Spring Quarter 2018
Supermassive black hole binaries (SMBHBs) should be common products of the hierarchical growth of galaxies and the loudest expected sources of low-frequency gravitational waves. Periodic quasar variability has been predicted as an observational signature of SMBHBs, due to modulated mass accretion or relativistic Doppler boosting. We have conducted a systematic search for periodically varying quasars in the Pan-STARRS1 Medium Deep Survey and identified 26 candidates from ~9,000 color-selected quasars in a ~50 deg^2 sky area. We further extend the baseline of observations via our imaging campaign with the Discovery Channel Telescope and the Las Cumbres Observatory network telescopes. We then reevaluate the candidates using a more rigorous, maximum likelihood method by searching for a periodic component in addition to red noise, which is modeled as the Damped Random Walk process or a broken power law power spectrum. We also apply our method to the well-known periodic quasar and SMBHB candidate PG1302-102 using extended data from ASAS-SN and find evidence for the lack of persistence over a baseline of ~15 years. I will conclude with a discussion of the exciting capabilities of LSST to discover SMBHBs that can potentially be detected by the next generation of pulsar timing arrays.
One of the major goals in observational cosmology today is to understand how our Universe transitioned from the “dark ages”, following recombination, into the ionized universe we can observe today. For this purpose we compiled a new data set of high redshift (5.8<z<6.5) quasar spectra that enables new insights into the early evolutionary phase of our Universe and the early stages of quasar and galaxy formation traced by the intergalactic gas. I will discuss the imprints of the intergalactic medium (IGM) on the spectra of these quasars that constrain the evolution of the IGM opacity as well as its neutral gas fraction. I will highlight the implications of this analysis regarding the timing of the epoch of reionization, the morphology of the UV background radiation, and the temperature-density relation of the high-redshift IGM. Additionally, I will present an analysis of the proximity zones of the quasars in our data set, i.e. the regions surrounding the quasars that have been ionized by their own radiation, in order to set constraints on the onset and duration of the reionization process as well as the lifetime of these quasars. We identified several objects showing exceptionally small proximity zones and argue that only a very short quasar lifetime (~10,000 yr) can be causing these small zones by comparing our measurements to radiative transfer simulations. I will discuss the consequences of such short lifetimes on the quasar’s ionizing power, their black hole accretion rates and highlight tensions with current theoretical models for black hole formation.
The formation of supermasssive stars in globular clusters and the origin of the multiple populations
I will present a new model for explaining the strange abundance patterns observed in globular clusters. This model includes the creation of a supermassive star (SMS) with M >10^3 Msun by runaway collisions in the dense core of massive globular clusters during their formation. The SMS is highly convective so its surface layers are constantly polluted by the nuclear products from its core. The SMS has a very high mass loss rate. The chemically enriched wind from the SMS mixes with the pristine original gas of the cluster, resulting in the formation of chemically enriched low mass stars. The SMS works like a “conveyor belt” because it is continuously rejuvenated by stellar collisions and produces a huge amount of enriched wind material. The predicted abundances agree with the observed chemical abundance patterns of low mass stars in globular clusters.
The Data Release 2 from the ESO Gaia mission has provided unmatched positions and astrometric data for over 1.4 billion stars in our Galaxy. This remarkable dataset represents a new era for studying the Milky Way, stars, and even exoplanets. In the past month since the release of Gaia DR2, the astronomical community has seen a flood of exciting new results, and the public has been able to watch the process of discovery happen in real time. In this informal talk I will show highlights from Gaia DR2, as well as new community-driven projects using Gaia DR2 in conjunction with other datasets. I am particularly excited about the possibilities of combining Gaia DR2 with ZTF, Kepler/K2, and TESS. UW will also be hosting a “Gaia Sprint” from June 4-8 in conjunction with the CCA/Flatiron Institute. We invite you all to participate in this event, and I will welcome discussions during this lunch talk about projects ideas, useful resources, and collaborations.
Winter Quarter 2018
Cosmological simulations consistently find that halos of low-redshift ~L* galaxies are filled with hot gas shocked to the virial temperature (∼10^6 K), out to at least the virial radius of the dark matter halo (~250 kpc). This result follows since ~L* galaxies are expected to reside in halos with masses of ~10^12 M_sun, above the threshold halo mass for the formation of a pressure-supported virial shock. I will demonstrate that observations of gas around ~L* galaxies (mainly by HST) can in many respects be explained with an alternative scenario, where the hot gas is limited to the inner dark matter halo (<100 kpc), while further out the gas is predominantly cool (~10^4.5 K) and has a low thermal pressure. Such a scenario is expected if the virial shock around ~L* galaxies cannot form due to lack of pressure support, and instead inflows from the intergalactic medium shock against outflows from the galaxy. I will discuss theoretical uncertainties which may explain why this alternative scenario is not realized in current cosmological simulations, and also predictions which can be used to test this scenario in upcoming observational campaigns.
Multiphase gas structure is ubiquitous in our universe. Recent observations suggest that large quantities of cold gas with temperature of a few 10^4 K are found in circumgalactic medium (CGM), which extends up to a few times of galactic virial radius. However, the origin and fate of such cold gas still remain unclear. In this talk, I will mainly explore magnetized thermal instability as a promising mechanism for cold gas formation. In addition, I will discuss warm gas generation from MHD turbulent mixing layers, and preliminary particle-fluid simulations of cold cloudlets, which paves the way for final two-fluid model of cold gas in cosmological simulations.
The ESA Euclid space mission core science goals rely on complementary deep ground-based surveys covering both the northern and southern galactic caps. This talk will present an overview of the space survey design and the current status of the ground-based survey campaign, with a focus on its first element, CFHT’s Canada-France Imaging Survey.
The gravitational-wave spectrum of astrophysical sources extends down 9 orders of magnitude in frequency below the signals observed by ground-based detectors. As the wavelength increases so does the detector size. Using the pulses of very stable millisecond pulsars (MSP) it is possible to detect these larger scale gravitational waves. I will discuss the observing campaign of 70 of these MSPs by the North American Nanohertz Observatory for Gravitational waves (NANOGrav). The most recent limits, using our first 11 years of data, on the stochastic background of super massive black hole binaries and cosmic string cusps will be presented. The recent efforts to model the solar system ephemeris will be highlighted as well. Lastly, I will describe how we characterize an instrument, the other end of which we are unable to inspect. Specifically, I will highlight our recent efforts to come up with objective red noise models for our pulsars.
During the HI reionization, the UV radiation from the first luminous sources injected vast amount of energy in the intergalactic medium, photo-heating the gas to tens of thousands of degree Kelvin. This increase in temperature has left measurable `imprints’ in the thermal history of the cosmic gas: a peak in the temperature evolution at the mean density and a smoothing out of the gas in the physical space by the increased gas pressure following reionization (i.e. Jeans smoothing effect). The structures of the HI Lyman-alpha forest at high redshift are sensitive to both these effects and therefore represent a powerful tool to understand when and how reionization happened.
I will present a novel investigation of the IGM thermal history using the Lyman-alpha forest flux power spectrum at z~5. For the first time, we extended the measurement down to the smallest scales currently detectable by high-resolution quasar spectra. These scales have been proven to be the most sensitive to different reionization scenarios and allow new constraints on the timing and the heat injection by reionization.
I will show the new results and I will discuss their consistency with different possible sources of reionization.
Fall Quarter 2017
The flow of gas through the circumgalactic medium (CGM) regulates galaxy growth over cosmic time. Observations have recently revealed a complex multi-phase structure in the CGM that has challenged many of the established theories—highlighting significant gaps in our understanding of this critical aspect of galaxy formation. The spatial scales relevant to the CGM span a huge range with its structure and evolution determined by small-scale processes, such as the launching of galactic winds by clustered supernovae and thermal instability in the hydrostatic halo, and large-scale processes, such as cosmological accretion. I will describe my attempts to understand the details and interplay of these multi-scale processes in order to develop a coherent picture of the CGM that is consistent with observations.
The Origin of Stellar Species: constraining stellar evolution scenarios with Local Group galaxy surveys
Current mainstream theories of stellar evolution make simple approximations for fundamental processes like convective mixing or mass transfer in close binaries. These approximations can lead to systematic errors in our understanding of the physical properties of galaxies, both local and at high-redshifts. A critical reappraisal of stellar evolution theories is in order. In this talk, I will discuss a method to recover delay-time distributions (DTDs), which can pose powerful observational constraints on stellar evolution scenarios for many classes of stellar objects. To calculate DTDs, one needs a catalog of objects and a map of the star-formation histories of the host galaxy. The technique is particularly effective in the Local Group, where reliable star-formation histories from resolved stellar populations, and high quality surveys are available. I will discuss the application of this method to pulsating variable stars like RR Lyrae and Cepheids. I will also discuss the progress of my group towards the calculation of a DTD from Local Group supernova remnants, with the hope of constraining the long-standing progenitor problem of thermonuclear and core-collapse supernovae.
At UNSW we have built the world’s most sensitive astronomical polarimeter, capable of measuring stellar polarisation down to levels of a few parts per million. I will describe the instrument and the research we have done with it since its commissioning in 2014. One of the main goals of such instruments has been to contribute to the characterisation of exoplanet atmospheres by detecting polarised light reflected from hot-Jupiter type planets. However, we can also learn much about stars using these techniques. We have made some of the first polarimetric observations that directly probe stellar atmospheres and allow us to constrain stellar properties. In order to interpret these observations we have developed methods for modelling the atmospheres of both planets and stars including full polarisation in the radiative transfer. I will also describe future plans including a version of the instrument we are developing for Gemini North.
In the Milky Way now, star formation proceeds almost exclusively in Molecular Clouds (MCs) under 106 Msun . This is not necessarily true at higher redshifts. The CANDELS survey has shown us a fraction of galaxies host massive kpc-scale stellar clumps (Guo et. al 2015). This suggests that star formation may proceed in objects at least 100 times more massive than present day MCs. However, lensing source reconstruction, which has enhanced resolution, has shown us that star formation can still proceed in MC-scale objects at these redshifts (e.g. Johnson et al. 2017). A natural way to piece two opposing datasets together would be to turn to simulations of clump formation. However, this produces a similar dichotomy where both large (e.g. Inoue et al. 2016) and small (Tamburello et al. 2016, Behrendt et al. 2016) objects can be formed. With a wealth of conflicting observational and theoretical data, we find ourselves at a crossroad. We propose a new approach to studying clump formation in simulations. We seed clump formation events in isothermal simulations of galaxy disks. In this way, we can explore a large parameter space in both perturbation size and strength. We can find a space of likely clump masses for a given galaxy. We propose this method as a way to stitch together the wealth of data we now have.
In the last decade, the potential impact of galaxy-scale outflows driven by quasars on their environment has become widely recognized. Quasars not only provide radiative feedback in the form of pressure and photo heating, they also affect the ionization state of the gas in and around the host galaxies. In this talk I show that there is strong evidence of a different ionization state close to the quasar along and across the line of sight. I have exploited the spectra of 100 quasars at emission redshift zem = 3.5 – 4.5 to construct a large sample of narrow absorption line (NAL) systems. The observations have been carried out with VLT/X shooter in the context of the XQ-100 Legacy Survey. I statistically study their physical properties and distribution on different scales. I also present results from stacking Lyman alpha forest absorbers in the XQ-100 sample to look for metals signal at large velocity separation from the zem.
Finally, I briefly talk about the Extremely Red Quasars, a unique red-quasar population with exotic physical conditions. They are candidates to be young objects in a transition stage between dusty starbursts and unobscured blue quasars: the perfect laboratory where to study powerful outflows through their peculiar emission lines.
Low-surface-brightness galaxies (LSBGs) are a significant component of the galaxy population, which provide a unique testing ground for theoretical predictions of galaxy and star formation, stellar feedback processes, and the distribution and nature of dark matter. However, their defining characteristic—central surface brightnesses that are fainter than the night sky—makes them difficult to detect and study, leading to their underrepresentation in previous optical surveys and biasing our view of the full galaxy population. I will present a new view of these elusive galaxies from the Hyper Suprime-Cam (HSC) Survey, a 300-night imaging survey using the 8.2-meter Subaru Telescope on Mauna Kea. After giving an overview of the HSC Survey, I will present our source-detection pipeline and initial catalog of LSBGs within the first ~200 deg^2 of the survey, which will grow to 1400 deg^2 upon survey completion. Our LSBG catalog will facilitate follow-up efforts (which we have already started) to study the physical properties and number densities of these galaxies as a function of environment. Pushing such studies to lower surface brightnesses will be necessary to form a more complete census of the galaxy population, which will ultimately provide one of the strongest tests of the standard LCDM framework.
Starting in the early 2020s, the Large Synoptic Survey Telescope will carry out a decade long survey of the southern sky. Offering a unique combination of breadth, depth and cadence, this survey will enable us to address some of the most profound questions in modern astronomy.
It will also present huge challenges: how can we collect, store, process and — most importantly! — understand the hundreds of petabytes of data which will be produced? How can we combine the statistical and algorithmic rigor needed to enable the next generation of precision cosmology with the speed and agility needed to identify and respond to transients and variable sources? How can we make vast volumes of data available to the community in a way that enables your particular science case?
In this talk, I will briefly review the design and scientific goals of LSST, provide an update on the current status of construction, explore some of the algorithmic and data processing challenges that the LSST Data Management team faces, and describe the key role that members of the Department of Astronomy here at UW are playing in making it all possible.
Constraining the Movement of the Spiral Features and the Locations of Planetary Bodies within the AB Aur System
The circumstellar disk around the Herbig Ae star AB Aur has many interesting features, including spirals, asymmetries, and non-uniformities. However, comparatively little is known about the envelope surrounding the system. Recent work by Tang et al (2012) has suggested that the spirals in the disk may instead be due to areas of increased density in the envelope and projection effects. I will report polarimetric modeling results of AB Aur designed to begin to place constraints on properties of the envelope such as infall rate and cavity opening angle, compare our results to observations in order to determine the origin of the spiral structures, and place constraints on the location of planetary bodies within the system.
The Lyman-alpha damping wing from neutral hydrogen in the intergalactic medium is predicted to be a key signature of the reionization epoch in the spectra of high-redshift quasars. There are substantial challenges in measuring and interpreting this signal, however: the intrinsic spectrum of the quasar near its Lyman-alpha emission line is highly uncertain, and the strength of the damping wing depends on the patchy structure of reionization and the age of the quasar. We have developed a Principal Component Analysis-based machine-learning approach to predict the intrinsic quasar spectrum, and have combined semi-numerical simulations of reionization topology with 1D radiative transfer through hydrodynamical simulations to predict the range of possible damping wing morphologies. Using a Bayesian statistical formalism calibrated with forward-modeled mock spectra we can then translate an observed quasar spectrum into constraints on the global neutral fraction and the length of the luminous quasar phase. I will demonstrate the application of these methods to the highest redshift quasars known to date, and discuss the potential for existing quasar spectra to constrain the reionization history at z > 6.
Spring Quarter 2017
Astronomers are entering a new era of exoplanet imaging with the Gemini Planet Imager (GPI), an extreme adaptive optics system at Gemini South. I will present some highlights of the achievable science with high contrast imaging instruments, including the GPI Exoplanet Survey (GPIES), an ongoing 890 hour survey of 600 young nearby stars. I will also present an overview of the technical limitations that prevent GPI from reaching planet masses and separations below ~2 Jupiter masses and ~10 AU, namely contrast and resolution, respectively. To further improve sensitivity to lower exoplanet masses and smaller separations, I will also discuss my ongoing work on new data processing algorithms, as well as possible future upgrades to the instrument in between leaving Gemini South in mid-2018 (when GPIES is planned to finish) and a possible move to Gemini North. For the former, I present an analysis from a new PSF subtraction algorithm that can improve contrast by up to ~45% at angular separations near the diffraction limit, while for the latter I present simulations showing that upgrading GPI with a new focal plane wavefront sensing technique, called the Self-Coherent Camera, could improve contrast by up to a factor of ~20, reaching ~Saturn mass sensitivity.
I will present an overview of Synthetic UniveRses For Surveys (SURFS), the next generation of mock observations, following in the footsteps of Millennium and Bolshoi simulations. The SURFS simulation set consists of N-body/Hydro simulations in the Planck concordance LCDM cosmology, sampling scales & halo masses down to 1 kpc and 100 million solar masses in 210 Mpc/h cosmological volumes. These simulation parameters are optimised to understand the galaxy formation physics governing satellite galaxies and chosen so as to produce synthetic analogues to upcoming surveys like WAVES and WALLABY. We use state-of-the-art Halo Finders, Trackers and Semi-Analytic Models (SAM) of galaxy formation to follow not just the evolution of central galaxies/haloes but the active lives of satellites/subhaloes spanning group to low cluster mass scales. I will present preliminary results on the evolution on the cosmic growth and gas accretion history of haloes and how the cosmic web ties into it.
Anchoring the distance scale and providing new insights into stellar physics using high-precision observations of classical Cepheids
Classical Cepheid variable stars (henceforth: Cepheids) are best-known for their crucial role in calibrating the cosmic distance scale, and thus, for investigating dark energy. Yet, Cepheids continue to be objects of high interest for stellar physics and rank among the most-studied types of variable stars.
This talk presents recent observational work aimed at increasing the accuracy of extragalactic distance measurements as well as providing new insights into stellar pulsations via highly precise observations obtained with state-of-the-art instrumentation from the ground and from space. Specifically, I present how high-precision radial velocity measurements of Cepheids a) support unprecedented parallax accuracy, b) reveal systematic uncertainties of Baade-Wesselink-type distances, and c) have enabled the discovery of atmospheric velocity field perturbations that are presently not understood. Related irregular variability patterns discovered via high-precision photometric and interferometric observations are also discussed.
The ongoing ESA mission Gaia is expected to revolutionize stellar astrophysics and provide a highly accurate anchor for the extragalactic distance scale. As this talk shows, high-precision observations can expose secrets of seemingly well-understood stars and play a crucial role for leveraging Gaia’s full potential.
Winter Quarter 2017
I will present the first coronagraphic spectroscopy of the AU Mic debris disk system obtained with HST/STIS as part of GO-12512. Spectra of the system were taken by placing a long slit in the disk direction while blocking out the central star with an occulting bar. A naked star of similar spectral type was likewise observed for a PSF subtraction. This procedure results in a two dimensional spectrum as a function of disk position between 5200 and 10,200 angstroms for the system. I will report the results of these AU Mic spectra, which can be used to help determine the dust grain composition of the system by characterizing the disk’s color as a function of radial distance along the its midplane. In addition, I compare the spectra on either side of the disk in order to probe the presence of any compositional and structural asymmetries. This reveals the dynamical perturbations and chemical processing occurring within the disk and traces the potential composition and architecture of any planetary bodies in the system.
I will present an overview of the identification and investigation of the handful of “young solar system analogs” — star/disk systems — that lie within a mere ~100 pc of Earth. I describe advances in our understanding of protoplanetary disk structure and evolution enabled by these systems, with particular emphasis on the new discovery space that is now being opened by high-resolution imaging with ALMA as well as extreme AO cameras on large ground-based optical/IR telescopes.
I will review a few recent results on theoretical models of galaxy formation in non standard DM models, with a focus on Warm Dark Matter and Self Interacting DM.
Galactic chemical evolution is a multidisciplinary topic that involves nuclear physics, stellar evolution, galaxy evolution, and cosmology. Observations, experiments, and theories need to work together in order to build a comprehensive understanding of how the chemical elements synthesized in astronomical events are ejected and spread inside galaxies and recycled into new generations of stars. Nuclear physics provides nuclear reaction rates, stellar models provide the composition of stellar ejecta, galaxy models follow the evolution of chemical species driven by multiple stellar populations, cosmological simulations dictate how galaxies form and evolve in general, and observations provide constraints to test and improve numerical recipes driven by theories. During this talk, I will address the topic of galactic chemical evolution and present our efforts to create permanent connections between different fields of research (including nucleosynthesis and gravitational wave physics). Our ultimate goal is to better understand the origin of the elements in the universe and to explain the diverse chemical evolution patterns observed in nearby galaxies.
Autumn Quarter 2016
After briefly reviewing what is known and what we still need to know in order to correctly identify the supernovae Ia progenitors in different populations, I will talk about massive white dwarfs that are accreting mass in binary systems and are burning hydrogen in shell. I will review what we know about symbiotics and Be+white dwarf systems as possible progenitors of type Ia supernovae, and I will present new observational data about some of the hottest and massive white dwarf binaries in the Local Group.
Spring Quarter 2016
Rotation and Activity in Low-mass Hyades and Praesepe Members, and the Implications for Gyrochronology
Winter Quarter 2016
Galactic winds blow through galaxies of all shapes and sizes, but they are particularly ubiquitous in high-redshift star-forming galaxies, where they may be driving galactic evolution by regulating the baryon cycle. Due to recent advances in the modeling of stellar feedback, cosmological simulations of galaxy formation can now generate galactic winds explicitly, while also matching many observed properties of galaxies at various epochs. We can therefore study simulated winds as an emergent phenomenon, and derive insights into galaxy evolution to compliment current observational knowledge. In my talk, I will discuss my progress in characterizing galactic outflows in the Feedback in Realistic Environments (FIRE) simulations. I will quantify the overall prevalence and intensity of galactic winds, their connection to physical galactic properties, and the observational implications of wind-driven evolution for galaxies and the circumgalactic medium.
Please join us in the reading room on Tuesday Jan 12 at 4pm for a general meeting about APO, including new instrumentation on the 3.5m telescope, planning that is starting now for the 2.5m Sloan telescope in the 2020’s time frame (“After Sloan 4”), and current usage and availability of ARCSAT, the 0.5m photometric telescope.
You are particularly encouraged to participate if you have input about the new spectrograph (currently in planning stage) to replace DIS on the 3.5m. A committee is actively soliciting input from the users community for science requirements to determine design goals.
Join us for 10 min talks by Chris Laws, Nicole Silvestri Kelly, Toby Smith, Oliver Fraser, Ana Larson, and Joe Huehnerhoff!
10 min astro lunch talks by graduate students.
The Fluctuating UV Background Across Cosmic Time
10 min astro lunch talks by graduate students.
Autumn Quarter 2015
Oscillating stars in eclipsing binaries are powerful tools for testing stellar models because binarity allows for independent computation of physical stellar parameters. Thanks to advances in asteroseismology, red giants have become astrophysical laboratories for studying stellar evolution and probing the Milky Way. In this talk, I highlight an interesting pair of oscillating red giants in the eclipsing binary KIC 9246715, and I discuss work underway to characterize the 20 known red giants with eclipsing companions observed by Kepler. These are rapidly becoming some of the best-studied stars and an important benchmark for asteroseismology.
This is the first of a series of astro lunches featuring short (10 minute) talks by the faculty in October. This event will contain talks by Julianne, Matt, Emily, Fabio, and Paula. See you there!
I will discuss my work to determine the compositions of small planets. The density-radius distribution of 72 exoplanets smaller than 4 Earth radii peaks at 1.5 Earth radii. Planets smaller than 1.5 Earth radii usually increase in density with increasing planet radius, suggesting that planets up to 1.5 Earth radii can have rocky surfaces. However, planets larger than 1.5 Earth radii typically decrease in density with increasing planet radius, suggesting that at around 1.5 Earth radii, planets begin to accrete volatile envelopes that reduce their bulk density. This trend is exemplified in two systems for which I present updated planet masses. By simultaneously fitting radial velocities and transit timing variations with TTVFast, I determined that (1) Kepler-11 has six planets with volatile envelopes; and (2) Kepler-10 has one rocky planet, one planet with a volatile envelope, and one non-transiting planet
10 minute faculty research talks at noon in the reading room. Join us!
10 minute post-doc/research scientist research talks (+ 1 faculty member).
10 minute post-doc/research scientist research talks (+ 1 faculty member)
10 minute post-doc/research scientist research talks