Fall Quarter 2019 (current)
Millisecond pulsars (MSPs) are formed by being spun-up (recycled) to rapid spin periods through accretion of matter and angular momentum from a non-degenerate companion. Studies of these systems provide essential insights into the physics of low-level accretion onto magnetized compact objects and the interactions between MSPs and their stellar and gaseous environment. The MSP population in the Galactic field is dominated by wide orbit MSP-white dwarf binaries, representing the end products of this recycling process. However, recent multiwavelength follow-up observations of unassociated Fermi γ-ray sources have revealed a subclass of MSP binaries in which recycling is apparently not yet complete. In this talk, I will outline some of the basic steps we take to identify and characterize candidate MSP binaries in the late stages of the recycling process, and will show a few systems whose unusual phenomenology and inferred evolutionary tracks suggest they are the first known systems in an expanding class of compact objects that are the progenitors of typical field MSP binaries.
Luminous Blue Variables show a curious and erratic photometric behaviour. In addition to giant outbursts, ala P Cygni in 1600 and Eta Carinae in 1837, they experience large irregular variations, called S Dor variations, when the stars increase their size by as much as a factor 3 to 10 with Teff dropping by a factor 1.5 to 3 at about constant luminosity. During their small and hot phase the stars have Teff in the range of 11000 to 30 000 K, but in the cool phase they all reach about the same Teff of about 9000 K. The variations are erratic and unpredictable (?) and occur on a timescale of years to decades. In this talk I will discuss some recent work to explain their curious properties. I will concentrate on the proposed Geyser mechanism and the role of rotation.
The huge data volume generated from the ongoing and upcoming astronomical survey programs has pushed the astronomers to shift from the traditional methods to more sophisticated and scalable approaches for data reduction and analysis without compromising with the precision and accuracy of the results. To address these concerns, Machine Learning (ML) algorithms have been considered in Astronomy, like other domains, and have been employed successfully for a wide range of astronomical problems in the last couple of decade. Classifying astronomical objects is one such field and in this talk, we will consider:
(1) stellar spectral classification using machine learning (ML) and deep learning (DL) techniques like ANN, RF, and CNN based on their spectra in the optical region of EM spectrum. We show that using CNNs, we are able to lower the RMS error up to 1.23 spectral sub-classes and apply the final model on stellar spectra from the SDSS.
(2) classification of X-ray spectra of Low Mass X-ray Binary (LMXB) systems which consist of a main-sequence star and a compact object that could be a Black Hole (BH) or a Neutron Star (NS). Using random forest (RF) algorithm, we are able to identify the type of compact object in the LMXB system (BH or NS) by feeding the X-ray energy spectrum to the model with an average accuracy of 87+/-13%.
Perks and Pitfalls of Pebble Accretion — Implications for Planet Formation in the Inner and Outer Disk
Much recent work on planet formation has focused on planetary growth through accretion of particles whose aerodynamic properties make them marginally coupled to the local nebula. Growth of planets through accretion of these “pebbles,” often termed “pebble accretion,” has several notable features when contrasted with “traditional” growth that relies on purely gravitational interactions. In this talk, I will give an overview of pebble accretion and discuss why the theory has garnered so much interest, and then go on to describe several novel and important features of growth by pebble accretion that have emerged from my work. Firstly, I demonstrate that growth by pebble accretion qualitatively changes for core masses above a minimum mass scale, above which far more pebble sizes are able to be accreted, and particles can be captured on scales comparable to the planet’s hill radius. This change in behavior implies that the early stages of planetary growth must be fueled by processes other than pebble accretion, which can bring planets up to the minimum masses needed for pebble accretion to take hold. I then discuss the semi-major axes where gas giant formation can occur if these early stages are dominated by accretion of planetesimals. Finally, I show that consideration of the smallest sizes of particles that can be captured by pebble accretion leads naturally to an upper planetary mass limit known as the “flow isolation mass.” This mass scale naturally results in planetary growth ending at super-Earth masses in the inner disk, where, in the absence of flow isolation, rapid pebble accretion rates imply that planets either stall at sub-Earth masses or run away to become gas giants.
Spring Quarter 2019
The Large Synoptic Survey Telescope will generate a data deluge: millions of transients and variable sources will need to be classified from their light curves. Photometric classification has long been a problem of interest in the astronomical community, but the Photometric LSST Astronomical Time-series Classification Challenge (PLAsTiCC) brings a wide range of models together, simulated under LSST-like conditions for the first time. PLAsTiCC was delivered to the community through a Kaggle challenge, designed to stimulate interest in time-series photometric classification and deliver methodologies that will advance the LSST science case. I will give an overview of the road to PLAsTiCC, the models and the validation of the data, discuss some of the results from PLAsTiCC, and discuss the science impact of classification on photometric cosmology with Type Ia supernovae.
I’ll present new efficient and accurate techniques for including massive neutrinos in N-body simulations, using a linear response (to the cold dark matter) approximation for the neutrinos. Then I’ll talk about the potential for massive neutrinos to resolve some cosmological tensions within CMB observations galaxy clusters. Finally, I’ll discuss how to detect features in quasar spectra using a Gaussian Process based machine learning technique.
Dark matter not only forms an invisible cosmic scaffolding within which galaxies form, its distribution in the universe also contains a wealth of information about neutrinos, dark energy, and physics at the earliest, inflationary times. Measurements of gravitational lensing in the cosmic microwave background (CMB) allow this matter distribution to be directly seen and mapped. In my talk, I will explain why ongoing and upcoming measurements of the lensing signal from CMB experiments such as AdvancedACT and Simons Observatory will be remarkably powerful probes of cosmology and fundamental physics. Lensing is not only a signal, however, but also a source of noise that limits how much we can learn about the very early universe. With illustrations from recent work, I will explain why de-lensing – removing the lensing effect to reveal the primordial sky – is crucial for the future of CMB cosmology.
We are made of stardust—or, at least in significant parts, of material processed in stars. Hot, massive giant stars can drive the chemical evolution of galaxies and trigger and quench star formation through their strong winds and their final demise as supernovae. Yet optical and X-ray measurements of the wind mass loss strongly disagree and can only be reconciled if the winds are highly structured, with colder, dense clumps embedded in a tenuous hot gas. In (quasi-)single stars, however, wind properties are inferred for the whole clump ensemble. No measurements of individual clumps or clump groups are possible. Luckily, nature provides us with perfect laboratories to study clumpy winds: high mass X-ray binaries. These are systems where a neutron star or a black hole accretes matter from the wind of a giant stellar companion and emits the liberated gravitational potential energy mainly in the X-ray band. This radiation is quasi-point like and effectively X-rays the wind, in particular the clumps crossing our line of sight towards the neutron star or black hole.
In this talk, I will show how we have used a variety of observations of some of the brightest X-ray binaries to constrain wind properties: Low resolution, high cadence observations reveal the dynamics of clump movements as well as wind porosity. Time- and absorption-resolved analysis of high resolution X-ray spectra give insights into the composition of the multicomponent wind plasma and unveils the layered temperature profile and comet-like structure of wind clumps. Comparisons of observations with the newest hydrodynamical simulations reveal the influence of large-scale wind structure introduced by the presence of the compact object, such as accretion wakes. Finally, future X-ray telescopes such as XRISM and Athena will revolutionise the field, allowing us to observe individual clumps in bright sources and, for the first time, make faint sources accessible for high resolution spectroscopy. This will provide us with a sample of HMXBs that will allow us to compare wind properties in massive stars of different stellar (sub-)types and at different radii, thereby constraining theories of clumpy wind formation and evolution.
By the end of the next decade LSST will provide 10-year light curves of billions of sources, looking at regions of the sky with a diverse range of stellar density – from galactic poles to the near-plane regions. We show two examples of LSST science, looking at quasar light curves and stellar crowded fields.
Stripe82 is a relatively low-density region which has been repeatedly observed by SDSS and PS1 surveys. We use S82 PS1 and SDSS data to model quasar variability as a damped random walk (DRW). We show how the DRW parameters are improved by extending light curve baseline, including the predictions for ZTF and LSST data. We look for correlations with the black hole mass and quasar luminosity to describe physical drivers of variability.
We also quantify the performance of the LSST pipeline for processing crowded fields, using images obtained from DECam and comparing to a specialized crowded field analysis performed as part of the DECAPS survey. We find that the LSST pipeline easily handles regions of density up to 200 thousand per sq.deg., and then there is a gradual degradation, mostly in completeness, when progressing towards higher densities.
Debris discs around main sequence stars are the tenuous, dusty remnants of primordial protoplanetary discs. They are composed of dusty and icy planetesimals (asteroids and comets) that produce dust in mutual collisions, hence the name ‘debris disc’. The presence of circumstellar dust is most commonly inferred by the presence of excess emission, above that predicted for the host star’s photosphere, at infrared and millimetre wavelengths, and also through imaging of scattered light and detection of polarization at optical and near-infrared wavelengths. Combining broadband photometry, spectroscopy, and multi-wavelength resolved imaging of debris discs provides tight constraints on both the spatial distribution of the circumstellar dust, and also the properties (grain size, albedo, composition) of the constituent grains. Debris discs are an easily visible marker of a planetary system around their host stars, analogous to the Asteroid and Edgeworth-Kuiper belts in the Solar system. Tracing the dust-producing planetesimal belts with millimetre wavelength imaging observations enables a search for planets revealed through the disc architecture (eccentricity, warps) or sub-structure (gaps).
In this talk I will discuss the current efforts in debris disc observations, the interpretation of their emission and spatially resolved structure through modelling, and the implications for identifying the presence of planets through their interaction with the dust-producing planetesimal belts.
NASA’s Stratospheric Observatory for IR Astronomy (SOFIA) is now in its 7th observing cycle and is the only facility enabling observations across the entire IR wavelength range. The cycle 8 Call for Proposals will be issued by early June with an expected submission deadline of Sep 6. In this short lunch talk I will present some recent results that showcase some of the observatory’s capabilities, including some recent polarization observations from the newest instrument, HAWC+. Topics will include planetary science, circumstellar disks, astrochemistry, the interstellar medium, star formation, and active galactic nuclei. After the showcase, I will provide a brief overview of the current instrument suite as well as a look at our next instrument, HIRMES: a mid- to far-IR spectrometer that will offer extremely high spectral resolution (R ~ 10^5). HIRMES is currently under development at GSFC and is expected to be available to Guest Observers in 2021. Finally, I will give a short overview of the (expected) Cycle 8 Call for Proposals, including funding possibilities and available instruments
In parallel to the recent success of ground based gravitational wave (GW) observatories, is the ongoing effort to identify binary supermassive black hole systems which should be detected by space-based GW observatories and pulsar timing arrays. One promising avenue towards early identification of these systems is to determine the electromagnetic signatures of binaries when they feed on gas in the center of merging galaxies. I will describe our recent progress in producing the first ever general relativistic 3D magnetohydrodynamic simulations capable of evolving the full accretion structure around each black hole for 10s of orbits using a new multi-patch code, and the spectral and variability signatures of these systems. As an example of outstanding fundamental questions concerning single black hole accretion which are important to making the choice of what to include in simulations of binary accretion, I will also discuss some significant improvements in modelling single black hole accretion via the inclusion of self-consistent radiative transport and describe outflow signatures of magnetically arrested disk (MAD) systems.
Winter Quarter 2019
Feedback and Chemical Enrichment in Low Mass Dwarf Galaxies: Insights from Simulations Tracking Individual Stars
Galactic chemical evolution is driven by the complicated interplay of gas accretion, galaxy mergers, star formation, stellar feedback, mixing and turbulence in the ISM, and galactic outflows. Stellar feedback is fundamental in this evolution. How metals — ejected in stellar winds and supernovae — mix with a multi-phase ISM and couple to galactic winds depends sensitively on feedback physics that is poorly understood. Improving our theoretical understanding of both stellar feedback and galactic chemical evolution is becoming increasingly important as number and quality of observations of stellar and gas phase abundances in nearby galaxies continues to grow. We use high resolution, hydrodynamics simulations of isolated, low mass dwarf galaxies to better understand the complex relationship between feedback and galactic chemical evolution. By following stars as individual star particles, we can model both stellar feedback and stellar yields in unprecedented detail. Our star-by-star feedback model includes stellar winds from massive stars and AGB stars, photoelectric heating, stellar ionizing radiation followed through a ray-tracing radiative transfer method, core collapse supernovae, and Type Ia supernovae. We have used these simulations to explore differences in how metals with different nucleosynthetic origins mix within the ISM and couple to galactic winds. I will summarize these results to-date and present ongoing work in understanding the role each component of our multi-channel stellar feedback model plays in driving the chemical evolution of galaxies.
Magnetic accretion in cataclysmic variables: search for new objects and simultaneous modelling using X-ray emission and optical polarization
Magnetic cataclysmic variables are compact binary systems in which mass transfer occurs from a low-mass star onto a magnetic white dwarf. In polars and intermediate polars subclasses, the magnetic field of the white dwarf is strong enough to disrupt the inner accretion disk or even completely prevent disk formation. SW Sextantis stars are a type of novalike cataclysmic variable, which show observational characteristics that indicate matter outside the orbital plane is distributed asymmetrically in azimuth. Many scenarios have been proposed to explain the SW Sex behavior, one of them is magnetic accretion. As a first part of the project, we intend to search for signs of magnetic accretion in SW Sex systems such as circular polarization and/or coherent photometric and/or polarimetric variability that can be associated with the white dwarf spin period. In this project, we aim for the discovery of magnetic cataclysmic variables from large surveys (such as ZTF, CRTS). Magnetic cataclysmic variables candidates or confirmed objects can be further studied using the CYCLOPS code.This code performs multi-wavelength fitting of the accretion column flux using simultaneous photometric and polarimetric light curves and X-ray spectra and light curves. It considers cyclotron and free-free emission from a 3D post-shock region, which is non-homogeneous in terms of density, temperature and magnetic field. This kind of research is inline with my PhD, which is about modelling the optical and X-ray data of IPs.
We describe the discovery of 2015 BP519, an extreme trans-Neptunian object in our solar system detected by the Dark Energy Survey. Its current orbit, with a semi-major axis of roughly 450 AU, an eccentricity of 0.92, and an inclination of 54 degrees, places it as one of the most extreme TNOs. 2015 BP519 displays rich dynamical behavior due to its interactions with the four known giant planets, including rapid diffusion in semi-major axis, but its inclination is expected to remain confined to its current value in the known solar system. We also discuss the BP519’s behavior within the context of the Planet Nine hypothesis and find that 2015 BP519 adds to the circumstantial evidence for the existence of this proposed new member of the solar system, as it would represent the first member of the population of high-i, ϖ-shepherded TNOs. We also briefly discuss other solar system objects discovered by the Dark Energy Survey, and plans going forward for getting the most solar system science out of a cosmology survey.
The Lyman-alpha forest is a series of absorption lines seen in quasar spectra and is a powerful tool for probing the thermal state of the intergalactic medium (IGM). At intermediate redshifts (2< z <5), the statistical properties of the Lyman-alpha forest predicted by recent hydrodynamical simulations are in good agreement with a range of spectroscopic data. However, at lower and higher redshifts this is still not the case. A range of reionization models remain unconstrained and the precise timing of reionization remains elusive. The integrated heating during reionization has a measurable impact on the Lyman-alpha flux power spectrum. We adopt Markov Chain Monte Carlo approach to recover the integrated heating during reionization. This method can distinguish between early and late reionization models.
The ubiquitous supermassive black holes observed at the centers of all massive galaxies are believed to have grown via luminous accretion during quasar phases in the distant past, emitting 10% of the accreted rest-mass energy as light according to the Soltan argument. This 10% “radiative efficiency” defines a maximum growth rate via the Eddington limit. But the supermassive black holes powering quasars at z > 7 would require extremely massive initial seeds or super-Eddington accretion in order to grow to billion solar-mass scales when the Universe was less than 750 million years old. I will show that one can measure the radiative efficiencies of the most distant quasars using surrounding neutral intergalactic gas as a cosmological-scale ionizing photon counter, exclusively during the epoch of reionization. From the Lyman-alpha absorption signatures observed in the two highest redshift quasars, we find evidence that the radiative efficiencies of these quasars could be much lower than the canonical 10% value. I will discuss the consequences of the low efficiencies we measured for black hole growth in the early Universe, and possible alternative scenarios.
Given a set of observations, one often tries to infer probability distributions for model parameters using Bayesian inference. One common method is Markov Chain Monte Carlo (MCMC) sampling, where each MCMC step, one computes the likelihood of the data given the model parameters, typically using a Chi^2 metric under the assumption of Gaussian uncertainties. In practice, MCMC analyses can require over 100,000 likelihood evaluations, depending on the complexity of the model and the dimensionality of the problem. When using a slow forward model to compute the likelihood, running an MCMC analysis can become intractable. I present approxposterior, an open-source Python implementation of “Bayesian Active Learning for Posterior Estimation” by Kandasamy et al. (2015), that derives accurate approximations to the true underlying posterior probability distribution for inference problems with computationally expensive models. approxposterior trains a Gaussian Process (GP) as a surrogate model for likelihood evaluations and iteratively improves its performance by leveraging the inherent uncertainty in the GP’s predictions. Each iteration, approxposterior identifies high-likelihood regions in parameter space where the GP is uncertain, evaluates the forward model at these points, and then re-trains the GP to maximize its predictive ability while minimizing the number of model evaluations. approxposterior derives accurate posterior distributions using 100-1000s model evaluations, orders of magnitude less than is typically required by MCMC methods. The derived marginal posterior distributions have medians that are typically within 1-5% of the true values, with similar uncertainties to the true distributions. approxposterior has been validated in 2-5 dimensions and is under active development to scale to higher-dimensional inference problems.
Autumn Quarter 2018
Exploring and Refining Methods to Efficiently Measure the concentration-Mass Relation of Strong Lensing Galaxy Clusters
We are entering an age of large astronomical surveys where sizable data sets will be collected, from which thousands of galaxy clusters will be detected, leading to opportunities to perform complete statistical analyzes. Galaxy clusters are prime candidates to learn about the evolution of structure in the universe, constrain cosmological parameters, and explore the properties of baryonic matter, dark matter, and dark energy. The concentration-Mass (c-M) relation across cosmic time can be used to describe the evolution of the matter distribution of galaxy clusters. To constrain the mass density profile, a combined mass estimate from the core and a large scale of the galaxy cluster are needed. Strong lensing is the best method to estimate the mass at the core of the clusters. Computing lens models requires extensive human and computational resources. We explore and refine techniques to streamline this process, making it feasible to examine larger samples of clusters. I will discuss our efforts to characterize the uncertainties using the mass enclosed by the Einstein radius as zeroth order, and the use of “basic” lens models as a first approximation of the mass estimate at the core of strong lensing clusters. Last, I will describe the future application of the developed techniques to measure the c-M relation for simulated and observed galaxy clusters spanning a broad redshift and mass range.
Astronomers are interested in delineating boundaries of extended sources in noisy images. An example is finding outlines of a jet in a distant quasar. This is particularly difficult for jets in high redshift, X-ray images where there are a limited number of pixel counts. Using Low-counts Image Reconstruction and Analysis (LIRA), Stein et al. 2015 and McKeough et al. 2016 propose and apply a method where jets are detected using previously defined regions of interest (ROI). LIRA, a Bayesian multi-scale image reconstruction, has been tremendously successful in analyzing low count images and extracting noisy structure. However, we do not always have supplementary information to predetermine ROI and the size and shape can greatly affect flux/luminosity. LIRA is also unaware of correlations that may exist between adjacent pixels in the real image. In order to group similar pixels, we impose a successor or post-model on the output of LIRA. We adopt the Ising model as a prior on assigning the pixels to either the background or the ROI. From the posterior of this model, we are able to delineate probabilistic boundaries. This method has been applied to the jet data as well as simulations and appears to be capable of picking out meaningful ROIs.
Across almost all scientific disciplines, the instruments that record our experimental data and the methods required for storage and data analysis are rapidly increasing in complexity. This has been particularly true for astronomy, where current and future instruments produce data sets of a size and complexity not accessible with traditional methods. Within the Astronomy Department at UW, researchers at the Institute for Data-Intensive Research in Astrophysics and Cosmology (DiRAC) are working on a whole range of subjects related to the question of how to turn data into physical understanding. In this talk, I’d like to give an introduction into why DiRAC exists, who we are, what we work on, the resources we can provide, and why you might want to get involved with DiRAC (hint: we have cookies!).
Despite their importance for understanding the mutual build-up of supermassive black holes and their host galaxies, and the emission of low frequency gravitational waves, there is yet no definitive evidence for sub-parsec separation supermassive black hole binaries (SBHBs) in galactic nuclei. This is partly because the presently employed methods for identifying such SBHBs rely on indirect evidence that takes many years to accumulate. I will discuss new techniques that could provide the first definitive evidence for SBHBs at sub-pc separations using electromagnetic radiation.
Simulations suggest that the turbulent process of merging galaxies powers black hole growth. In the process of merging, as two galaxies spiral around each other, violent shocks disrupt the structure of the galaxies enabling gas to fall onto the black holes creating a dual AGN and also heavily obscuring them behind a screen of gas and dust. In the subsequent black hole merger, gravity waves are predicted which can result in a recoiling black hole that leaves the center of the galaxy. While the theoretical model is clear, recent observational studies have provided dramatically different scenarios and contradictory results. I will review recent advances using high resolution AO imaging and high energy X-rays that have enabled the detection of emission from these black holes even behind large amounts of obscuring gas and dust. I will also discuss how future missions such as high resolution X-ray imaging, JWST, and large diameter AO systems (TMT, E-ELT) may improve our understanding of galaxy mergers and black hole growth.
The Kepler mission and its continuation as K2 have revolutionized both the study of exoplanets and of stellar astrophysics, providing high-precision light curves of hundreds of thousands of stars. Kepler saturates at the eleventh magnitude, however, and naked-eye stars are far too saturated to observe conventionally. I will describe the ‘halo’ and ‘smear’ photometry algorithms, based respectively on Total Variation minimization and a subtle CCD bias, which nevertheless recover nearly normal quality light curves of stars as bright as the first-magnitude Aldebaran and Spica, the Pleiades, and hundreds of other bright stars in both Kepler and K2. Although we have not detected any transiting planets in this sample, we have revealed these bright stars to be ubiquitously variable, detecting classical pulsations, solar-like oscillations, binary and rotational modulation in nearly all stars. I will describe highlights of this survey, including a detailed study of bright red giants for use as spectroscopic benchmark stars, the discovery that Maia is not a Maia variable, and that the planet Aldebaran b may have been temperate in the distant past.
Summer Quarter 2018
Sowing black hole seeds: Direct collapse black hole formation with realistic Lyman-Werner radiation in cosmological simulations
We study the birth of supermassive black holes from the direct collapse process and characterize the sites where these black hole seeds form. In the pre-reionization epoch, molecular hydrogen (H2) is an efficient coolant, causing gas to fragment and form Population III stars, but Lyman-Werner radiation can suppress H2 formation and allow gas to collapse directly into a massive black hole. The critical flux required to inhibit H2 formation, Jcrit, is hotly debated, largely due to the uncertainties in the source radiation spectrum, H2 self-shielding, and collisional dissociation rates. Here, we test the power of the direct collapse model in a self-consistent, time-dependant, non-uniform Lyman-Werner radiation field using an updated version of the SPH+N-body tree code Gasoline. We vary Jcrit from 30 to 10^3 in units of J21 to study how this parameter impacts the number of seed black holes and the type of galaxies which host them. We focus on black hole formation as a function of environment, halo mass, metallicity, and proximity of the Lyman-Werner source. Massive black hole seeds form more abundantly with lower Jcrit thresholds, but regardless of Jcrit, these seeds typically form in halos that have recently begun star formation. Our results do not confirm the proposed atomic cooling halo pair scenario; rather, we find that black hole seeds predominantly form in low-metallicity pockets of halos which already host star formation.
Sowing black hole seeds: Direct collapse black hole formation with realistic Lyman-Werner radiation in cosmological simulations
I will address two of the major open questions about the first billion years of the universe: 1.) Exactly how and when did cosmic reionization occur? and 2.) How did the first supermassive black holes form? In the first part of my talk, I will discuss how Lyman-alpha intensity mapping can be used to probe reionization. Galaxy line intensity mapping is a new technique to measure the large-scale three-dimensional clustering of high-redshift galaxies. I will describe the first Lyman-alpha intensity mapping simulations which include the complex radiative transfer effects of Lyman-alpha photons scattering through neutral gas in the intergalactic medium. Utilizing a Monte Carlo approach to radiative transfer, we find Lyman-alpha scattering smooths the intensity mapping signal on small scales. The amplitude and scale-dependence of this effect depend strongly on the mean neutral fraction of the universe, making Lyman-alpha intensity mapping a very promising probe of reionization. In the second portion of my talk, I will describe how the existence of billion solar mass supermassive black holes only a billion years after the Big Bang presents an interesting puzzle and discuss one possible solution, the formation of massive (~100,000 solar mass) black hole seeds formed through direct collapse of gas in pristine dark matter halos. I will conclude by presenting recent work on how these “direct collapse black holes” can be identified observationally by combining future X-ray and infrared observations.
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