary climate and habitability, most of which are currently unconstrained by observations, and their effects must be understood to accurately determine a planet’s habitability potential and prioritize planets for observational follow-up. Professor Shields will describe the methods used by her research group to quantify the effects on planetary climate of a range of factors important for planetary habitability, and share recent results from this work, which demonstrates how the unique interactions between a star and a planet’s atmosphere and surface can produce either a recipe of successful ingredients for habitable surface conditions, or one that reveals less favorable planetary prospects for life.
Captioned recording available here.
Rapid progress in data science promises to deeply transform how we extract meaning from complex data sets. New powerful analysis and simulation techniques such as deep learning (DL) are quickly emerging and impacting all data-based sciences. Astrophysics is no exception, as illustrated by some of our work: Ravanbakhsh et al. (2016) were the first to estimate cosmological parameters from 3D dark matter simulations directly achieving ~3-5 times stronger constraints than power-spectrum alone; He et al. (2018) used a convolutional neural network to generate large scale structure dark matter simulation million times faster than our traditional simulation techniques; Yip et al. 2019 used U-net to predict the position of all types of galaxies given a dark matter density map, by-passing the need of large volume full hydrodynamic simulation of the Universe. Zamudio-Fernandez et al. 2019 used a generative adversarial network to produce new neutral hydrogen simulation with Illustris as a training set.
All of these examples illustrate how deep learning can be used to tackle astrophysical challenges, and all of them point to an end goal of extracting the maximum amount of information from the observations of our Universe by directly comparing our theoretical understanding in the form of simulated Universes to the observables.
In this talk, I will discuss this vision and how deep learning techniques help our quest in understanding the Universe.
Please contact the department office for Zoom information. This colloquium will not be recorded.
Spring Quarter 2020
[Virtual Colloquium] Mapping Matter in Strong Gravity: Spectral-Timing of Black Holes and Neutron Stars
One of the best laboratories to study strong-field gravity is the inner 100s of kilometers around black holes and neutron stars in binary systems with low-mass stars like our Sun. The X-ray light curves of these binary systems show variability on timescales from milliseconds to months — the shorter (sub-second) variability can appear as quasi-periodic oscillations (QPOs), which may be produced by general relativistic effects. My research looks at QPOs from black holes and neutron stars (as well as coherent X-ray pulsations from neutron stars) by fitting the phase-resolved energy spectra of these signals to constrain their physical origin and track their evolution in time. In this talk, I will introduce why black holes and neutron stars are interesting and discuss state-of-the-art “spectral-timing” analysis techniques for understanding more about them. I will also highlight open-source astronomy research software and the importance of mental wellbeing in academia.
Recording and transcript available here.
Astronomical data grow in quality and quantity, and yet many scientific questions cannot be answered by a single data set alone. While independent analyses are currently the norm, they miss out on substantial scientific synergies offered by the spatial overlap of current and future surveys. In my talk I will show how to build a coherent hyperspectral model from multiple data sets of the same celestial scene: imaging and spectroscopy observed from ground and space. By combining several novel methods from constrained optimization, statistics and machine learning, my group is building the analysis framework to carry out this data fusion. I will present its benefits for cosmological analyses such as gravitational lensing and studies of galaxy clusters, AGNs, and the evolution of galaxies. I will also discuss the challenges that lie ahead, which will require data-driven decision making. And I will conclude with an outlook on how the upcoming surveys LSST and WFIRST will support data fusion for the entire astronomical community, enabling inference and discovery at an unprecedented scale.
Recording and transcript available here.
A crow that mimicked gravitational waves. An astronomer that discovered microwave ovens. A telescope that got shot. Observational astronomy is rife with true stories and tall tales of the misadventures that accompany our exploration of the universe. However, these stories can do much more than serve as entertainment during conference dinners and colloquium coffee; oftentimes, telling them also means sharing the science behind our work and putting a human face on what can seem to many people, like a farfetched and remote field of study. By sharing our experiences as astronomers we can also preserve the unusual quirks of our rare and strange jobs and map the rapid technological expansion of our field, the new possibilities offered by tomorrow’s telescopes, and the evolving role of humans as observers and scientists. In this colloquium I’ll share some of astronomy’s best observing stories and explore their utility as tools for science communication and supporting our field.
Recording and transcript available here.
Winter Quarter 2020
Star clusters are invaluable astrophysical laboratories, providing discrete tracers of the Milky Way’s chemical evolution and star formation history, and enabling stringent tests of stellar formation & evolutionary models. Accurately diagnosing the structure and dynamics of these clusters, as well as their likelihood to persist as a bound structure, requires precise (sub-km/s) velocity measurements for a significant fraction of the cluster’s membership, an achievement that has only recently become observationally feasible with Gaia astrometry and precision radial velocities from multiplex instruments such as the SDSS-APOGEE spectrograph. Analysis of their spatio-kinematic structure confirms that most clusters and associations within 1 kpc are unbound; several clusters show clear signs of hubble-flow-like expansion, suggesting a strong role for shock-triggered star formation and/or modification of the local gravitational potential by mechanical feedback on molecular gas. Spatially, kinematically, and temporally coherent structures can be identified throughout the extended solar neighborhood, however, even on spatial and temporal scales larger than the typical cluster size & lifetime: analysis of the large scale demographics of such structures identifies a typical de-coherence timescale of 300 Myrs, and the prevalence of spatially extended stellar structures, which may indicate the fossil remains of past star formation events along Galactic spiral arms.
In a reprise of the famous 1919 solar eclipse experiment that confirmed Einstein’s general relativity, the nearby white dwarf Stein 2051B passed very close to a background star in March 2014. As Stein 2051B passed by, the background star’s position was relativistically deflected. Measurement of this deflection with the Hubble Space Telescope (HST) allowed us to determine the mass of Stein 2051B, which confirms the physics of degenerate matter and provides a new tool for mass determinations of isolated stars.
Another consequence of the relativistic bending of light is the temporary brightening of the background star, commonly known as gravitational microlensing. The combination of brightening and deflection provides a powerful technique to detect isolated black holes, and measure their masses. I will discuss the technique, and our HST programs aimed at the first detections of stellar-mass black holes through this technique.
The wealth of observational data on light curves, compact object masses, and chemical abundances hold critical clues on how massive stars live and die. However, the utility of these observables were severely hampered due to our limited understanding of the explosion mechanism in core-collapse supernovae. We address this problem by developing a novel and efficient method of simulating supernovae through calibrated neutrino-driven explosions. Instead of the usual arbitrary mass cuts and tuned energies, we employ neutrino-transport along with a proto-neutron star contraction model. In this talk, I will review some of the most exciting results we have found from the application of this approach to a population of single massive stars, including possible physical explanations to the “Red Supergiant Problem”, and the birth mass function of black holes and neutron stars. I will end the talk by presenting new results from its application to massive stars in binary systems that have lost their envelope to its companion. Surprisingly, it turns out the explosions of stripped stars do not make enough Nickel-56 to explain the observed Type Ib/c light curves, which goes against the decades long assumption that their optical displays are powered by radioactivity.
Despite tremendous recent progress, gaps remain in our knowledge of our cosmic history. For example, we have yet to make direct observations of Cosmic Dawn or the subsequent Epoch of Reionization. Together, these represent the important period when the first stars and galaxies were formed, dramatically altering their surroundings in the process. Radio telescopes targeting the 21cm line will open up these crucial epochs to direct observations in the next few years, filling in a missing chapter in our cosmic story. I will review our recent results from the field, including the latest developments from the Hydrogen Epoch of Reionization Array (HERA), giving an overview of its promise to provide exquisite constraints on reionization astrophysics. I will additionally discuss our parallel efforts to build a more complete and data-driven model of the low-frequency radio sky, as well as possible ways to leverage machine learning to perform large-scale analyses with our data.
Feedback from young, super star clusters (SSCs) is a major driver for galaxy evolution at all cosmic epochs, but may be especially relevant for cosmic reionization. The standard scenario for massive-star feedback has been that superwinds from SSCs clear pathways for Lyman continuum (LyC) and Ly-alpha radiation to escape from host galaxies. However, theoretical predictions indicate that for the most massive and compact SSCs, superwinds are actually suppressed by catastrophic cooling and pressure confinement. These extreme conditions are rare, but observational evidence from local starbursts increasingly supports this scenario. I will present our work in establishing this new paradigm and its consequences for LyC and Ly-alpha escape.
In the past year, the longstanding goal of imaging a black hole has become reality. Two long baseline interferometry experiments operating at submillimeter and near-infrared wavelengths can now achieve microarcsecond scale angular resolution with sufficient sensitivity to detect synchrotron radiation from the Galactic center black hole, Sgr A*, and the supermassive black hole in M87. I will discuss the first results from each experiment, focusing on the opportunity to study accretion and jet physics in the immediate vicinity of an event horizon. I will outline the challenge of pushing towards tests aimed at determining whether black holes in the Universe are those predicted by General Relativity.
In this talk I discuss our efforts to increase the precision of modeling and analysis for upcoming line intensity mapping (LIM) science. LIM probes the physics of galaxies and large-scale structure (LSS) by mapping the aggregate line emission of star-forming galaxies and the intergalactic medium, which is much faster than traditional observations of individual galaxies. While previous efforts have been focused on detecting LIM candidates in the diffuse background due to LSS, current and upcoming surveys seek to use LIM to probe galaxy physics and cosmology, requiring accurate line emission models and precise analysis tools. I will first give a introduction to LIM and its applications. Next, I will present our work constructing realistic, multi-line LIM mocks and predicting how they can improve LIM galaxy and cosmology science. I will also discuss EXCLAIM, an LIM survey that maps CO and [CII] emission, as well as how our LIM mocks will be used to meet EXCLAIM’s science goals. Finally, I will briefly discuss accounting for a curved sky in large-area LIM analyses, specifically in the context of weak lensing LIM surveys.
The NASA Kepler mission has provided its final planet candidate catalogue, the K2 mission has contributed another four years’ worth of data, and the NASA TESS mission has just started producing planet candidates of its own. The demographics of the exoplanet systems probed by these transiting exoplanet missions are complemented by the demographics probed by other techniques, including radial velocity, microlensing, and direct imaging. I will walk through the progress of the Kepler occurrence rate calculations, including some of the outstanding issues that are being tackled. I will present some new results from K2, and outline how K2 and TESS will able to push the stellar parameter space in which we can explore occurrence rates beyond that examined by Kepler. Finally, I will highlight some of the pieces of the larger demographics puzzle – occurrence rate results from the other techniques that probe different stellar and exoplanet regimes – and how we can start joining those pieces together.
The discovery of the first neutron star merger, GW170817, signaled a wealth of firsts in physics and represents only the tip of the iceberg of the discovery potential we can achieve in multi-messenger astronomy. In this talk, I present late-time observations of GW170817 extending until 2 years after the merger, which provide deep insight into the geometry of the jet, energetics, and the merger’s local environment. I then connect these observations to the existing population of cosmological short GRBs, demonstrating that the primary difference between the two populations is viewing angle. I also present deep Hubble Space Telescope imaging of GW170817, which allow us to place the first direct constraints on the dynamical formation of the neutron star binary progenitor. Finally, I discuss our ongoing observational campaign to analyze the host galaxies and demographics of short GRBs, yielding new surprises such as a growing population of high-redshift events.
Fall Quarter 2019
Stars escaping globular clusters form thin, long and kinematically-cold tidal streams. In pristine conditions, these streams have nearly uniform density, however, new Gaia observations of one such structure in the Milky Way halo have revealed a likely site of perturbation. In this talk I will show that the on-sky morphology suggests a recent, close encounter with a massive and dense perturber. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. This discovery opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Milky Way halo. I will discuss the observational and modeling efforts I am leading to follow up this feature, review prospects for discovering evidence of similar events in other stellar streams, and forecast the sensitivity of stellar streams to low-mass perturbers in the era of LSST.
The orbital structure of the Kuiper Belt is dynamically excited, with most objects found on highly inclined and eccentric orbits. It is this orbital structure that has provided the main evidence for the early migrations of the gas-giant planets, and the violent dispersal of the outer proto-planetesimal disk. Many observational properties have been used as constraints on numerical efforts to uncover the details of the migratory history, including the detailed orbital distribution of Kuiper Belt Objects (KBOs), and the compositional classes KBOs which are known to correlate with the different orbital classes in which KBOs reside. This chemo-dynamical structure appears to be the result of the dynamical processes experienced by KBOs during the dispersal of the early disk, and the compositional heterogeneity of that disk. As a dynamical tracer however, the chemo-dynamical structure of the Kuiper Belt remains largely untapped. It was around this idea that the Col-OSSOS project was created. The Colours of the Outer Solar System Origins Survey is a large program that is nearly done executing on the Gemini-North and CFHT telescopes aimed to gather high quality optical-NIR colours of ~100 KBOs, in a brightness complete sample, and has designed to overcome various weaknesses of colour surveys that came before it. Col-OSSOS has been running since 2014 and has resulted in many exciting findings to date. With some anecdote about working very closely with Gemini and the excellent work they have done, I will present some of the main results of the Col-OSSOS, including a discussion about what the observed colour distribution informs us of the chemical structure of the early disk. Our results have provided some critical context for a number of otherwise curious objects, including the first silicate bearing objects in the Kuiper Belt, some odd blue binary KBOs found where no blue objects should be, and a compositional interpretation that implies most KBOs share a primordial source with the C-type asteroids. I will present a cosmogonic history that draws all of these disparate pieces together.
I will describe a pair of sharp puzzles suggesting a possible mismatch between our theory of galaxy evolution and recent observations. First, current models predict that the z ~ 4-8 universe should be a time in which the most massive galaxies are transitioning from their initial halo assembly to the later baryonic evolution seen in star-forming galaxies and quasars. Instead, massive galaxies appear to exist impossibly early, before their halos should even have been able to assemble. Second, there is increasing observational evidence that history of many galaxies may not be driven by environment but rather by cosmic factors. I will then discuss whether theory and observation are truly inconsistent or whether analytical techniques might be responsible for the current tension. I will also outline a possible theoretical solution.
The detection of gravitational waves, gamma-rays, and multi wavelength radiation from the binary neutron star merger GW170817 has been an enormous breakthrough in astrophysics. It has confirmed some theoretical expectations and opened new riddles. I will extensively introduce the observations and physics of the event and then concentrate on the non-thermal component of the emission and on the possible the association of binary neutron star mergers with short-duration gamma-ray bursts. I will show that the most likely scenario is the one in which GW170817 produced a canonical short gamma-ray burst jet that was misaligned with our line of sight, resulting in unusual behavior. I will finally discuss the results of the O3 observation period of LIGO and what to expect from the future.
For the first time in the history of our planet its inhabitants have the means to avert catastrophic impacts of near-Earth asteroids. The two-pronged approach currently pursued by national agencies and their international partners is to take stock of potentially hazardous objects on the one hand, and to test technologies that alter the trajectories of large impactors on the other hand. In this seminar I will discuss current and future capabilities of finding and deflecting near-Earth asteroids. In particular, I will focus on the Large Synoptic Survey Telescope (LSST) currently under construction at Cerro Pachón, Chile, as well as the Double Asteroid Redirection Test (DART), a NASA asteroid deflection demonstration mission to be launched in 2021.
One of the most mysterious components of galaxies is dust. Dust not only plays an important role in the physics and chemistry of the ISM, but also shapes our views of galaxies by absorbing and scattering UV and optical light and re-emitting it in longer IR wavelengths. Despite its importance, dust is not well characterized beyond the Milky Way and a few other well-studied local galaxies. I will talk about what we know about dust and obscured star formation in galaxies at redshift of z~2, the peak epoch of cosmic star formation activity, using our current data and facilities, such as ALMA and Keck, and the prospects with the future JWST. JWST/MIRI will be revolutionary in advancing our knowledge about the dust-obscured star formation in typical high-redshift galaxies, owing to its significantly higher sensitivity and resolution compared to those of its predecessors. On the other hand, the unprecedented power of ALMA gives us the opportunity to probe the cold dust content of galaxies at z>1, and the state-of-the-art optical and near-IR instruments on 10-m class telescopes add important pieces of information about the UV-optical dust attenuation at high redshifts. The synergies among these powerful facilities will give us a more complete picture of the dust content of galaxies during the peak epoch of star formation activity in the universe.
The Zwicky Transient Facility (ZTF) started science operation in March 2018. 40% of ZTF observing time is dedicated to two public surveys: one covering the entire Northern sky every three nights in g and r passbands and one visiting the Galactic Plane every night in g and r. In addition to the public Galactic Plane survey, ZTF has conducted a dedicated high-cadence survey of selected Galactic Plane fields to study the ultra-short variable sky. So far we have covered about 4900 sqd. In this talk, I will present an overview of the ZTF Galactic Plane observations and present some science highlights from the first year, including the discovery of a new class of compact pulsating stars as well as the first accreting helium star binary in an ultracompact orbit.
The Sloan Digital Sky Survey (SDSS) is one of the most successful and prolific projects in the history of Astronomy. In its fifth iteration SDSS-V (2020-2025) will provide a more comprehensive, global picture of the local universe by studying the interplay between galactic genesis, stellar and black hole processes, and the physics of the ISM. I will review the main science goals of the project, the exciting new hardware being implemented (robot fibre positioner, large IFU systems), and the operational challenges. In particular, I will describe the work that the Telescope Engineering Group at UW is carrying out to make SDSS-V a reality. As in its previous incarnations, SDSS-V remains committed to providing high-quality data products for the astronomical and educational communities. I will discuss some of the new ideas being developed for SDSS-V with regards to data reduction, release, archival, and visualisation.
Spring Quarter 2019
Feedback from massive stars plays a central role in shaping the evolution of entire galaxies. Despite a solid qualitative understanding of feedback, our quantitative knowledge remains poor. Currently, only a small number of HII regions have adequate observational information on both gas and stars needed for detailed feedback studies. However, the growing availability of integral field unit (IFU) instruments and the novel analysis techniques we’ve developed for them, now allow the study of stellar feedback in orders-of-magnitude more HII regions than previously possible, i.e. the numbers needed to fully quantify the effects of feedback over a large dynamic range of stellar and ISM properties, and to connect the results to state-of-the-art star formation and galaxy evolution models.
I will discuss the first results of resolved stellar feedback studies from a MUSE IFU legacy dataset covering the nearby Sculptor galaxy NGC 300, as well as results from MUSE observations of HII regions in the Magellanic Clouds. By merging the MUSE NGC300 data with HST resolved stellar photometry, I demonstrate that ground-based IFU data of nearby galaxies is ideally suited to quantify feedback from massive stars all the way down to individual cloud scales. Moreover, I will discuss the MUSE NGC 300 dataset in terms of a pathfinder to ongoing and next-generation IFU nearby galaxy surveys and instruments such as the Local Volume Mapper. Finally, I will highlight serendipitous discoveries only possible thanks to the 3D nature of IFU data.
Secondary eclipses of gaseous exoplanets (when a host star occults its planet) may be diagnostic of atmospheric temperatures and dynamics, and variations in the planetary phase curves (light reflected and emitted from the planets) may point to active meteorology, as advective processes sweep thermal structures and aerosols into and out of view. The Kepler Mission’s very long observational baseline, spanning hundreds of days for some planets, makes its dataset especially well suited to search for such variability, and datasets for several gas giants remain unexploited. A perfect example, Kepler-76b is a 2 Jupiter-mass gas giant in a 1.5-day orbit around an F-star and was observed for nearly 1,400 days by Kepler. These data reveal substantial variability in the planet’s phase curve, with the dayside brightness temperature fluctuating by hundreds of degrees K of timescales of tens of days. Although the much shorter baseline of TESS observations will make such variability analysis difficult, the mission is likely to find many more hot Jupiters suitable for detailed characterization. Moreover, TESS’s increased sensitivity in red wavelengths as compared to Kepler means that planetary eclipses observed by TESS will probably be more sensitive to thermal emission from the planets, likely probing different aspects of their atmospheres.
The evolution of massive stars is heavily influenced by the amount of mass they lose, both as OB stars in the main-sequence, as well as later in their lives. This mass loss can happen either by stellar winds, or by Roche-lobe overflow in close binary systems. One of the great uncertainties in understanding massive star evolution is knowing which mechanism dominates. I will describe a number of recent observational tests we have been carrying out addressing this question (along with others) using observations of massive stars in the star-forming galaxies of the Local Group as our laboratories.
Jupiter’s innermost large satellite, Io, is like the Moon in size but with a very different history and surface. Extreme tidal heating from its close proximity to Jupiter and an orbital resonance between Io, Europa and Ganymede has left Io’s interior hot and vigorously convecting. This makes Io a good analogue to the early Earth, where heat flow was likely around modern Io’s, about 10x that of today’s Earth. The tidal heating also informs our understanding of Europa, Io’s neighbor, and Enceladus, moon of Saturn, where liquid water is found in the interiors, making them important places to search for life outside Earth. The real allure of Io, however, is in the hundreds of active volcanoes, sky-high plumes and yellow-red hydrothermal areas that litter the surface. Io loses most of its heat through giant lakes of lava, one of which is twice as long as Puget Sound and releases as much as ¼ the total heat from the interior. Our studies of the temperatures and changes over time of lava lakes on Earth reveal that eruption temperatures, related to compositions, are visible from remote sensing and show that cooling is rapid for Earth conditions. These studies also help us understand Io’s behemoth lava lakes, as do Earth-based telescopic observations, but to really understand how hot Io’s volcanoes are, how much heat is flowing from Io, and how Io’s present and past help inform the history of Earth and other Solar System – and extrasolar – planets, we plan and hope for a dedicated Io space mission.
Galaxies are not lonely islands floating in the Universe. They host large gaseous envelopes of baryons, a.k.a., the circum-galactic medium (CGM), that exist far beyond a galaxy’s visible extent. Baryonic inflows from CGM replenish star-forming fuel in galaxies, whereas outflows from galaxies enrich the CGM. In this talk, I will describe the theoretical and observed distribution and flows of baryons in the MW’s CGM, including how the MW’s disk hides up to half of its CGM from direct observation. I will then describe new techniques to generate synthetic observations of the CGM using the Enzo & FOGGIE cosmological simulations, and show how these can be used to reveal the hidden baryons in the MW’s CGM. Finally, I will briefly highlight the connections between low- and high-redshift CGM studies, including new applications that rely on fast radio bursts (FRBs).
Supermassive black holes are ubiquitous in massive galaxies, but it remains unclear how many sub-Milky Way mass galaxies have central black holes. If low mass galaxies commonly host central black holes, these will make up the majority of massive black holes in the local universe. From the demographics of these black holes we can place constraints on how supermassive black holes form in the early universe. I will discuss my group’s discoveries of 10 black holes in nearby galaxies using adaptive optics kinematics observations. These include the first discovery of massive black holes in stripped galaxy nuclei, and the first dynamical measurements of central BHs with masses below one million solar masses.
The AAVSO was formed in 1911 as a group of US-based amateur observers obtaining data in support of professional astronomy projects. Now, it has evolved into an International Organization with members and observers from both the professional and non-professional astronomical community, contributing photometry to a public photometric database of about 25,000 variable objects, and using it for research projects. As such, the AAVSO’s main claim to fame is that it successfully engages backyard Astronomers, educators, students and professional astronomers in astronomical research. I will present the main aspects of the association and how it has evolved with time to become a premium resource for variable star researchers. I will also discuss the various means that the AAVSO is using to support cutting-edge variable star science, and how it engages its members in projects building a stronger international astronomical community.
It is well-established that fast-spinning millisecond pulsars are neutron stars recycled through accretion from binary companions. For most millisecond pulsars the accretion process has permanently ceased, and they are in binary systems with low-mass white dwarf companions. Follow-up observations of newly discovered gamma-ray sources from the Fermi Gamma-Ray Space Telescope have revealed a substantial population of “spider” millisecond pulsars with hydrogen-rich companions; these systems had mostly been missed in radio pulsar surveys. I will discuss the properties of neutron stars in these binaries and and the implications for the formation and evolution of millisecond pulsars.
Data for better and sometimes worse is granted authority. My colleague, Carl Bergstrom, and I created a class that teaches students when to Call BS on numbers, statistics and algorithms. The course, or parts of the course, are being adopted at more than 70 universities around the country. In this talk, I will survey some of these lessons, common pitfalls, and habits of mind that we have found useful in our research, in the classroom, and in our personal lives where misinformation, disinformation and deep fakes are becoming a daily occurrence.
Quasars are among the most luminous objects in the sky. These very energetic regions lie at the center of massive galaxies and are powered by a supermassive black hole. While it has been found that there is a correlation between the mass of these supermassive black holes and the mass of the surrounding galaxies, the co-evolution of galaxies and quasars is barely understood. Outflows launched from the vicinity of supermassive black holes are a key piece in this puzzle, potentially linking the small and the large-scale phenomena.
We have discovered that some of this gas is outflowing at very high speeds (speeds between 0.1c and 0.2c). I will present a survey of these extremely high velocity outflows observed as broad absorption lines in Sloan Digital Sky Survey Data. This realm of the parameter space of quasar outflow’s velocity has not been included in previous surveys of quasar spectra, and might pose the biggest constraints for theoretical models. Moreover, the kinetic luminosity of outflows at 0.2c is two orders of magnitude larger than those speeding at high velocities (~10,000 km/s). Studying extremely high velocity outflows can help us understand the interaction between the central supermassive black hole and the host galaxy, so I will discuss the characteristics and properties of the found sample.
Winter Quarter 2019
While the Kepler and TESS missions were designed to search for exoplanets, their impact has reached far beyond the statistics of planet occurrence. Over the past decade, the Kepler/K2 platform has collected the most precise light curves ever produced for a half million stars. With these data we are discovering new astrophysics of stars, and in turn revealing clues about the formation history of our own Galaxy. Here I will show how high precision time domain data from Kepler and TESS, combined with astrometric data from Gaia, have revealed new mysteries about stars and the stellar populations within our Galaxy. Along the way I’ll try to highlight work from many remarkable UW students, and discuss opportunities for new projects and collaboration.
We are seeking both light and gravitational waves from binary supermassive black holes, the biggest, meanest discrete binary systems in the Universe. When two supermassive binary black holes pair up as a binary at the center of a merger remnant, during their inspiral and coalescence phases, they will produce intense gravitational radiation, which we expect to detect with gravitational-wave observatories like Pulsar Timing Arrays and LISA in the coming ~decade. Pulsar timing arrays use distributed networks of pulsars to sense these waves as they pass through our galaxy; in effect, they are an observatory on a Galactic scale.
If black holes in-spiral in an isolated environment, we should by now have already detected nanohertz-frequency gravitational waves with pulsar timing arrays… or so we thought. This talk will discuss pulsar timing arrays and what our latest, most stringent limits on gravitational waves mean for galaxy evolution and supermassive binary black holes. I will also briefly discuss efforts to discover both gravitational and electromagnetic waves from binary supermassive black holes.
Many observed triple systems in our Universe are in a hierarchical configuration: two objects orbit each other in a relatively tight inner binary while the third object is on a much wider orbit. In this case, the secular approximation (i.e., phase-averaged, long-term evolution) can be applied, where the interactions between two non-resonant orbits are equivalent to treating the two orbits as massive wires. Thus, the orbits may change shape and orientation, on timescales longer than the orbital periods, but the semi-major axes are constant. This approximation has been proven to be very useful in many astrophysical contexts, from planetary to triple-star systems and even black holes. I will discuss recent developments in that field and will show that hierarchical triple systems are richer and far more exciting than considered of before. In particular, the tight orbit can reach extremely high eccentricities and undergo chaotic flips of its orientation. This behavior has important implications for the evolution of many systems, and I will present some seminal examples, such as retrograde hot Jupiters, blue stragglers, and black-hole binaries.
The reionization of hydrogen was a landmark event in cosmic history. Within one billion years of the Big Bang the first galaxies emitted enough ultraviolet photons to ionize the gas in deep space, permanently transforming the Universe. Determining exactly when and how reionization occurred is therefore central to our efforts to understand these early sources, as well as the physics that governs the interaction between galaxies and their environments. I will describe what we know about reionization from the study of quasar absorption lines and other probes of the high-redshift Universe, focusing especially on what we’re learning about the intergalactic medium (IGM) shorty after reionization is believed to end. By combining observations of high-redshift quasars with wide-field galaxy surveys we are beginning to better appreciate the complexity of the IGM at this epoch, and recognize how it may help us to construct a more complete model of reionization.
Over time star-forming galaxies transform into passively-evolving red galaxies. These transformations are due to a combination of internal processes, like the presence of a bar or active galactic nucleus, as well as environmental processes. I will review the ways in which host environment can affect the properties of galaxies and then present some of our recent work on group and cluster galaxies in the local universe. Previous work has found that the properties of satellite galaxies depend on the mass of their host halo. We explore how galaxy star formation rates and morphologies depend on the evolutionary state of their host environment as traced by both dynamics and X-ray richness. It is only with large samples of well-studied galaxies in the local universe that these subtle environmental effects can be measured. I will discuss how these trends can be used to constrain the mechanisms at play in environmentally-driven galaxy evolution.
Using mainly bulge red clump giants (RCG) and giants as tracers, a picture has emerged of the Galactic bulge consisting of a rotating peanut-shaped structure made up largely of old and metal-rich stars (~10 Gyr, [Fe/H] falling between -0.5 and +0.5 dex). However, there is a more metal-poor population of stars present in the bulge, a scarcer, lesser explored population of stars. I will present results from our spectroscopic survey of bulge RR Lyrae variables, tracers of possibly the oldest and most metal-poor stars in the bulge ([Fe/H] peaking at -1.0 dex). To date, we have obtained spectra of ~2000 OGLE bulge RR Lyrae stars in eight 2 degree windows. Our results indicate that the RR Lyrae stars exhibit hot kinematics and null or negligible rotation and are therefore members of a separate population from the bar/pseudobulge that currently dominates the mass and luminosity of the inner Galaxy. Our RR Lyrae stars predate these structures, and have metallicities, kinematics, and spatial distribution that are consistent with a “classical” bulge, or an inner halo.
Einstein’s Microscope: Caustic Transiting Stars in Lensing Clusters as Probes of Dark Matter Granular Structure
The physical nature of the Dark Matter (DM) has remained a mystery. Despite the triumph of the Cold Dark Matter (CDM) paradigm on large scales, empirical evidence for the DM structure on sub-galactic scales is scarce. Thanks to progress in observations, strong gravitational lensing has provided a promising avenue to address this fundamental question. As has recently been uncovered by the Hubble Space Telescope (HST), massive galaxy clusters, Nature’s most powerful gravitational magnifiers, can lens individual luminous stars at z~ 1–2 that lie fortuitously near the lensing caustic in a caustic straddling lensed galaxy, by extreme magnification factors ~ 10^2–10^3. I will discuss several important aspects about this remarkable phenomenon. First, these highly magnified stars have dramatically enhanced observability because they are inevitably subject to intermittent microlensing due to intracluster stars, and they are sensitive to stellar-mass compact DM objects inside the cluster halo. Second, their image positions in the critical curve vicinity can be perturbed by intervening low-mass DM subhalos inside the cluster halo, making them powerful targets for testing a key prediction of the CDM model. In addition, at the culmination of microlensing events with a magnification factor ~ 10^3 – 10^4, the star’s flux becomes susceptible to minuscule fluctuations in the lens surface density, which allows to probe mini-halos if the DM is made of the QCD axion. In the forthcoming years, the James Webb Space Telescope and possibly the giant ground-based optical/IR telescopes will play a huge role in fully realizing the scientific potential in the study of caustic transiting stars.
The circumgalactic medium (CGM; non-ISM gas within a galaxy virial radius) regulates the gas flows that shape the evolutionary paths of galaxies. It likely contains most of the metals lost in galaxy winds and enough material to sustain star-formation for billions of years. Owing to the vastly improved capabilities in space-based UV spectroscopy with the installation of HST/COS, observations and simulations of the CGM have emerged as the new frontier of galaxy evolution studies. In this talk, I will describe observational constraints we have placed on the origin and fate of this material by studying the gas kinematics, metallicity and ionization state of gas 10 – 200 kpc from galaxies’ stars. I will highlight the exciting new results from my group that constrain the cosmic baryon cycle on different physical scales, from black holes to the intergalactic medium. I will introduce several exciting new techniques for resolving the gaseous structures in the CGM, and will conclude by posing unanswered questions about the CGM that will be addressed with future survey data and hydrodynamic simulations in a cosmological context.
You can make a difference: Incorporating inclusive practices into Astronomy and your day-to-day work life
It is widely recognized that not all groups of people succeed in equal numbers in academia despite our best intentions to make the profession a fair and objective meritocracy. I will give a brief review of the historical context of oppression in the US and some statistics which demonstrates the current situation in science and astronomy. I will spend the majority of the talk discussing ways in which individuals at all seniority levels can take actions to increase the inclusion of marginalized groups in our profession.
Autumn Quarter 2018
An overview of UW Astronomy, announcements, and a welcome to new members of the department.
Asteroid families are the remnant fragments of asteroids broken apart by collisions. There are only a few known Main Belt (MB) asteroid families with ages greater than 2 Ga (Brož et al., 2013; Spoto et al.,2015). Estimates based on the family producing collision rate suggest that the lack of >2 Ga-old families may be due to a selection bias in classic techniques used to identify families. Family fragments disperse in their orbital elements, semi-major axis, a, eccentricity,e, and inclination, i, due to secular resonances and the non gravitational Yarkovsky force. This causes the family fragments to be more difficult to identify with the hierarchical clustering method (HCM), which attempts to find cluster in orbital element space, when applied to family fragments’ elements as the fragments age. We have developed a new technique that is insensitive to the spreading of fragments in e and i by searching for V-shaped correlations of family members in an asteroid diameter, D. A group of asteroids is identified as a collisional family if its boundary in the a vs. 1/D plane has a characteristic V-shape which is due to the size dependent Yarkovsky effect. The V shape technique is demonstrated on the known families and families difficult to identify by HCM, and used to discover a 4 Ga-old family linking most dark asteroids in the inner MB not included in any known family (Delbo’ et al., 2017). The 4 Ga-old family reveals asteroids with D > 35 km that do not belong to any asteroid family implying that they originally accreted from the protoplanetary disk and support recent theories on the formation of asteroids (Morbidelli et al., 2009).
As science educators, researchers, communicators, and/or supporters, we cannot deny the connection between science and government. The ability to send missions to Mars, to study star formation in a galaxy, and to model the early universe primarily depends on both government — i.e., taxpayer — money and public — i.e., not just scientist — support. Scientists can and do engage in work to determine the direction of our field and how society prioritizes science. As we approach the 2018 midterms, two years in to a new administration, how is science faring on a national stage? In this talk, I will discuss the current environment for science and space policy in general, and current events dominating policy discussions in the astronomical sciences in particular, including NASA flagship mission development, sexual harassment in the sciences, NSF facilities support, and how Congress has responded to the Trump administration’s science priorities. I will discuss the role of the American Astronomical Society (AAS) in such discussions and in advocating for the astronomical sciences in Washington and what individual scientists can do to effectively engage in the political process.
The success of ground-based transit and RV surveys, and the Kepler/K2 mission, has shifted the exoplanet field from pure discovery to a combination of discovery, demographic analysis, and detailed characterization, especially for exoplanet atmospheres. Unfortunately, most known transiting exoplanet hosts are too faint to permit atmospheric characterization. We are using data from the TESS, K2, and ground-based transit surveys like the Kilodegree Extremely Little Telescope (KELT) project to find planets around bright stars while addressing specific questions about planet formation and evolution. We are also studying the birthplaces of planets by searching for occultations of newly formed stars by their protoplanetary disks with our Disk Eclipse Search with KELT (DESK) survey. These systems provide insight into the conditions required for planet formation. I will describe our results and discuss how we will search for these kinds of objects in future surveys such as LSST.
The depth of next generation surveys poses a great data analysis challenge: these surveys will suffer from crowding, making their images difficult to deblend and catalog. Sources in crowded fields are extremely covariant with their neighbors and blending makes even the number of sources ambiguous. Probabilistic cataloging returns an ensemble of catalogs inferred from the image and can address these difficulties. We present the first optical probabilistic catalog, cataloging a crowded Sloan Digital Sky Survey r band image cutout from Messier 2. By comparing to a DAOPHOT catalog of the same image and a Hubble Space Telescope catalog of the same region, we show that our catalog ensemble goes more than a magnitude deeper than DAOPHOT. We also present an algorithm for reducing this catalog ensemble to a condensed catalog that is similar to a traditional catalog, except it explicitly marginalizes over source-source covariances and nuisance parameters. We also detail efforts to make probabilistic cataloging more computationally efficient and extend it beyond point sources to extended objects. Probabilistic cataloging takes significant computational resources, but its performance compared to existing software in crowded fields make it a enticing method to pursue further.
Stratospheric Observatory for Infrared Astronomy (SOFIA): Overview, The New Science Results, and Future Opportunities
The joint U.S. and German Stratospheric Observatory for Infrared Astronomy (SOFIA), to develop and operate a 2.5-meter infrared airborne telescope in a Boeing 747SP, has now successfully obtained first science with 7 instruments. The present instruments include the FORCAST camera in the 5 to 40 micron spectral region, the GREAT heterodyne spectrometer in the 63 to 240 micron spectral region, the FIFI-LS 50-210 micron imaging spectrometer, the EXES 5-28 micron high-resolution spectrometer and HAWC+ a 50 to 240 micron imaging polarimeter. An overview of the flying observatory, the instruments and new science results will be presented. Emphasis will be given to several observations of the Galactic Center Region and spectacular HAWC+ imaging polarimeter results. Future observing opportunities and participation in future instrument developments, over the 20 year lifetime of the observatory will be discussed.
Sandia National Laboratories is one of three DOE NNSA research and development laboratories whose primary mission is stockpile stewardship. These laboratories engage in many research areas, including bio-engineering, alternative energy, inertial confinement fusion. The Z-machine facility at Sandia is the world’s largest X-ray generator and is used to study high-energy-density physics. The Z-facility is ideal for studying atomic physics in the same conditions as those observed in astrophysical objects; this is the foundation for the Z-Astrophysical-Plasma-Properties (ZAPP) collaboration. The collaboration (funded in part by the Wootton Center for Astrophysical Plasma Properties) is actively trying to seek solutions to several problems: “Is opacity responsible for the discrepancy between the standard solar model and helioseismology?”, “Is resonant Auger destruction (RAD) the reason why certain spectral lines are not observed in black-hole accretion disks?”, “How accurate are the current line-broadening models (of neutral hydrogen) that are used to determine white dwarf masses?”. The iron-opacity experiment has revealed that there may be an unknown source of opacity for Fe at the base of the solar convection zone; experiment and data analysis methods are currently under scrutiny. RAD has been shown to not be 100% efficient and we are currently working on more fully characterizing the physics behind RAD. The white-dwarf-photosphere experiment found consistency between models for the broadening of H-beta. However, there is an unexplained discrepancy between emission and absorption line shapes, and significant discrepancies on the modeling the high-n lines of hydrogen.
The accelerated expansion of the Universe is the most surprising cosmological discovery in decades. It has inspired a new generation of ambitious surveys to determine the fundamental nature of this acceleration. I will introduce the different measurement techniques used by today’s cosmologists, describe the landscape of current and near future wide-field galaxy surveys, and present cosmology constraints from the first year of the Dark Energy Survey (DES Y1). This analysis constrains the composition and evolution of the Universe through a combination of galaxy clustering, galaxy-galaxy lensing, and cosmic shear. These three measurements yield consistent cosmological results, and in combination they provide some of the most stringent constraints on cosmological parameters. I will describe the validation of measurements and modeling from pixels to cosmology and I will give an outlook on cosmology analysis plans and challenges for future, much larger experiments such as LSST and WFIRST.
Ultra-compact binaries (UCBs) are systems containing compact or degenerate stars with orbital periods less than one hour. Tens of millions of UCBs are predicted to exist within the Galaxy emitting gravitational waves (GWs) at mHz frequencies. Combining GW searches with electromagnetic (EM) surveys like Gaia and LSST will yield a comprehensive, multimessenger catalog of UCBs in the galaxy. Joint EM and GW observations enable measurements of masses, radii, and orbital dynamics far beyond what can be achieved by independent EM or GW studies. GW+EM surveys of UCBs in the galaxy will yield a trove of unique insight into the nature of white dwarfs, the formation of compact objects, dynamical interactions in binaries, and energetic, accretion-driven phenomena like Type Ia superonovae.
50 years after their discovery, neutron stars are poised to take center stage in this era of multi-messenger astrophysics. I will highlight advances in theory and mention some key observations that have provided fundamental new insights about neutron star properties and their central role in nuclear astrophysics. I will discuss how neutron stars, and extreme phenomena involving them, can serve as laboratories to study dense matter, neutrinos, axions and dark matter.
Spring Quarter 2018
Following the Big Bang the Universe was homogeneous in matter, energy and barren of chemistry. It is the stars which built up the periodic table. Astronomers have now identified several classes of cosmic explosions of which supernovae constitute the largest group. The Palomar Transient Factory (PTF) consisting of the 48-inch Oschin Schmidt-optics telescope (hosting a large field-of-view mosaic detector) and the Palomar 60-inch robotic telescope (initially equipped with an imaging CCD photometer) was designed to explicitly undertake a systematic survey of the optical night sky. The speaker will talk about the returns and surprises from this project: super-luminous supernovae, new classes of transients, new light on progenitors of supernovae, detection of gamma-ray bursts by purely optical techniques and troves of pulsating stars and binary stars. The successor to PTF is the Zwicky Transient Factory (ZTF) with a wide field-of-view imager (47 square degrees) and an IFU spectrograph aimed squarely at classification on the 60-inch. ZTF is poised to become the stepping stone for the Large Synoptic Survey Telescope.
A significant fraction of heavy elements in the universe spend some time in the interstellar medium as dust grains. But how much do we really know about the origin and evolution of dust in the Universe? I will review the current state of astromineralogy and describe how observations of X-ray bright compact objects can yield key insights into the evolution of Milky Way dust. With high resolution X-ray spectroscopy, we directly measure the abundance of gas-state metals and mineral composition of dust in the interstellar medium (ISM). With imaging, we timing and imaging, we obtain dust grain size and spatial distributions from X-ray scattering halos around bright point sources. Understanding the scattering component of X-ray extinction is also important for interpreting accretion by compact objects, including the supermassive black hole at the center of our galaxy. I will review the most recent exciting dust scattering discoveries, many of which draw on multi-wavelength observations. Finally, I will explain how future X-ray observatories can tackle current open questions about the dusty Universe.
Estimating the mass of complex dynamical systems such as star clusters, galaxies, and galaxy clusters is important for testing astrophysical theories at a variety of scales, but is difficult in practice. The Milky Way Galaxy’s mass in particular is not well known within a factor of two. Popular techniques use position and velocity measurements of tracer objects, such as globular clusters and halo stars, to constrain the total gravitational potential of the Galaxy. This approach presents many challenges, including: different degrees of measurement uncertainties, incomplete data, and how best to use (or not use) multiple tracer populations in the analysis. I will discuss a hierarchical Bayesian method that estimates the mass of the Milky Way while attempting to overcome these challenges. We hope the method provides a concrete step forward in our understanding of the Milky Way’s total mass and mass distribution. I will describe the results as it applies to the Milky Way, as well as results from a series of blind tests on simulated data from galaxies made in cosmological simulations.
Supermassive black holes are in place by the first billion years of the universe’s existence. Several promising channels have be proposed for their formation, but forming supermassive black holes in the requisite time-frame remains a theoretical puzzle. One promising channel is that of direct collapse, in which a cloud of gas collapses to a massive seed black hole that then grows to supermassive size via accretion. In this talk I will give an overview of potential supermassive black hole formation channels and outline a suite of 3D hydrodynamical simulations probing conditions conducive to formation of a massive seed black hole via direct collapse.
Does the star and planet forming environment matter for protoplanetary disks and stellar properties? What are the observational signatures and theoretical implications? UV radiation from massive stars and stellar density may affect the protoplanetary disk properties including disk mass, disk size, accretion rate, and disk lifetime. The final mass of young stellar objects (YSOs) in high UV radiation and rich cluster environment may also be influenced due to early loss of circumstellar materials. In this talk I will present our on-going multi-wavelength studies of the Orion A region to probe these questions. I will focus on protoplanetary disks in three star forming regions within the Orion A cloud to probe the role of UV radiation and stellar density. The Orion Nebula Cluster, NGC 1977, and Lynds 1641 are all located in the Orion A cloud having similar ages, yet their environments are different. I will discuss our findings from our current surveys of these three regions in Orion A, and compare our findings to some low density, weak radiation environments, such as Taurus star forming region. This study is also a part of one of the NASA NExSS program, Earth in Solar System (EOS, PI Apai). I will also briefly talk about the EOS program.
Cosmological simulations can now make specific and detailed predictions for the shapes, masses, and substructure fractions in galactic dark matter halos that depend on the dark matter model assumed. Comparing these predictions to the observed mass distributions of galaxies should in principle lead to constraints on the nature of dark matter, but observable dynamical tracers can be scarce in regions where the dark matter distribution is best able to discriminate between models. One such region is the distant outskirts of galaxies, where the influence of baryonic matter on the dark matter halo is limited and the effect of dark substructures most prominent. New surveys of Milky Way stars like Gaia, alongside next-generation instruments and giant telescopes, are for the first time providing accurate positions, velocities, and chemical abundances for large numbers of stars in faint tidal streams: remnants of tidally-disrupted satellite galaxies that trace out the mass distribution in the distant reaches of galaxy halos. I will show how state-of-the-art simulations play a crucial role in interpreting and analyzing this wealth of new information about stellar halos, and how stellar halo observations over the next decade will characterize the dark matter distribution in galaxies, test theories of the nature of dark matter, and illuminate the role of dark matter in galaxy formation.
Observations from the WISE and NEOWISE missions have provided invaluable information about the diameter, visible and IR albedos, and other properties of ~164,000 asteroids—more than 16 times as many as all previous infrared asteroid observations combined. No other four-band space observing mission is currently planned. So it is imperative that we understand the strengths and weaknesses of this data and have clearly documented, reproducible methods of analyzing them. Dr. Nathan Myhrvold will discuss what he has learned from his independent, two-year empirical examination of the NEOWISE methods and results. His investigation has found that despite numerous preliminary papers published by the NEOWISE group, analysis of these data is far from a solved problem. Much work remains to be done to address systematic errors and inconsistencies in the results, to correct poorly fit properties, and to develop open, transparent analytical methods that can be reproduced for all of the data sets.
The circumgalactic medium (CGM; non-ISM gas within a galaxy virial radius) regulates the gas flows that shape galaxies. Owing to the vastly improved capabilities in space-based UV spectroscopy with the installation of HST/COS in 2009, observations and simulations of the CGM have emerged as the new frontier of galaxy evolution studies. In the last decade, we have learned that the CGM of Milky Way mass galaxies likely contains enough material to harbor most of the metals lost in galaxy winds and to sustain star-formation for billions of years. Remarkably, this implies that most of the heavy elements on earth cycled back and forth multiple times through the Milky Way’s own CGM before the formation of the solar system. I will describe constraints we have placed on the origin and fate of this material by studying the gas kinematics, metallicity and ionization state. I will conclude by posing several unanswered questions about the CGM that will be addressed with future survey data and hydrodynamic simulations in a cosmological context.
Winter Quarter 2018
The intergalactic medium (IGM) plays a unique role in constraining the (small scale) matter power spectrum, since the low-density, high redshift IGM filaments are particularly sensitive to the small scale properties of dark matter. The main observable manifestation of the IGM, the Lyman-alpha forest, has provided important constraints on the linear matter power spectrum, especially when combined with cosmic microwave background data. This includes, most notably, the tightest constraints on warm dark matter (WDM) and fuzzy dark matter (FDM) models, that I will present in this talk.
Galaxies are complicated beasts – many physical processes operate simultaneously, and over a huge range of scales in space and time. As a result, creating accurate models of the formation and evolution of galaxies over the lifetime of the universe presents tremendous challenges. In this talk I will discuss these challenges and their solutions, and will explain how large-scale computational models can be used to gain insights into the very first galaxies that formed in the universe (over 13 billion years ago!), and how we can use both these computational models and observations of the Milky Way and its neighbors to infer how galaxies have grown and evolved in the intervening time.
Although thermonuclear (Type Ia) supernovae and neutron star mergers are some of the most important astrophysical events, our understanding of these explosions is vague. I will present abundance measurements of elements across the periodic table (Mg, Fe, Co, Ba, and others) that address the nature of both types of explosions. The measurements are based on Keck/DEIMOS spectroscopy of red giants in dwarf galaxies, which experienced a large number of Type Ia supernovae. The iron-peak elemental abundances strongly suggest that the majority of Type Ia supernovae in dwarf galaxies exploded below the Chandrasekhar mass, i.e., the double-degenerate model or the single-degenerate, double-detonation model. The DEIMOS spectra also reveal that barium comes from the r-process and appears in the dwarf galaxies on a timescale similar to iron (at least 100 Myr). Therefore, the mostly likely origin is not supernovae but neutron star mergers. The evolution of the [Ba/Fe] ratio indicates a neutron star merger rate consistent with results from LIGO.
What happened after inflation? Pretty much everything. But if we make this statement more precise and ask, “What happened immediately after inflation?” the answer is that we actually don’t know. The era immediately following the universe’s first period of accelerated expansion is known as reheating, and it is a part of the cosmic timeline for which we have no observables. In this talk, I will give some background on inflation, explain what reheating is, and describe my work on related questions at the non-dark matter intersection of particle physics and astrophysics.
One of the key objectives of modern astrophysics is to understand the formation and evolution galaxies. In this regard, the Milky Way is a fantastic testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical, chemical, and age distributions of its stellar populations. Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.
The NASA Juno Spacecraft is the ninth spacecraft launched from Earth to reach Jupiter and only the second to orbit the largest planet in the Solar System. It is also the first solar-powered spacecraft to orbit Jupiter. Launched in 2011 and currently in polar orbit around Jupiter, Juno’s primary goal is to improve our understanding of the formation and evolution of Jupiter. Juno has now completed 11 perijove passes over a wide range of Jovian longitudes. This talk will provide an overview of the Juno mission and present a few early results.
Precise and accurate star formation histories from galaxy spectra and the path to life finding NIR detectors
Recent discoveries of black hole mergers and the collision of neutron stars by the Laser Interferometer Gravitational wave Observatory (LIGO) have opened up the era of gravitational wave astronomy and mutimessenger astronomy. The Advanced LIGO detectors are currently being upgraded in preparation for their third observing run (O3), looking forward to more gravitational wave detections including the possibility of the first un-modeled gravitational wave signals. Beyond Advanced LIGO, the future of gravitational wave astronomy includes the Laser Interferometer Space Antenna (LISA) mission led by the European Space Agency (ESA). Meanwhile, Pulsar Timing Arrays (PTAs) such as the North American Nanohertz Observatory for Gravitational waves (NANOGrav) are poised to make detections of gravitational waves in the nanohertz range. The historic first detections of gravitational waves have opened a new way to study our universe, with discovery potential across the entire gravitational wave spectrum.
Autumn Quarter 2017
I will describe our development of a convolutional neural network (CNN) to learn to search for and characterize absorption lines in quasar spectra. Specifically, the algorithm discovers and measures the redshift and Hydrogen column density of damped Lya systems (DLAs). These systems dominate the neutral hydrogen gas of the universe, trace the interstellar medium of distant galaxies, and offer cosmological constraints on the build up of gas and heavy elements across cosmic time. I will discuss the lessons learned employing CNN techniques on large spectral datasets and the prospects for future analysis.
Line Intensity Mapping has emerged as a powerful tool to probe the large-scale structure across redshifts, with the potential to shed light on dark energy at low redshifts and the cosmic reionization process at high redshift. Multiple spectral lines, including the redshifted 21cm, CO, [CII], and Lyman-alpha transitions, are promising tracers with several experiments on-going or in the planning. I will describe current pilot programs and cross-correlation sciences, and present future prospects.
A prediction of axion dark matter models is they can form Bose-Einstein condensates and rigid caustic rings as a halo collapses in the non-linear regime. In this talk, I will present results from the first study of the caustic ring model for the Milky Way halo (Duffy & Sikivie 2008), focusing on observational consequences. I will describe the formalism for calculating the gravitational acceleration of a caustic ring halo. The caustic ring dark matter theory reproduces a roughly logarithmic halo, with large perturbations near the rings. I will show that this halo can reasonably match the known Galactic rotation curve. We explored the effects of dark matter caustic rings on dwarf galaxy tidal disruption with N-body simulations. N-body simulations of the Sagittarius (Sgr) dwarf galaxy in a caustic ring halo potential, with disk and bulge parameters that are tuned to match the Galactic rotation curve, match observations of the Sgr trailing tidal tails as far as 90 kpc from the Galactic center. Like the Navarro-Frenk-White (NFW) halo, they are, however, unable to match the leading tidal tail. None of the caustic, NFW, or triaxial logarithmic halos are able to simultaneously match observations of the leading and trailing arms of the Sagittarius stream. I will further show that simulations of dwarf galaxies that move through caustic rings are qualitatively similar to those moving in a logarithmic halo.
The sky in X-rays is incredibly dynamic. Black holes and neutron stars vary on time scales ranging from milli-seconds to decades, their brightness occasionally changing by several orders of magnitude within seconds or minutes. Studying this variability is one of the best ways to understand key physical processes that are unobservable on Earth: general relativity in strong gravity, extremely dense matter and the strongest magnetic fields known to us are just a few examples. In this talk, I will give an overview of the state-of-the-art of time series analysis in high-energy astronomy. I will present key statistical methods and machine learning models we have been developing recently as well as point out the opportunities and challenges of the spectral-timing revolution we are moving toward with data from current and future space missions. At the same time, this talk also chronicles my path from conventional astronomical research into data science and data-intensive astronomy, and thus touches upon some of the occasionally surprising turns that path has taken.
The Dawn of Multi-Messenger Astrophysics with Gravitational Waves: the first joint detection of GWs and Light
I present the results from the first detection of gravitational waves and light from the same cosmic source, GW170817. GW170817 is the merger of two neutron stars, detected in Gravitational Waves (GWs) by Advanced LIGO/Virgo on August 17, 2017. The GW detection was followed by the discovery of electromagnetic emission across ten orders of magnitude in wavelength, which allowed the localization of GW170817 to a galaxy in our own cosmic neighborhood. In this colloquium I will present the results from the first successful hunt for electromagnetic radiation from a GW source across the electromagnetic spectrum, from the X-rays to the radio band. I discuss the open questions raised by GW170817 and the bright future ahead for multi-messenger Astrophysics.
How well you communicate your research to the media and the public can make or break your career in astronomy. But the imperatives that drive science media can seem mysterious. What makes research projects and results newsworthy? What are the ingredients of a compelling science story? How can you help journalists get it right? When is it better to bypass the media and tell your story yourself? Two veteran science writers will pull back the curtain on the inner workings of science publications and share tips on how to convey your work compellingly without sacrificing clarity or accuracy. W. Wayt Gibbs, a contributing editor for Scientific American and editorial director at Intellectual Ventures, will dissect the anatomy of an astronomy cover feature and explain how he successfully pitched an astronomy story to the New York Times, Science, Scientific American, and other major outlets, while simultaneously exploiting social media channels. And Alan Boyle, an award-winning space journalist for MSNBC and currently aerospace and science editor at Seattle’s GeekWire tech news site, will illuminate how science news works and the challenges of writing short and fast for a general audience.
Since Hubble’s early classification of galaxies a century ago, it has been clear that galaxies fall into 2 broad categories – those that are blue and characterized by spiral disks, and those that are red and elliptical in shape. With the advent of modern spectroscopic surveys, this galaxy ‘bimodality’ has been understood in the context of star formation, in which galaxies can be broadly classified as either star forming or passive. However, the physical processes that regulate star formation, transforming galaxies between these populations, remain hotly debated. In this talk, I will review recent results that investigate some of the mechanisms proposed to play a role in the enhancement and suppression of star formation, including galaxy mergers, bars and the role of active galactic nuclei (AGN). I will also present new results from the SDSS MaNGA survey that uses spatially resolved spectroscopy to map where in the galaxy these processes are occurring. Taken together, I will show that whilst a wide variety of mechanisms can alter a galaxy’s global star formation, these processes seem to generally both boost and quench star formation from the inside-out.
The local expansion rate of the Universe, the Hubble constant, is one of the fundamental parameters in our current concordance cosmology and one that anchors the expansion history of the Universe. The resolution of the historical factor-of-two controversy in the Hubble constant nearly two decades ago (e.g., the Hubble Space Telescope Key Project; Freedman et al. 2001) has evolved into a 3.4-sigma tension between the traditional Cepheid-distance ladder measurements (Riess et al. 2016, Freedman et al. 2012, Freedman et al. 2001) and that determined from modelling anisotropies in the cosmic microwave background (CMB; Planck Collaboration et al. 2016). At the heart of the tension, is not only a difference in method, but also a fundamental difference in the state of the observed Universe: the distance ladder measures the local rate in the nearby universe (e.g., z~0), whereas the CMB anisotropy measurements uses the very young Universe (z ~1100). Resolution of the tension requires (i) a full scale evaluation of the systematic effects in either technique or (ii) “new physics” added to the standard cosmological model. The trigonometric parallaxes provided by Gaia in the near term permit an unprecedented opportunity to use alternative standard candles and construct a full end-to-end distance ladder without Cepheids. The Carnegie-Chicago Hubble Program is doing just that; we are in the middle of building a new distance ladder that relies on the tip of the red giant branch (TRGB; Beaton et al. 2016). As I will demonstrate, this not only provides a direct cross-check on the Cepheid path, but there are numerous advantages to using a distance indicator that, as a standard candle from old stellar populations, is nearly ubiquitously present low-crowding and low-extinction components of galaxies. More specifically, by being able to calibrate every ‘local’ SNe Ia and easily probing ever-larger volumes with JWST and WFIRST, the TRGB-based distance ladder paves a clear path to a 1% measurement within the foreseeable future.
The Zeeman effect is the only observational tool that allows us to directly measure the magnetic field strength and direction in the interstellar medium. We provide an overview of ongoing projects in which we are using Zeeman splitting of the 21-cm hydrogen line and the 18-cm hydroxyl (OH) transitions in order to probe astrophysical magnetic fields. We will highlight the first detection of extragalactic Zeeman splitting in the OH megamaser emission from starburst galaxies. Results will be shown from a survey of Zeeman splitting of OH masers in our Milky Way’s spiral arms that show large-scale field directions conflicting with those probed by Faraday rotation. We will also discuss previous measurements of the Zeeman effect in Galactic 21-cm radio emission, some of the instrumental challenges involved in such measurements, and plans for a large-scale survey using the 26-m John A. Galt Telescope at the Dominion Radio Astrophysical Observatory.
Spring Quarter 2017
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Future constraints on the cosmological initial conditions and the different constituents of the cosmic energy budget will be driven by large-scale structure surveys, as we have extracted much of what is possible from our workhorse, the cosmic microwave background. I will discuss some challenges with using these surveys to understand how to relate what we observed to the cosmic initial conditions. I will mainly focus on a mini-revolution in how we understand the late time cosmos perturbatively, which potentially enables us to extract more information from the large-scale matter distribution. I will also briefly comment on some work my group is doing that is relevant to large weak lensing surveys (the era of large weak lensing surveys is dawning!) and the Lyman-alpha forest (a large-scale structure probe that has never been as successful as one might hope – we are working to understand why).
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Karl-Heinz Bohm and Erika Bohm-Vitense arrived at the University of Washington Astronomy Department in 1968. Over the next decades their research programs made important contributions to various aspects of stellar evolution. Karl-Heinz worked on Herbig-Haro objects and other types of pre-main sequence stars. Erika made major contributions to stellar abundances, convection and rotation. Both of them served as advisors for the Ph.D. research of many graduate students and were known particularly for their teaching of graduate courses. In this colloquium we will highlight the backgrounds and some of the research contributions of this UW Astronomy Dynamic Duo.
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Gravitational lensing represents a unique tool to study the dark Universe. In the weak lensing regime small distortions in the images of galaxies caused by the large-scale structure can be detected over the whole sky. Measuring these coherent distortions yields cosmological insights complementary to other probes like the cosmic microwave background (CMB). Ongoing wide-field imaging surveys exploit this to come up with competitive constraints on important cosmological parameters. In this colloquium I will concentrate on recent results from the ongoing European Kilo Degree Survey (KiDS) and show a mild tension of these results with CMB measurements from the Planck mission. Possible future developments will be discussed that could help make cosmic shear measurements even more robust and lead to an answer to the question whether this tension is real or not. I will conclude with an outlook towards future missions like Euclid, LSST, and WFIRST.
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At the outskirts of the solar system, beyond the orbit of Neptune, lies an expansive field of icy debris known as the Kuiper belt. The orbits of the individual asteroid-like bodies within the Kuiper belt trace out highly elongated elliptical paths, and require hundreds to thousands of years to complete a single revolution around the Sun. Although the majority of the Kuiper belt’s dynamical structure can be understood within the framework of the known eight-planet solar system, bodies with orbital periods longer than 4,000 years exhibit a peculiar orbital alignment that eludes explanation. What sculpts this alignment and how is it preserved? In this talk, I will argue that the observed clustering of Kuiper belt orbits can be maintained by a distant, eccentric, Neptune-like planet, whose orbit lies in approximately the same plane as those of the distant Kuiper belt objects. In addition to accounting for the observed grouping of trajectories, the existence of such a planet naturally explains other, seemingly unrelated dynamical features of the solar system.
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Massive stars have a profound astrophysical influence throughout their tumultuous lives and deaths. Stellar feedback – the injection of energy and momentum by stars to the interstellar medium (ISM) – occurs through a variety of mechanisms: radiation, photoionization heating, winds, jets/outflows, supernovae, and cosmic-ray acceleration. Despite its importance, stellar feedback is cited as one of the biggest uncertainties in astrophysics today, stemming from a dearth of observational constraints and the challenges of considering many feedback modes simultaneously. In this talk, I will discuss how a systematic approach to multiwavelength observations can be used to overcome these issues. I will summarize results from application of these methods to massive-star regions in the Milky Way and nearby galaxies, where feedback processes are best resolved. Finally, I will highlight exciting prospects of using current and upcoming facilities to explore feedback in diverse conditions.
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Polarimetry provides unique information about planetary atmospheres and surface properties complementing more conventional observations. However, polarimetry is currently an under- utilized technique for exoplanet characterization, especially for smaller planets, where its utility is poorly understood. Models of polarised light from terrestrial planets can allow for the detection of biosignatures and habitability markers. Combining the abilities of the Virtual Planetary Laboratory’s SMART radiative transfer code (glint capabilities from a Cox-Munk Formalism ) and the University of New South Wales’ VSTAR radiative transfer code (polarimetric capabilities) we explore the detectability of ocean glint for an Earth-like planet in polarised light. This is compared to theory (e.g.   ) and assessed in observational contexts.
The practical utility of polarimetry in determining cloud species, identifying biomarkers, and detecting ocean glint is assessed with a first order example in the form of the Earth as an exoplanet. In a similar vein to photometric surface mapping (see ), polarimetric surface mapping can provide detailed information about terrestrial exoplanets which may be crucial to exotic worlds such as super Earths and planets around red dwarfs. We explore the capabilities of polarimetry in the context of state-of-the-art Earth-based imaging and aperture polarimeters (e.g. SPHERE  or HiPPI ) and next era space telescopes (e.g. HabEx or LUVOIR). This research is relevant to upcoming large ground-based and future NASA exoplanet characterization missions, such as the proposed HabEx and LUVOIR telescope concepts, particularly in the context of coronography.
References:  Cox, C. and Munk, W. (1954) JOSA, 44, 11  Bailey, J. (2007) Astrobiology, 7, 2  Seager, S. et al. (2000) ApJ 540, 1, 504-520  Zugger, M.E. et al. (2010) ApJ, 723, 1168-1179  Cowan, N. et al. (2009) ApJ, 700, 2, 915-923  Thalmann et al. (2008) SPIE, 7014  Bailey, J. (2015) MNRAS, 449, 3, 3064-3073
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My collaboration recently argued that Kepler space telescope data includes “golden” stars whose luminosities vary quasiperiodically, with two frequencies nearly in the golden ratio, and whose secondary frequencies exhibit power-law scaling with an exponent near -1.5, suggesting strange nonchaotic dynamics. Fractal state space structure without sensitive dependence on initial conditions is a distinctive but underappreciated dynamics between order and chaos. While familiar strange chaotic attractors famously exhibit complicated and unrepeatable dynamics, strange nonchaotic attractors exhibit complicated but predictable dynamics. In this talk, I describe how spectral scaling can distinguish between different sub classes of RR Lyrae variable stars in both Kepler and Optical Gravitational Lensing Experiment (OGLE) photometry. I then use a series of phenomenological models to make plausible the connection between golden stars and fractal spectra. I thereby suggest that some features of variable star dynamics may reflect universal nonlinear phenomena common to even simple systems.
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The James Webb Space Telescope has just passed two huge milestones; completion of: 1.) the telescope itself; and 2.) its entire instrument package. The project has held schedule (and budget) since being replanned in 2011 and is heading toward launch in October, 2018. Although there are many more steps to take before then, most notably a full system test in the mid-2017, it is urgent to plan how to use our new flagship space telescope. The guaranteed time observational programs will have been defined, an Early Release Science program will allocate 500 hours roughly in July, 2017, and the call for General Observer proposals will be released in November 2017. This talk will focus on the capabilities of the observatory, emphasizing the advances over present (and even some future) facilities, with examples of the science it will enable. JWST will redefine our views of topics ranging from exoplanets to the first galaxies and super-massive black holes.
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In the last several years, the disruption and accretion of stars by super-massive black holes (SMBHs) has been linked to tens of luminous flares observed in the cores of nearby galaxies. Our theoretical understanding of these tidal disruption events (TDEs), however, remains incomplete. While recent simulations have provided unprecedented detail on the dynamics of the disruption, we still do not have a good understanding of how infalling gas circularizes and accretes onto the SMBH, or how or where the thermal emission we observe is generated. The art of modeling the tidal disruption of stars by massive black holes forms the main theme of my talk. Detailed simulations should tell us what happen when stars of different types get tidally disrupted, and what radiation a distant observer might detect as the observational signature of such events.
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Disk initial conditions combined with formation processes determine the bulk composition of a planet. I will discuss new work showing that the masses of observed protoplanetary disks may be larger than previously thought and suggest that the features of high-mass disks, combined with the efficacy of pebble accretion in turbulent gas, may explain the observed distribution of giant planets. Within this context, I will suggest a possible source for the observed correlation between planet mass and metallicity among giants, discuss implications for planetary system architectures, and provide predictions for correlations between the compositions of low-mass planets discovered by Kepler/K2 and the likely presence and properties of giant planets orbiting the same stars on more distant orbits.
Winter Quarter 2017
Galaxy mergers drive central inflows of gas, which are an important triggering mechanism for AGN. Kpc-scale separation supermassive black hole pairs in ongoing galaxy mergers are ideal tracers of this link between galaxy mergers and AGN. In dual AGN systems both black holes are fueled as AGN, whereas in offset AGN systems only one of the black holes is an AGN. I will present multiwavelength approaches to building catalogs of dual AGN and offset AGN, and show the results of our observing campaigns with HST, Chandra, VLA, and Keck. Finally, I will discuss what our results show about whether galaxy mergers preferentially fuel the most luminous AGN, which black hole in a merger is more efficient at accreting gas, and where in a merger the AGN fueling occurs.
Galaxy clusters provided the first observational evidence of dark matter. They remain important cosmological probes today, as evidenced by recent controversy about the apparent deficit of clusters observed in Planck studies of the Sunyaev-Zeldovich (SZ) effect compared to the number of clusters expected based on observations of the microwave background. One possible solution would be that multiple cluster mass estimators are systematically underestimated by ~60%. There are now three methods to identify large samples of clusters for cosmological studies: optical richness, X-ray flux, and SZ decrement. I will describe a series of spectroscopic surveys we have conducted of cluster samples identified with each of the three methods using MMT/Hectospec. These surveys show that dynamical mass estimators such as velocity dispersion agree well with most mass proxies, albeit with substantial scatter. In particular, Planck-SZ mass estimates (calibrated from hydrostatic X-ray masses) agree well with measured velocity dispersions. This agreement suggests that SZ mass bias is too small to resolve the Planck SZ-CMB tension. I will discuss possible explanations for this tension and how we can test some of them in the near future.
The stellar disks of galaxies are enormous, but they are only the tip of the iceberg: galaxies are embedded in much larger structures known as halos, roughly ten times as big and ten times as massive as the galaxy disk. Galaxy halos are expected to be filled with hot, diffuse plasma; this hot plasma plays a crucial role galaxy formation, and fundamentally determines the mass-scale of galaxies. As new observational techniques begin to probe halo gas for the first time, however, they have found that their halos are also full of cold gas. This cold gas is unexpected because it cannot be hydrostatically supported against gravity. Moreover, these observations typically indicate a relatively modest total fraction of cold gas (~0.01% by volume), yet find it in essentially every sightline through the galaxy. It’s hard to understand how so little gas manages to be present everywhere. I will show that cold gas clouds are prone to “shattering” into fragments, producing a mist of tiny, distributed cloudlets which naturally reproduce these unexpected observations. This same effect dramatically enhances the drag force coupling the dynamics of cold and hot gasses; I will also discuss potential applications to entrainment of cold gas in galaxy winds, and the possibility of using cold gas to constrain the kinematics of the hot halo gas.
Our current story for the origin of the heavy elements has at its core a recycling program on cosmological scales. Using telescopes on the ground and in space, we have traced baryons as they flow from the intergalactic medium into galaxies, where they are incorporated into stars, undergo fusion, and are expelled in supernovae and other types of stellar death. The process can repeat many times, raising the cosmological heavy element contribution to the baryon budget. In principle, if we run the clock backwards far enough, we could observe the very first stars, aka ‘Population III stars’: those formed from only those elements created in nucleosynthesis shortly after the big bang. As of yet, however, the Population III stars have eluded direct detection. In this talk, I will present a new suite of tools and techniques used in the hunt for Population III stars, and will discuss how the future generation of telescopes may finally reveal these first stars in the universe.
It’s Alive, Alive! The Commissioning of APOGEE-2 South Instrument and the Use of Data-Driven Techniques for Analysis of APOGEE Results
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is one of the cornerstone projects of the Sloan Digital Sky Survey (SDSS). APOGEE is a large-scale (> 10^5), near-infrared (1.51-1.69 μm), high-resolution (R~22,500) spectroscopic survey that generates highly accurate radial velocity and element abundance values for stars of the Milky Way Galaxy. The APOGEE data are used to generate metallicity gradient, metallicity distribution function, and star formation rate information that in turn can be used to shed light upon Galactic assembly and growth.
Over the 2008-2011 period, APOGEE-1 observations were taken at the 2.5m Sloan Foundation Telescope of the Apache Point Observatory (APO). In 2014, APOGEE-2 continued data acquisition at APO (APOGEE-2N). Additionally, the build of the second APOGEE instrument began in earnest. This second spectrograph is located at the 2.5m du Pont Telescope of the Las Campanas Observatory (APOGEE-2S). First light of the APOGEE-2S instrument is set for February 15, 2017. The “all sky” view of APOGEE-2N and APOGEE-2S extends the APOGEE-1 chemodynamical examination of the Milky Way to include: the four disk quadrants, the inner and outer halo, the full expanse of the bulge, tidal streams, and local satellite galaxies.
I will provide an overview of the APOGEE-2 Project. I will describe the (herculean) set-up process of SDSS operations in the Southern Hemisphere and relay details of the instrument build, infrastructure modifications, instrument reassembly and reintegration at LCO. Furthermore, I will highlight a few APOGEE scientific efforts including those which harness APOGEE data via “data-driven” techniques (e.g. The Cannon).
Over the past two decades ongoing surveys have detected thousands of new planetary systems around nearby stars. These systems include apparently single gas giant planets on short period orbits, closely packed systems of up to 5-6 “mini-Neptunes”, and solar-system-like architectures with either one small planet or no planets interior to 0.5 AU. Despite our success in cataloguing the diverse properties of these systems, we are still struggling to develop narratives that can explain their divergent evolutionary paths. In my talk I will describe two promising new avenues of investigation, including constraints on the compositions of short-period planets and statistical studies of the frequency of outer gas giant and stellar companions in these systems. Taken together, these observations provide important clues that can be used to determine whether or not the observed population of short period exoplanets formed in situ or migrated in from farther out in the disk.
Galaxy clusters contain large amounts of cold dark matter, hot ionized gas, and tens to hundreds of visible galaxies. The abundance of clusters as a function of mass and redshift can be used to probe the growth of structure and constrain cosmological parameters. Dynamical measurements probe the entire mass distribution, but standard analyses yield unwanted high mass errors. First we show that modern machine learning algorithms can improve mass measurements by more than a factor of two compared to using standard scaling relations. Support Distribution Machines are used to train and test on the entire distribution of galaxy velocities to maximally use available information. Second we show that cluster abundance can be quantified with the distribution of direct observables rather than inferred mass to avoid uncertainties in the mass-observable relation. A novel statistic called the velocity distribution function (VDF) is constructed by stacking the probability distribution of galaxy velocities for a select number of clusters in a given volume. The VDF can be measured directly and precisely, and produces unbiased constraints on cosmological parameters. Finally we discuss how our approaches can be generalized to multi-wavelength observations of gravitational lensing, SZ effect, and X-ray emissions.
As a result of the high precision and cadence of surveys like MOST, CoRoT, and Kepler, we may now directly observe the very low-level light variations arising from stellar granulation in cool stars. Here, we discuss how this enables us to more accurately determine the physical properties of Sun-like stars, to understand the nature of surface convection and its connection to magnetic activity, and to better determine the properties of planets around cool stars. Indeed, such sensitive photometric “flicker” variations are now within reach for thousands of stars, and we estimate that upcoming missions like TESS will enable such measurements for ~100,000 stars. We present recent results that tie “flicker” to granulation and enable a simple measurement of stellar surface gravity with a precision of ~0.1 dex. We use this, together and solely with two other simple ways of characterizing the stellar photometric variations in a high quality light curve, to construct an evolutionary diagram for Sun-like stars from the Main Sequence on towards the red giant branch. We discuss further work that correlates “flicker” with stellar density, allowing the application of astrodensity profiling techniques used in exoplanet characterization to many more stars. We also present results suggesting that the granulation of F stars must be magnetically suppressed in order to fit observations. Finally, we show that we may quantitatively predict a star’s radial velocity jitter from its brightness variations, permitting the use of discovery light curves to help prioritize follow-up observations of transiting exoplanets.
Autumn Quarter 2016
Join us as we welcome new folks, address the state of the department, and highlight plans for another year of UW astronomy.
The Latte Project is a new suite of cosmological zoom-in baryonic simulations that model the formation of Milky Way-like galaxies at parsec-scale resolution, using the FIRE (Feedback in Realistic Environments) model for star formation and stellar feedback. Using these simulations, I examine the roles of accretion and stellar feedback in driving the formation and structure of disk galaxies like the Milky Way. The Latte simulations also self-consistently resolve the dwarf galaxies that form around each host, and I will discuss the relative impacts of internal stellar feedback and external environment on their star formation histories, stellar kinematics, and chemical enrichment histories, including progress in addressing the long-standing “missing satellites” and “too-big-to-fail” problems of LCDM cosmology. Finally, I discuss promising new avenues for using upcoming observations to test the role of feedback in dwarf galaxies.
Over the past decade it has become increasingly clear that supermassive black holes are essential components of galaxies, as demonstrated by the correlations connecting black hole masses and galaxy bulge properties. Although ~100 dynamical black hole mass measurements have been made to date, the local black hole mass census is highly incomplete. Gaining a more complete picture of black hole demographics and a deeper understanding of the mechanisms that drive black hole/galaxy evolution requires the measurement of black holes in a wider range of galaxy types with diverse evolutionary histories. In this talk, I will discuss the observation and measurement of black holes in an exciting population of local compact galaxies that appear to be relics of the z~2 quiescent galaxies. I will also describe our new Gemini Large and Long Program aimed at addressing a bias in the types of galaxies for which dynamical black hole mass measurements have been made.
The environments extending several hundred kiloparsecs from galaxies contain the fuel that feeds galactic star formation, and act as the reservoir into which ejecta from stellar and AGN feedback are driven. Observations of the cool hydrogen and metal content, kinematics, and morphology in these regions (i.e., the circumgalactic medium, or CGM) can therefore provide incisive tests of our understanding of these processes. Background quasar spectroscopy has long been the tool of choice for such observations; however, spectroscopy of bright galaxies can also be used to study the halos of galaxies in the foreground (as well as their own). Moreover, galaxies offer background beams many orders of magnitude larger than those provided by QSOs and may be spatially resolved with modern IFUs. I will discuss the first systematic survey to use this technique to characterize cool gas in individual foreground galaxy halos, reporting a unique constraint on the morphology of MgII absorption in the z~0.5 CGM. I will then describe new measurements of gas flows made possible with the spatially-resolved galaxy spectroscopy being obtained by the SDSS-IV/MaNGA survey, demonstrating MaNGA’s potential for establishing the frequency and cross section of gas accretion onto galaxies. Such three-dimensional study is crucial to understanding the thermodynamics of the gas flows which regulate galaxy growth.
Massive stars are the “cosmic engines” of the Universe, proving most of the UV ionizing radiation of galaxies, while also powering their far-IR luminosities. And, massive stars serve as the primary source of carbon and oxygen enrichment of the ISM, as well as manufacturing the elements heavier than Fe during their core-collapse deaths as SNe. Population III massive stars likely played a key role in the re-ionization of the Universe, and their black-hole remnants may have formed the seeds of the super-massive black holes found in the centers of many galaxies today. However, the physics of massive star is complicated, and feedback from observations is crucial to advancing stellar evolution modeling. Massive star evolution depends heavily on the metallicity of the gas out of which these stars form owing to the importance of mass loss via their strong stellar winds. Thus we can use the galaxies of the Local Group as our laboratories for closing the loop in our understanding of massive star evolution. In this talk I will discuss how we identify OB stars, Luminous Blue Variables, yellow and red supergiants, and Wolf-Rayet stars, and what we have so far learned from these studies.
Discussion of the scientific as well as public policy challenges related to potential asteroid impact scenarios we will be faced with when the LSST comes online. At that point, the discovery rate of Near Earth Asteroids will increase by more than a factor of 20 over the current rate, and the list of asteroids with worrisome probability of hitting the Earth will also become much larger.
For most of the history of the universe, quasars have been the dominant source of ionizing photons. Most of the baryons in the universe reside in the intergalactic medium (IGM), even at the present. The most dramatic interaction between these two was the last major phase transition of the universe, when quasars fully reionized helium at z~3. I will discuss our current observational understanding of the helium reionization epoch, as well as what these observations reveal about quasars themselves in the extreme UV (EUV). The EUV quasar spectrum can be modeled as a power law, but the average spectral index and its dispersion are poorly understood, leading to large uncertainty in theoretical models of IGM ionization. Restframe EUV observations with HST are allowing us to finally constrain quasar continuum emission in the EUV, and disentangle it from weak emission lines.
The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) is undertaking an all-sky thermal infrared survey to both discover new near-Earth asteroids and comets, and characterize previously known NEOs. NEOWISE provides simultaneous imaging at 3.4 and 4.6 microns, measuring the thermal emission from NEOs and allowing their diameters
to be computed. I will give an overview of the NEOWISE mission, and present some recent results from this dataset.
We are currently in the midst of several sky surveys and on the cusp of more in the future. Both targeted and wide surveys have provided much needed information on the numbers and origin of cataclysmic variables. I will review some of the results from the past and ongoing surveys, with emphasis on UV and optical wavelengths, and point out what is needed to make future progress in this field.
In this talk I will discuss the observational techniques I use to understand the progenitor systems of Type Ia supernovae (SN Ia), a powerful cosmological standard candle. The characteristics of a SN Ia explosion strongly indicates the thermonuclear detonation of a carbon-oxygen white dwarf star, but how and why it explodes, and the role of a binary companion, are not well constrained. We will explore several unique case scenarios and investigate the evidence for and against non-degenerate companion stars, such as the presence of circumstellar material and emission from the impacted secondary star. We may also delve into the reverse problem for candidate Type IIn SN 2009ip, a transient for which the progenitor system was well characterized but the true nature of the explosion remains shrouded in mystery.
Spring Quarter 2016
Where do new telescopes come from? To start this process NASA has recently convened a group of Science and Technology Definition Teams. These teams will explore the science yield and engineering challenges for exciting new astronomy flagship mission concepts to follow the upcoming James Webb Space Telescope. UW Astronomy professors Julianne, Vikki and Eric will describe aspects of planning, designing and campaigning for these missions, focusing primarily on the LUVOIR concept – a 12 meter-class Large UV Optical Infrared Telescope. They will also engage the audience in a discussion of the political climate, science case, engineering challenges and potential science yield for these missions, which could revolutionize our understanding of habitable zone exoplanets, as well as Galactic and extragalactic science.
Mass loss is important for our understanding of the late evolutionary phases of massive stars, including the formation of Wolf-Rayet (WR) stars and determining what type of supernovae various systems will become. However, small changes in mass loss rates cause very different theoretical predictions for the evolutionary path and supernovae end state for a massive star. For example, massive star winds are expected to be clumped, but this lowers overall mass loss rate estimates to a point where theoretical models cannot explain the observed number of WR stars. Additionally, massive binaries further complicate the mass loss picture because they lose material and interact with their local environments in several ways that single stars cannot: through mass streams, accretion, and mass loss via outflows in Roche-lobe overflow systems; and through stellar winds in colliding-wind systems. In this talk I will focus on presenting results from studies of a small number of systems that provide an in depth look at the circumstellar structures that can form during periods of heavy mass loss. In particular, I will show data from many observing techniques and wavelength regimes that, when viewed all together, present a three dimensional view of where circumstellar material is located in massive systems.
Kepler revealed the common existence of tightly packed super Earth systems around solar type stars, existing entirely inside the orbit of our Venus. Those systems must be stable for the ages of their host stars (~109 years); their formation mechanism must provide interplanet spacings that permit long term stability. If one postulates that most planetary systems form with tightly packed inner planets, their current absence in some systems could be explained by the collisional destruction of the inner system after a period of metastability. We posit that our Solar System also originally had a system of multiple planets interior to the orbit of Venus. This would resolve a known issues with extent of and the energy/angular momentum of our inner planet system; in our picture the disk material closer to the Sun also formed planets, but they have since been destroyed. By studying the orbital stability of systems like the known Kepler systems, we demonstrate that orbital excitation and collisional destruction could be confined to just the inner parts of the system. In this scenario, Mercury is the final remnant of the inner system’s destruction.
Primitive meteorites and cometary dust particles contain preserved interstellar dust grains that predate the origin of the Solar System. These grains are recognized as pristine presolar stardust by large isotopic anomalies indicating that they originated in the outflows or explosions of evolved stars. They were witnesses to stellar, interstellar and protosolar environments and thus serve as probes of a wide array of astrophysical and cosmochemical processes. This talk will provide numerous examples of how presolar grains are identified, studied in the laboratory and used to gain important information about astrophysics with a focus on stellar nucleosynthesis and galactic chemical evolution.
From its flight through the Pluto system in July, a spectacular trove of data was recorded on the solid state data recorders aboard the New Horizons spacecraft. It will be late 2016 before all is transmitted back to Earth, but already there have been many remarkable discoveries giving a sense of the complexities in the system. This talk will show some of the scientific highlights and puzzles that the New Horizons science team is investigating. It will also briefly touch on plans for January 2019 when New Horizons will get the first up-close look at a small Kuiper belt object.
Supernova impostors are optical transients that, despite being assigned a supernova designation, do not signal the death of a massive star or an accreting white dwarf. Instead, most impostors are likely to be the result of a major eruption from a massive star. Although the physical cause of these eruptions is still debated, tidal interactions from a binary companion has recently gained traction as a possible explanation for observations of some supernova impostors. In this talk, I will discuss the particularly interesting impostor SN 2010da, which is (so far) unique among this class of objects due to its high-luminosity, variable X-ray emission. The X-ray emission is consistent with accretion onto a neutron star, making SN 2010da both a supernova impostor and likely high mass X-ray binary. If it is indeed a high mass X-ray binary, it is extremely young: CMD modeling of the coeval stellar population revealed by HST imaging constrains the age of SN 2010da system to <5 Myr. This age is consistent with the theoretically predicted first onset of X-ray production in X-ray binaries, making SN 2010da an important object for understanding massive binaries and high mass X-ray binary formation.
Massive early-type galaxies are central to our understanding of large-scale structure, stellar populations, and supermassive black holes, but their detailed formation histories remain poorly understood. The recently initiated MASSIVE Galaxy Survey is a volume-limited, integral-field spectroscopic and photometric survey of the structure and internal dynamics of the ~100 most massive early-type galaxies within a distance of 100 Mpc. The combination of integral-field spectroscopy on sub-arcsecond and large scales (out to two effective radii) allows us to perform simultaneous dynamical modeling of the supermassive black holes, stars, and dark matter. We also have an ongoing Hubble program to image a high-priority subsample of the MASSIVE galaxies. The ultimate goals of the survey include understanding variations in dark matter fraction and stellar IMF, the connection between black hole accretion and galaxy growth, and the late-time assembly of galaxy outskirts. I will describe the survey design and observational strategy and present first results on black hole mass measurements, stellar populations, and molecular gas detections in MASSIVE Survey galaxies.
Understanding the origin of the elements is one of the major challenges of modern astrophysics. Elements along the bottom two-thirds of the periodic table — including arsenic, selenium, barium, europium, lead, thorium, uranium, and others — are mainly produced by neutron-capture reactions. Some had not been detected previously in late-type stars, and the origins of all are not fully understood at present. My work focuses on abundances derived from ultraviolet and optical high-resolution spectroscopic data of dwarf galaxies, globular clusters, and field stars in the stellar halo. I will present recent observations of these elements that successfully muddy our understanding of when and how they were first produced in the early Universe.
Sniffing Alien Atmospheres: Exoplanet spectrophotometry (from ground-, airborne- and space-based observatories)
In my presentation I will give a short introduction to the science of extrasolar planets, in particular the technique of transit, eclipse and phasecurve spectro-photometry. I will describe my various projects in this emerging field using state of the art spectroscopic and photometric instruments on the largest ground based telescopes, the ‘flying telescope’ SOFIA (Stratospheric Observatory for Infrared Astronomy) and the Kepler and Hubble space telescopes.
NOTE: This colloquium occurs on a Friday.
Winter Quarter 2016
The initial mass function (IMF) for stars above ~1 Msun is essential to testing and validating theories of star formation, constraining chemical enrichment models, the frequency of core-collapse supernovae, and interpreting the stellar populations of galaxies across cosmic time. Yet, despite more than 60 years of research, observational constraints on the high-mass IMF remain remarkably uncertain. Widely used high-mass IMFs (e.g., Kroupa) have associated uncertainties approaching an order-of-magnitude, making it virtually impossible to determine if the high-mass IMF varies with respect to environment (e.g., metallicity or star formation intensity) or is “universal”. In this talk, I will present the most precise measurement of the high-mass IMF to date. Using ~100 young, resolved star clusters imaged as part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey, we find the high-mass IMF slope in M31 to be Gamma=1.45+/-0.03. Compared to the canonical Kroupa IMF (Gamma=1.3+/-0.7), the high-mass IMF in M31 is 0.15 dex steeper (i.e., fewer massive stars) and represents a factor of ~20 improvement in precision. There are no significant trends between the cluster IMF slopes and their ages, masses, and sizes, indicating that the IMF is remarkably “universal” in this sample of ~100 clusters. I will illustrate some of the broader implications of a steeper IMF slope (e.g., on star formation rate indicators, core-collapse supernovae rates) and will conclude by discussing the prospects for precision IMF measurements in other environments.
A radical new scenario has recently been suggested for the formation of giant planet cores that reports to solve this long-standing problem. This scenario, known as pebble accretion, envisions: 1) Planetesimals form directly from millimeter- to meter-sized objects (the pebbles) that are concentrated by hydrodynamic forces and then gravitationally collapse to form 100 – 1000 km objects (Cuzzi+ 2008, AJ 687, 1432; Johansen+ 2007, Nature 448, 1022). 2) These planetesimals quickly sweep up the remaining pebbles because their capture cross sections are significantly enhanced by aerodynamic drag (Lambrechts & Johansen 2012, A&A 544, A32; Ormel & Klahr (2010) A&A Volume 520, id.A43). Calculations show that a single 1000 km object embedded in a swarm of pebbles can grow to ~10 Earth-masses in less than 10,000 years. However, recent full-scale simulations of core formation with this process have failed to reproduce the giant planets in the Solar System (Kretke & Levison 2014, AJ 148, 19). I will discuss a new modification to the basic pebble accretion picture that appears to solve this problem and apply these new ideas to the terrestrial planet region.
When the first galaxies emerged, ~100 – 500 million years after the Big Bang, their starlight likely reionized and heated the intergalactic hydrogen that had existed since cosmological recombination. Much is currently unknown about this process, including what spatial structure it had, when it started and completed, and even which sources drove it. Recent observations of high-redshift quasars show large-scale spatial variations in the opacity of the z~5.5 intergalactic medium to Lyman-alpha photons. These spatial variations grow rapidly with redshift, far in excess of expectations from previous empirically motivated models. I will discuss possible explanations for the excess, as well as what they imply about the reionization process.
From Imaging Disks and Gas Giant Planets with the Gemini Planet Imager to Habitable Earth-like planets with the ELTs
On Thursday, February 11, 10:30 EST, scientists from Caltech, MIT and the LIGO Scientific Collaboration are scheduled to provide “a status report on the effort to detect gravitational waves – or ripples in the fabric of spacetime – using the Laser Interferometer Gravitational-wave Observatory (LIGO).” We will review astronomical sources of gravitational waves, and the key science questions that can be addressed by observing them. The implications for Astronomy of the results from current and future gravitational wave observatories will be discussed.
About half of observed exoplanets are estimated to be in binary systems. Thus, understanding planet formation and evolution in binaries is essential for explaining observed exoplanet properties. I will show how planet—disc interactions in a mildly inclined disc around one component of a binary can lead to the formation of highly eccentric and highly inclined planets.
The dust extinction curve is a critical component of many
observational programs and an important diagnostic of the physics of
the interstellar medium. Here we present new measurements of the dust
extinction curve and its variation towards tens of thousands of stars,
a hundred-fold larger sample than in existing detailed studies. We
use data from the APOGEE spectroscopic survey in combination with
ten-band photometry from Pan-STARRS1, 2MASS, and WISE. We find that
the extinction curve in the optical through infrared is well characterized by a one-parameter family of curves described by R(V). The extinction curve is more uniform than suggested in past works, with sigma(R(V)) = 0.18, and with less than one percent of sight lines having R(V) > 4. Our data and analysis have revealed two new aspects of Galactic extinction: first, we find significant, wide-area variations in R(V) throughout the Galactic plane. These variations are on scales much larger than individual molecular clouds, posing a challenge to existing paradigms of dust formation and processing: R(V) must be tracing much more than just grain growth in dense molecular regions. Indeed, we find no correlation between R(V) and dust column density up to E(B-V) ~ 2. Second, we discover a strong relationship between R(V) and the far-infrared dust emissivity.
Dusty star-forming galaxies host the most intense stellar nurseries in the Universe. Their unusual characteristics (SFRs=200-2000Msun/yr) pose a unique challenge for cosmological simulations and galaxy formation theory, particularly at early times. Although rare today, they were factors of 1000 times more prevalent at z~2-5, contributing significantly to the buildup of the Universe’s stellar mass and the formation of high-mass galaxies. However, an ongoing debate lingers as to their evolutionary origins at high-redshift, whether or not they are triggered by major mergers of gas-rich disk galaxies, or if they are solitary galaxies continually fed pristine gas from the intergalactic medium. Observational evidence has been mixed over recent years; some studies clearly point to chaotic kinematic histories and fast gas depletion times (~<100Myr), while other work may demonstrate secular (though active) disks can sustain high star-formation rates over long periods of time. Similarly, some works argue such extreme star-formers contribute very little to cosmic star-formation, while others find quite the opposite. Furthermore, their presence in early protoclusters, only revealed quite recently, pose intriguing questions regarding the collapse of large scale structure. I will discuss some of the latest observational programs dedicated to understanding their origins and frequency at early times, their context in the cosmic web, and future long-term observing campaigns that will reveal their relationship to “normal” galaxies, thus teaching us valuable lessons on the physical mechanisms of galaxy growth and the collapse of large scale structure in an evolving Universe.
Because faint, low mass galaxies are numerous at high redshifts, their impact on the Universe is expected to be significant. They may host a substantial fraction of the Universe’s star formation, provide many of the energetic photons needed to reionize the hydrogen gas surrounding galaxies, and affect their surroundings via powerful, starburst-driven galactic outflows. Because of their faintness, however, the properties of these galaxies are difficult to determine. I will discuss a variety of observations aimed at characterizing the physical conditions in low mass galaxies during the peak epoch of star formation, when the Universe was ~20% of its current age, with particular emphasis on the study of galactic outflows in faint galaxies.