Spring 2022 Schedule:
February 17th:
Riddhi Bandyopadhyay (Post-Doctoral Associate Research Scholar, Princeton)
Title: Geometry of Magnetic Fluctuations near the Sun from the Parker Solar Probe
Abstract: Solar wind magnetic fluctuations exhibit anisotropy due to the presence of a mean magnetic field in the form of the Parker spiral. Close to the Sun, direct measurements were not available until the recently launched Parker Solar Probe (PSP) mission. The nature of anisotropy and geometry of the magnetic fluctuations play a fundamental role in dissipation processes and in the transport of energetic particles in space. Using PSP data, we discuss measurements of geometry and anisotropy of the inner heliosphere magnetic fluctuations, from fluid to kinetic scales. The results are surprising and different from 1 au observations. We propose that this behavior is due to the nature of large-scale forcing and waves outside the solar corona.
February 10th:
Matthew Coleman (Post-Doctoral Associate Research Scholar, Princeton)
Title: Accretion Through the Boundary Layer
Abstract: Accretion is a ubiquitous process is the universe, shaping objects from planets to supermassive black holes. For accreters which posses a material surface, the accretion flow rapidly change configurations from a rotationally supported disk to the slowly rotating pressure supported accreter atmosphere. In the case where the magnetic field of the accreter is insufficient to interrupt the accretion disk, the accreted material is transported through the narrow boundary layer joining the accretion disk to its accreter. Due to a change in angular momentum gradients the magnetorotational instability is unable to operate in the boundary layer. Using recent high resolution simulations of the boundary layer I elucidate the physical processes transporting angular momentum and material through the boundary layer, and discuss the appearance of quasi-periodic oscillations in these simulations which my explain the observed dwarf nova oscillations.
February 3rd:
Benjamin Crinquand (Post-Doctoral Associate Research Scholar, Princeton)
Title: Kinetic modeling of black-hole magnetospheres
Abstract: A variety of astrophysical phenomena can only be explained as being powered by black holes. In particular, accreting supermassive black holes are responsible for launching relativistic plasma jets and for accelerating ultra-energetic particles. The mechanism that channels energy from the black hole to the particles remains a mystery.
Observations have come to help lately. The Event Horizon Telescope collaboration has been able to image the shadow of the supermassive black hole M87*, probing the magnetic structure almost down to the event horizon. The GRAVITY collaboration detected a hot spot in infrared orbiting Sgr A*, indicating the presence of a large-scale poloidal magnetic field. These observations give new clues to constrain theoretical models that are able to explain jets and particle acceleration. This problem involves complex interactions between collisionless plasma dynamics in high gravitational field, and pair creation due to the interaction of energetic particles with surrounding photons from the accretion flow. Only kinetic simulations can capture all these effects.
In this talk, I will present general relativistic particle-in-cell simulations of axisymmetric black-hole magnetospheres, which include self-consistent plasma supply and radiative processes. I will put the emphasis on extracting synthetic observables from these simulations, such as gamma-ray lightcurves and millimeter images, which can directly be compared to actual observations. This allows us to model very accurately the non-thermal radiation emitted from the innermost regions of black-hole magnetospheres.
Fall 2021 Schedule:
September 9th:
Tim Miller (Graduate Student, Yale University)
Title: Moving beyond the Sersic profile
Abstract: The modern standard for measuring galaxy morphology involves fitting parameterized models to galaxy images, typically one or more Sersic profiles. This framework underpins almost all morphological galaxy studies as well as photometry provided by most large surveys. In this talk I will discuss two projects in which we try to move beyond photometry and morphology measurements using traditional parameterizations. The first involves using the Dragonfly Telephoto array (DF) to measure photometry of nearby massive galaxies using a non-parametric method. The photometry of these galaxies is notoriously difficult due to their extended light profiles. When comparing to parameterized fits typically used to measure total flux we find good agreement with our method. However, we find bluer colors meaning that the M/L ratios of galaxies have been overestimated. The net effect is that galaxy masses in the literature have been slightly overestimated rather than underestimated, by ~10%. In the second part of, I will describe a new tool, imcascade, which is a bayesian application of the multi-gaussian expansion (MGE) method. The MGE method is a flexible way to model a galaxy’s profile, without the need for an a-priori choice of parameterization. I will describe the idea and initial tests of our method along with preliminary results from applying it to HST imaging from the CANDELS survey.
September 16th:
Biwei Dai (Graduate Student, University of California, Berkeley)
Title: Translation and Rotation Equivariant Normalizing Flow (TRENF) for Optimal Cosmological Analysis
Abstract: Our universe is homogeneous and isotropic, and its perturbations are translation and rotation equivariant. We develop a generative Normalizing Flow (NF) architecture which explicitly incorporates these symmetries, defining the data likelihood via a sequence of Fourier space based convolutions and nonlinear transforms, evaluating their Jacobian at each layer. This allows to train the NF by maximizing the data likelihood p(x|y) as a function of the labels y, such as cosmological parameters. In contrast to other generative models the NF approach has no loss of information since it preserves the full dimensionality of the data, and gives direct access to the data likelihood p(x|y). We apply this to outputs of cosmological N-body simulations and show that the summary statistics of the generated samples agree well with the simulations. We show that the reverse mapping is visually indistinguishable from a Gaussian white noise: when this is perfectly achieved the resulting p(x|y) likelihood analysis becomes optimal. On simple Gaussian examples we show that this approach maximizes the information in the data and saturates the Fisher information content in the labels y. On N-body simulation outputs we show that this leads to significant improvements in constraining power over the standard summary statistics such as the power spectrum. Finally, we develop a generalization of this NF that can handle effects that break the symmetry of the data, such as the survey mask.
September 30th:
Zhuo Chen (Graduate Student, University of California, Los Angeles)
Title: A new window on star formation history at the Galactic Center
Abstract: As the closest galactic nucleus, Milky Way's Nuclear Star Cluster (NSC) provides a unique opportunity to resolve the stellar population and to study its composition and star formation in this extreme environment. The limitation in our current understanding of the NSC star formation history is that previous studies assumed that all stars have solar metallicity. However, age and metallicity are degenerate parameters in star formation histories; by ignoring the effect of metallicities, the age estimates can be biased. Recent spectroscopic surveys showed a significant spread in the metallicity of stars in the NSC, which motivates us to revisit the star formation history and its implications on the formation and evolution of the NSC. In this talk I will present the star formation history of the NSC for the first time with metallicity constraints as obtained from large sample of stellar metallicity measurements from Keck, Gemini and VLT. The analysis shows significantly different star formation history than any previously published works. In addition, we model the initial mass function for the first time simultaneously, and present more accurate estimates on the number of compact objects at the Galactic center.
October 7th:
Andrés A. Plazas Malagón (Associate Research Scholar, Princeton University)
Title: Systematic errors in weak gravitational lensing for cosmological investigations
Abstract: Weak gravitational lensing of the large scale structure of the
Universe has been identified as a powerful way to learn about dark matter and dark energy, two largely unknown components of the Universe that make up 95% of its matter-energy contents. As such, weak lensing is one of the fundamental tools used by past, current, and future projects such as the Dark Energy Survey (DES), the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), and NASA's Nancy Grace Roman Space Telescope (Roman). Nevertheless, the extraction of the weak gravitational lensing signal from astronomical measurements is a challenging process, and great attention must be placed on the understanding and characterization of systematic errors. In this talk, I will discuss the challenges of Point Spread Function (PSF) modeling and shape measurement for weak gravitational lensing science with DES, LSST, and Roman. In particular, I will discuss the characterization and mitigation of detector effects such as the "tree rings" and the "brighter-fatter effect" in the fully-depleted, thick CCDs of the Dark Energy Camera and the LSST Camera, and in the near-infrared H4RG detectors of Roman. I will frame these investigations in the context of the main results from DES Year 1 and results from DES Year 3 data.
October 14th:
Dhruba Dutta Chowdhury (Graduate Student, Yale University)
Title: Constraining Dark Matter through Gravitational Heating and Cooling Processes
Abstract: Fuzzy Dark Matter (FDM), consisting of ultralight bosons, is an intriguing alternative to Cold Dark Matter (CDM). Unlike in CDM, FDM halos consist of a central solitonic core, surrounded by an envelope of order unity density fluctuations. The envelope density fluctuations also interact with the soliton causing it to wobble and oscillate. Using high-resolution numerical simulations of an FDM halo, corresponding to a particular boson mass, I will demonstrate that the gravitational potential fluctuations associated with the soliton's wobble, its oscillations, and the envelope density fluctuations dynamically heat nuclear objects (e.g., central star clusters and supermassive black holes) and galaxies. As a result, nuclear objects, initially located at rest at the soliton center, migrate outwards over time until the outward motion is counteracted by dynamical friction and an equilibrium is reached. Similarly, a galaxy undergoes significant size expansion and central density reduction over a Hubble time. Generalizing these results for other halo and boson masses and comparing them with observations (such as galaxy size-age relation, measured offsets of supermassive black holes and nuclear star clusters from the centers of their host galaxies) will be able to constrain the boson mass. After discussing FDM, I will also briefly present my work on the peculiar galaxy NGC 1052-DF2 and show what we can learn about its mass distribution from the dynamical friction-induced orbital decay of its globular clusters.
October 28th:
Tansu Daylan (Post-Doctoral Fellow, MIT & visiting Princeton)
Title: Using TESS Full Frame Images to find exoplanets transiting faint stars
Abstract: The Transiting Exoplanet Survey Satellite (TESS) is proceeding with its mission of discovering transiting, small exoplanets hosted by bright stars that are amenable to mass measurements, and hence, bulk and atmospheric characterization. Accordingly, exoplanet searches using TESS have so far been focused on bright host stars or a specific population (e.g., young) of stars. In this talk, I will provide an overview of our ongoing efforts in extending TESS's homogeneous survey of transiting exoplanets to fainter hosts with a limiting magnitude of T=13.5. Towards this purpose, we use the Quick Look Pipeline (QLP) light curves and construct various summary metrics from the FFIs to vet Threshold Crossing Events (TCEs) by excluding false positives such as nearby Eclipsing Binaries (EBs), stellar variability, and systematics. Preliminary results of this effort recently allowed the number of TOIs to surpass 4400 and, as we begin Cycle 4, we expect an additional ~1500 exoplanet candidates by the end of the extended mission. This projected yield is especially important to achieve a full-sky demographic survey of exoplanets with a well-characterized selection function.
November 4th:
Keming Zhang (Graduate Student, University of California, Berkeley)
Title: Fast 2-body microlensing inference with neural posterior estimation
Abstract: Owing to the pathological parameter space of binary microlensing which contains a multitude of local likelihood maximas that are narrow and deep, current analysis of such events are done on a case-by-case basis with computationally expensive "grid-searches" required as a prerequisite to MCMC posterior sampling. This status-quo approach creates a significant challenge on the scale of the Roman microlensing survey which is expected to discover thousands of such events. In this talk, I present a likelihood-free inference approach named neural posterior estimation, where a neural density estimator (NDE) learns a surrogate posterior as an observation-parameterized conditional probability distribution, from pre-computed simulations over the full prior space. After training on mere ~300,000 Roman-like 2L1S light-curve simulations, the NDE automatically produces accurate and precise posteriors for any future Roman light-curve within seconds, thus allowing for fast and automated inference. The NDE also captures expected posterior degeneracies.
November 11th:
Daniel Tamayo (NASA Hubble/Sagan and Lyman Spitzer, Jr. Postdoctoral Fellow, Princeton University)
Title: The long-term chaotic dynamics of exoplanet systems
Abstract: The vast majority of known exoplanet systems are billions of years old. Over these long lifetimes, chaotic gravitational interactions among the planets can lead to violent collisions and restructurings of their orbits. Indeed, the observed sample shows strong evidence of having been sculpted by such cataclysms, but the dynamical pathways to such instabilities, and the prediction of their associated timescales, are long-standing theoretical problems. I will describe our recent work elucidating the underlying dynamics that drive chaos in closely packed systems, and how we are using this new ability to predict the long-term stability of planetary systems to better understand the planet formation process.
November 18th:
Changhoon Hahn (Postdoctoral Research Associate, Princeton University)
Title: Accelerated Bayesian Galaxy SED Modeling
Abstract: Spectral Energy Distribution (SED) modeling is essential for galaxy evolution and cosmology to derive physical properties of galaxies, such as their stellar mass, star formation history, and chemical enrichment history, from observations. State-of-the-art SED modeling uses Bayesian inference, which requires sampling a high-dimensional parameter space. They take hundreds of CPU hours to analyze a single galaxy and, thus, are not scalable for the next-generation galaxy surveys. I will present how we can use new techniques in Machine Learning, such as neural emulation and neural density estimation, to dramatically accelerate Bayesian SED modeling. I will present how these techniques will be applied to the DESI Bright Galaxy Survey (BGS) to construct the PRObabilistic Value-Added BGS (PROVABGS), which will deliver full posteriors on the physical properties (e.g. stellar mass, star formation rate, metallicity, and age) of over 10 million galaxies out to z < 0.6. I will demonstrate the accuracy and precision of PROVABGS and discuss its scientific applications. Lastly, I will present new methods using neural posterior estimators that will accelerate Bayesian SED modeling even further and discuss their application to future surveys.
Spring 2021 Schedule:
Date: Jan 21
Speaker: Archie Bott
Title: Alfvénic turbulence in an expanding, collisionless, magnetized plasma
Abstract: An ever-expanding collection of astronomical observations indicate that strongly magnetized plasma turbulence is ubiquitous in many different astrophysical contexts, from the solar wind to the intracluster medium (ICM) of galaxy clusters. Understanding fundamental properties of such turbulence is a necessary component of explaining crucial aspects of these systems, including transport processes, heating and non-thermal particle acceleration. However, it remains an open question as to what extent established theories of magnetized turbulence first formulated in the framework of magnetohydrodynamics (MHD) apply to real astrophysical plasmas. In particular, it has been speculated that the infrequency of Coulomb collisions combined with the vast scale separations between macroscopic and dissipation scales can result in a complex interplay between the macrophysical evolution and microphysical response of astrophysically relevant plasma. In this talk, I will discuss the results of hybrid-kinetic particle-in-cell simulations that help address this question by initializing strong Alfvénic turbulence in a collisionless, magnetized plasma with comparable magnetic and thermal energies (ß~1), and then initiating that plasma's expansion. Our findings, which are directly relevant to turbulence in the near-Earth solar wind, show that, in spite of the co-presence of ion-Larmor-scale magnetic fluctuations amplified by the firehose instability, the key qualitative properties of strong MHD Alfvénic turbulence (e.g., critical balance) still persist an expanding, collisionless ß~1 plasma.
Date: Feb 4
Speaker: Philip Mocz
Title: Cosmological Simulations with Quantum Computers
Abstract: Cosmological simulations on classical computers are limited by time, energy, and memory usage. Quantum computers have the potential to exponentially improve computational costs and enable extremely large simulations that capture the whole dynamic range of structure in the Universe. However, not all computational tasks exhibit a 'quantum advantage'. Quantum circuits act linearly on quantum states, so nonlinearities (e.g. self-gravity in cosmological simulations) pose a significant challenge. In my Thunch talk, I will outline one potential approach to overcome this challenge and solve the (nonlinear) Schrodinger-Poisson equations for the evolution of self-gravitating dark matter, using a hybrid quantum-classical variational algorithm framework.
Date: Feb 18
Speaker: Jamey Szalay
Title: Moon-Magnetosphere interactions at Jupiter: New Insights from Juno
Abstract: Jupiter’s intense aurora provide a window into the complex plasma phenomena that occur throughout its vast magnetosphere. There are many distinct features in the Jovian aurora, which are typically categorized into: the oval-shaped main auroral emission, diffuse emissions equatorward of the main oval, polar emissions, and the Galilean satellite footprint spots and tails. While the source locations of most auroral features are difficult to determine due to uncertainties in magnetic field mapping, the footprints of the Galilean moons enable a precise determination of the magnetospheric source region for these phenomena. Of the many Jovian auroral features, Jupiter’s innermost Galilean satellite, Io, generates one of the most consistent and identifiable aurora. Io’s auroral signature itself has a rich morphology, including a long auroral tail trailing the Io footprint in the Jovian ionosphere. Auroral features at Jupiter associated with Europa and Ganymede have also shown similarities with those due to Io. The Juno spacecraft crossed flux tubes connected the tails of the Galilean satellites at a broad range of Jovian altitudes and longitudinal separations along the tails, enabling detailed measurements of the plasma populations sustaining their auroral footprints. This presentation will primarily summarize the current state of knowledge on Io’s auroral emissions, present new observations Europa’s and Ganymede’s footprint aurora, and discuss how recent in-situ Juno observations have shed light on these complex auroral processes.
Date: Feb 25
Speaker: Vladimir Zhadankin
Title: Numerical experiments on relativistic plasma turbulence
Abstract: Turbulence is believed to play an important role in high-energy astrophysical systems such as black-hole accretion flows and their jets. These systems contain plasmas that are collisionless, relativistic, and radiative; the properties of turbulence and the associated particle energization are largely unexplored in this physical regime. Understanding the interaction between turbulent fluctuations and relativistic particles is essential for interpreting observations (including the luminosity, spectra, and variability of systems). I will describe recent and ongoing work on studying relativistic plasma turbulence with kinetic particle-in-cell simulations. In particular, I will describe what we have learned about electron and ion heating, thermal coupling mechanisms, nonthermal particle acceleration, the effect of radiative cooling, and radiative signatures from driven turbulence simulations with varying setups.
Date: Mar 4
Speaker: Christopher Spalding
Title: The ancient Solar wind as a sculptor of terrestrial planetary formation
Abstract: The innermost region of our Solar system presents a number of enigmas. Mercury’s iron core is twice as massive as Earth’s relative to the planetary sizes, but no obvious meteoritic source provides the requisite iron to explain Mercury’s high density. Moreover, there is a conspicuous lack of material interior to Mercury’s orbit, contrasting sharply with the most common exoplanetary systems thus far detected. In this talk, I will explore the underestimated role played by the young Sun’s wind in sculpting both the inner Solar system and Mercury’s bulk composition. Specifically, the Sun possesses an outflowing wind of charged particles that generally weakens over time, as inferred from observations of young Solar analogues. During the epoch of planet formation, therefore, the Sun’s wind might have exceeded 100 times its modern value. Solar plasma would thus have permeated the early inner Solar system, directly affecting the orbits of planetary building blocks, potentially moving them out of the region interior to Mercury’s orbit. I discuss the consequences of this strong solar wind, with an eye to empirical expectations from future missions within the exoplanetary and Solar system fields alike.
Date: Mar 11
Speaker: Kassandra Anderson
Title: Exciting and Erasing Obliquities in Exoplanetary and Stellar Binary Systems
Abstract: Stellar spin-orbit misalignments (obliquities) are of great interest in planetary and stellar binary systems, as they shed insight into formation and migration histories. In planetary systems, several trends with stellar obliquity have emerged over the past decade, such as the correlation between obliquity and host star temperature: Hot Jupiters orbiting cool host stars tend to have low obliquities, while those orbiting hot stars have a broad range of obliquities. An enticing explanation for this pattern is tidal realignment of the cool host stars, but this requires that obliquity damping occurs faster than orbital decay, which is unclear. In the first part of this talk, I will briefly review mechanisms for exciting obliquities in hot Jupiter systems, before presenting the results of a recent study of tidal damping of stellar obliquities in hot Jupiter systems. In this study, we utilize a recent empirical estimate of the stellar quality factor to model the tidal dissipation, and explore various tidal histories for observed hot Jupiters, starting with a broad distribution of obliquities. Our model predicts efficient obliquity realignment (to less than 1 degree) for orbital periods less than 2-3 days, which may be tested with future observations. In the second part of the talk, I will shift gears to stellar binaries, focusing on the DI Herculis eclipsing binary system, which exhibits large spin-orbit misalignments. Starting from a spin-orbit aligned state, I will discuss a mechanism for exciting the obliquities in DI Herculis, due to the presence of an inclined circumbinary disk. As the disk loses mass through a combination of winds and accretion, a secular resonance may be encountered, causing the obliquities to be excited. We place constraints on the necessary disk properties for exciting the obliquities, finding that the disk must have been initially massive (~10 % of the binary mass) and inclined by at least 10 degrees relative to the binary orbit.
Date: Mar 18
Speaker: Keivan Stassum & Nina Hernitschek
Title: The Era of Big Data in Astronomy - Machine Learning for All-Sky Surveys
Abstract: In the era of large-scale astronomical surveys, methods to investigate these data, and especially to classify astronomical objects, are becoming more and more important. For example, various techniques for extracting information, such as structure function fitting and template-based period fitting, can be applied before a subsequent machine-learning classification searches for and classifies variable sources. We give a brief overview of state-of-the-art methods of data handling and machine learning techniques used in astronomy, as well as in more detail describe how to apply specific methods to typical problems occurring from large time-domain surveys such as Pan-STARRS1, ZTF, TESS, and finally the Vera Rubin Observatory (LSST).
Date: Mar 25
Speaker: Andrea Afruni
Title: Modeling the flows of circumgalactic gas from and to galaxies
Abstract: The evolution of galaxies is strongly linked to the gas residing in their halos, the circumgalactic medium (CGM), a multiphase gas with components at very different temperatures, that extends till the virial radii of galaxies. The origin and dynamics of this medium are however still debated, especially regarding its cool (T~10^4 K) phase, which is generally observed using UV absorption lines in the spectra of background quasars. In this talk I will present the results that we have obtained in the last few years by studying this medium with semi-analytical parametric models. In particular, we use our models to try to reproduce the data of recent observational surveys of this diffuse ionized gas, focusing on different samples of early- and late-type galaxies. I will show how, through a bayesian fitting analysis, we are able with our models to gain insights on the nature of this gas phase, and how we find that most of this cool medium seems to be part of cosmological gas accretion onto the halos of galaxies.
Date: April 8
Speaker: Scott Carsten
Title: Dwarf Galaxies: Probes of Galaxy Formation and Small-Scale Structure
Abstract: Low-mass galaxies are premier windows into many astrophysical processes, including dark matter on small scales, stellar feedback, and the effects of being a satellite galaxy of a massive host. However, to unravel the myriad of interrelated processes sculpting dwarf galaxies, it is critical to observe them in a range of environments. To address this, we have started the Exploration of Local VolumE Satellites (ELVES) Survey to characterize the dwarf satellites of all massive (M_Ks < -22.4 mag) hosts in the Local Volume down to dwarf luminosities of M_V<-9 mag. I will discuss early results from this survey, including new insights into the transformation of late-type dwarfs to early-type, how dwarf structure depends on environment, and the environmental dependence of the abundance of nuclear star clusters and globular clusters in dwarfs. I will also examine the use of dwarf satellite systems in constraining the stellar-to-halo mass relation and the implications for our understanding of small-scale structure formation.
Date: April 15
Speaker: Lachlan Lancaster
Title: A Fractal Theory of Stellar Wind Feedback in the Dense Turbulent ISM
Abstract: Winds from massive stars have velocities of 1000 km/s or more, and produce hot, high pressure gas when they shock. We present a theory for the evolution of bubbles driven by the collective winds from star clusters, which involves interaction with the turbulent, dense interstellar medium of the surrounding natal molecular cloud. A key feature is the fractal nature of the hot bubble's surface. The large area of this interface with surrounding denser gas strongly enhances energy losses from the hot interior, enabled by turbulent mixing and subsequent cooling at temperatures T ~ 10^4-10^5 K where radiation is maximally efficient. Due to the extreme cooling, the bubble radius scales differently from the classical Weaver solution and has expansion velocity and momentum lower by factors of 10-100, with pressure lower by factors of 100-1000. Our theory explains the weak X-ray emission and low shell expansion velocities of observed sources. We discuss further implications of our theory for observations of the hot bubbles and cooled expanding shells created by stellar winds, and for predictions of feedback-regulated star formation in a range of environments. We validate our theory with a suite of hydrodynamic simulations which will be used to explain key concepts throughout.
Date: April 22
Speaker: Erin Kado-Fong
Title: Disks, Halos, and Puffballs: On the origin and nature of dwarf galaxy stellar structure
Abstract: Low-mass dwarf galaxies (stellar masses less than ~5x10^9 solar masses) have often been invoked as ideal galactic laboratories to study the fundamental nature of dark matter and physics of star formation. However, due to their low luminosities, the detailed nature of dwarf galaxies beyond the nearby Universe has been an observational open question. Now, for the first time, current generation wide-field imaging allows for the detailed structural studies of dwarfs out to intermediate redshifts. I will first discuss the three dimensional shapes of dwarfs as a function of radius in both observations (HSC-SSP) and simulations (FIRE-2), with a special focus on the formation and maintenance of stellar disks and in-situ stellar halos in dwarf galaxies. Then, I will address the formation and evolution of the extreme low surface brightness end of the dwarf galaxy population, and show that the structure of such galaxies places novel constraints on the formation mechanism of this extreme subset of the dwarf population. Finally, I will discuss ongoing efforts to provide the first large and mass-complete sample of over 100,000 star-forming dwarfs beyond the nearby Universe.
Date: April 29
Speaker: Lucia Armillotta
Title: Cosmic-ray transport in simulations of star-forming galactic disks
Abstract: To date, most galaxy evolution simulations including a relativistic cosmic-ray fluid have shown that the latter can significantly contribute to the dynamics of the interstellar medium, aiding in the internal support against gravity and also in the launching of galactic outflows. However, the extent to which cosmic rays affect these phenomena is strongly sensitive to the way different cosmic-ray transport mechanisms are treated in the simulation. With the goal of understanding the details of the cosmic-ray transport on galactic scales, we have computed the propagation of cosmic rays in the solar-neighborhood environment reproduced by the MHD TIGRESS simulations. We have investigated a variety of cosmic-ray propagation models, from simple models based on a pure diffusive formalism and assuming space-independent cosmic-ray scattering to more realistic models treating the scattering of cosmic rays as a function of the properties of the background multiphase interstellar medium. I will review the main results of this analysis and discuss the properties of cosmic rays in conditions typical of the local interstellar medium.
Date: May 6
Speaker: Christian Aganze
Title: Disks, Halos, and Puffballs: On the origin and nature of dwarf galaxy stellar structure
Abstract: The structure, formation and evolution of the Milky Way has been largely probed with FGK main stars via wide-field spectroscopic, photometric, and astrometric surveys (e.g 2MASS, WISE, SDSS, LAMOST, GALAH, Gaia). These studies have revealed that our Galaxy is a dynamic, ever-evolving system with kinematically distinct disk, halo, and bulge structures. Additionally, the discoveries of stellar streams and other phase-space perturbations in the solar neighborhood reveals the Galactic merger history of our Galaxy (e.g. Gaia-Enceladus) and ongoing interaction with its satellites. Brown dwarfs and very low-mass stars (M<0.1 Msun, Teff< 3000K) provide a new avenue for studying the star formation history and chemical evolution of the Milky Way. These objects constitute over 50% of stars in the Milky Way, have lifetimes far in excess of the age of the Universe, have fully mixed interiors, and are either extremely stable (stars) or evolve continuously (brown dwarfs). Previous studies have focussed on nearby samples (d<100pc) given that these objects are intrinsically faint. Probing thick disk and halo populations requires considerably deeper surveys. I will present the discovery of 231 M7-T9 dwarfs in the WFC3 Infrared Spectroscopic Parallel Survey (WISPS) and the 3D-HST parallels program, spanning distances of 400 pc to 2 kpc. The identification of these sources was facilitated by Machine Learning methods, in particular Random Forest and Deep Neural Network classifiers. I model the spatial distribution of the sample using Monte-Carlo population simulations that incorporate assumptions about the stellar mass function, age distribution, brown dwarf evolutionary models, and Galactic structure, and place constraints on galactic scaleheight and population age as a function of spectral type. I will conclude with predictions of the yields of very low mass stars and brown dwarfs in the next generation of deep spectroscopic surveys with the James Webb Space Telescope, the Nancy Grace Roman Telescope, and Euclid.
Fall 2020 Schedule:
Date: Sep 3
Speaker: Song Huang
Title: Utilizing the Stellar Halo of Massive Galaxies for Cluster Cosmology
Abstract: The abundance and lensing signals of massive galaxy clusters contain valuable information about the underlying cosmology model. Although cluster cosmology has excellent potential in the near future, identifying clusters and the measurements of their halo mass are still very challenging tasks. Taking advantage of the high-quality deep images from the Subaru HSC survey and its unprecedented galaxy-galaxy lensing capability, we revisit the possibility of using massive central galaxy to find galaxy clusters and measure halo mass. And we propose straightforward tests to compare central galaxy-based cluster finder with the popular richness-based finder with the help of cosmological simulation. These tests demonstrate that a massive galaxy's outer stellar halo is a good proxy of halo mass and tracer of recent halo assembly.
Date: Sep 10
Speaker: Remy Joseph
Title: Astronomical Image modeling and applications: deblending, strong gravitational lensing and more
Abstract: Looking at imaging data of deep surveys, the problem of knowing where a galaxy ends and its neighbour begins does not have a simple solution. It often requires making a certain number of assumptions as to the shapes of these galaxies, their morphologies, colours, redshifts, obscuration, etc. These assumptions are used as priors to build models for galaxies that allow to make measurements of physical quantities. Not only the way we incorporate these priors in galaxy models impacts the quality of our measurements, but it also affects how galaxies are separated from their neighbours. In this talk, we will see a few avenues for building realistic models for galaxies based on our understanding of the instruments that acquire images and on modern techniques for free-form modeling of light profiles. The methods I will present can be used for a wide range of applications: source separation (deblending), strong lens modeling, low surface brightness modeling, crowded field detection and certainly more, so come with your own ideas!
Date: Sep 17
Speaker: Hiroki Nagakura
Title: Towards comprehensive understanding of core-collapse supernovae
Abstract: Core-collapse supernova (CCSN) explosion is a fiery death of massive stars. The explosion mechanism seems to involves complex interplay between micro- and macroscopic physics, and its detail is still uncertain. Numerical simulations have been used in an effort to understand the roles of each physics, and they have made a remarkable progress with increasing computational resources in the last several years. In this seminar, I will introduce the current status of the community by presenting our recent results of multi-dimensional numerical modeling of CCSN made by 2D/3D full Boltzmann simulations (in collaboration with a Japanese group) and another 3D simulations (in Princeton group). Based on the result of the former project, I will mainly discuss roles of asymmetric neutrino emissions on the origin of neutron star kick and on collective neutrino oscillations. For the latter, I will discuss the progenitor dependence of explodability, proto-neutron star convection and neutrino signals based on the results of systematic 3D simulations. If time allows, I will also present some preliminary results of currently on-going our studies; non-thermal neutrinos from CCSN and its observational consequence; how we will efficiently use observed data on multiple detectors (e.g., SuperKamiokande/HyperKamiokande and DUNE) to reconstruct neutrino spectra at a CCSN source.
Date: Sep 24
Speaker: Alex Chen
Title: Numerical Adventures into the Magnetospheres of Neutron Stars
Abstract: Pulsars are rapidly rotating, highly magnetized neutron stars that emit a wide spectrum of radiation from radio to gamma rays. Despite
abundant observational data in all wavelengths, theoretical understanding of the pulsar is still incomplete. In this talk, I will outline how numerical simulations have aided us in understanding the magnetosphere of pulsars. In particular, I will talk about two broadly different theoretical classes: the "strong" and "weak" pulsars, and some of the questions about these pulsars that we can answer with first-principle simulations. If time permits, I will also talk briefly about the magnetospheres of even more magnetized neutron stars: magnetars, and how it can produce simultaneous X-ray bursts and fast radio bursts.
Date: Oct 1
Speaker: Brandon Hensley
Title: Rethinking the Nature of Interstellar Dust
Abstract: The highly polarized emission from Galactic dust as seen by the Planck satellite has challenged our basic assumptions about the makeup of interstellar grains. I will first describe the "fingerprints" of specific materials composing dust manifest as mid-infrared (MIR) extinction features. A new determination of the MIR extinction curve using the highly-reddened sightline toward the hypergiant Cyg OB2-12 will be presented, including detection of absorption features from polycyclic aromatic hydrocarbons (PAHs). I will then overview how the polarization properties of grains depend on composition and how recent results from polarimetry are incompatible with the previous generation of dust models. Finally, I will introduce a new model of interstellar dust that posits that the silicate and carbonaceous materials largely reside on the same grains, an idealized mixture we term "astrodust." I will demonstrate the compatibility of the astrodust-based model with existing observations and highlight how it can be tested with future data.
Date: Oct 8
Speaker: Jamie Rankin
Title: Cosmic-rays Beyond the Heliosphere: A Survey of Discoveries and Mysteries
Abstract: Since August 2012, humankind has achieved historic in-situ measurements from just beyond the heliopause, through NASA's Voyager mission. Amongst many things, this unprecedented data set has enabled, for the first time, observations of the low-energy end of the cosmic ray spectrum, unfiltered by the solar wind. In this talk, I will discuss recent discoveries, mysteries, and open questions about these low-energy cosmic rays, including: (i) the discovery and evolution of an unexpected time-varying anisotropy, (ii) the impact of solar activity beyond the heliopause, and (iii) connections to the global heliosphere, as observed by NASA's Interstellar Boundary Explorer (IBEX) mission.
Date: Oct 15
Speaker: Guðmundur Stefánsson
Title: Detection and Characterization of M-dwarf Planets with Next Generation Instruments
Abstract: Planets orbiting nearby M-dwarfs—the most numerous stars in the galaxy and our nearest stellar neighbors—are advantageous for detailed characterization. The low mass of M-dwarfs increases the Radial Velocity (RV) semi-amplitude of planets orbiting in their Habitable-zones (HZ), making such planets detectable with current RV facilities. Further, the large planet-to-star radius ratios of M-dwarf planets make them favorable targets for atmospheric characterization. Despite these advantages, M-dwarf planets have been poorly studied until recently due to the intrinsic faintness of their host stars in optical band-passes. However, with new facilities operating at redder wavelengths, we are gaining further insights into their properties and occurrence rates. In this talk, I will discuss two new technologies and how we are using them to detect and characterize nearby M-dwarf planets. First, I will discuss Engineered Diffusers—nano-fabricated pieces of optics capable of molding the image of a star into a broad and stabilized top-hat shape—which we have used to obtain some of the highest precision transit observations from the ground (60ppm in 30min bins). Second, I will discuss two new Doppler spectrographs I am involved with, the near-infrared Habitable-zone Planet Finder (HPF) on the 10m Hobby-Eberly Telescope, and the NEID spectrograph on the 3.5m WIYN Telescope. I will describe the ongoing HPF survey to detect planets in the HZ around nearby (<25pc) mid-to-late M-dwarfs, and how we are using HPF to gain insights into the properties and orbital architectures of transiting M-dwarf planet systems recently discovered by the K2 and TESS missions.
Date: Oct 22
Speaker: Chris Hamilton
Title: Secular dynamics of binaries in stellar clusters
Abstract: Binary systems are found everywhere in astrophysics. If a binary is completely isolated, the relative motion of the two bodies describes a Keplerian ellipse which repeats itself indefinitely. However, when a binary is perturbed, its elliptical orbital elements may change with time. In particular, a wide class of perturbations can drive large-amplitude secular oscillations in a binary's eccentricity. As a result the binary may shrink or even merge, producing exotic phenomena like hot jupiters, blue stragglers and compact object mergers (i.e. LIGO/Virgo gravitational wave sources).
In my PhD I have developed a mathematical theory for the evolution of binaries orbiting in arbitrary axisymmetric potentials, and applied the theory to black hole merger physics. One key result of my theory is that if a black hole binary orbits inside a star cluster, then the gravitational tidal force from a star cluster is often sufficient to periodically drive a binary’s eccentricity e to very high values (say e ≈ 0.99 or higher). This drastically reduces the closest-approach (‘periastron’) distance p = a(1 − e) of the black holes, where a is the semimajor axis. Repeated close passages of the black holes allow for significant dissipative bursts of gravitational radiation, shrinking the semimajor axis quickly. I have shown that this effect leads to mergers of black hole binaries which could not have merged if they were isolated.
In this talk I will (a) introduce the three-body problem as the archetypal example of a perturbed binary, (b) describe the general theory for secular evolution of binaries orbiting arbitrary axisymmetric potentials, and (c) discuss the application of the general theory to the problem of compact-object mergers (LIGO/Virgo gravitational wave sources) in stellar clusters.
Date: Oct 29
Speaker: Kishalay De
Title: The dynamic lives and fates of accreting white dwarfs in wide-field time domain surveys
Abstract: The lives and fates of accreting white dwarfs are crucial for a broad range of issues in modern astrophysics. In this talk, I will present
new insights into their dynamic lives enabled by new wide-field time domain surveys. Helium accreting white dwarfs have been long proposed
as sub-Chandrasekhar mass progenitors of Type Ia supernovae, although there exist difficulties in explaining their observational properties.
I will present results from a systematic search for these elusive thermonuclear transients with the largest volume-limited sample of
supernovae using the Zwicky Transient Facility optical time domain survey. I will discuss observational evidence that suggests that
helium shell explosions are realized in nature over a broad range of white dwarf core and shell masses -- ranging from peculiar Type Ia
supernovae to the faintest class of 'Ca-rich' transients. I will discuss my work on building the first wide-field infrared time domain
survey called Palomar Gattini-IR. By scanning the entire sky every two nights, Palomar Gattini-IR is uncovering optically obscured
thermonuclear explosions in the Milky Way, and providing new clues to the Galactic classical nova rate, as well as the unique 'stable' fates
of helium white dwarf mergers.
Date: Nov 5
Speaker: George Wong
Title: Interpreting black hole observations with numerical simulations
Abstract: Astrophysical relativistic jets are likely powered by spinning black holes via the Blandford–Znajek (BZ) mechanism, in which spin energy is siphoned away from the hole and converted into a Poynting flux jet. Recent measurements by the Event Horizon Telescope and the GRAVITY Collaboration have enabled precise black hole observations, and the data provide a new means to directly investigate the BZ mechanism. In this talk, I will discuss how numerical simulations can be used to explore the black hole/jet connection in the context of theory and observation. I will show how black hole spin influences image features via instabilities in the jet–disk interface, and I will discuss how the horizon-scale magnetic field imprints on image polarization patterns. I will also discuss how strong gravity produces a set of measurable observables that track black hole mass, spin, and inclination and that are independent of plasma physics uncertainties.
Date: Nov 12
Speaker: Jeong-Gyu Kim
Title: Modeling Dispersal of Giant Molecular Clouds by UV Radiation Feedback
Abstract: Giant molecular clouds (GMCs) are the primary reservoir of cold molecular gas in the interstellar medium and sites of ongoing star formation. Massive stars formed inside GMCs produce copious UV photons that erode and expel molecular gas. While this UV radiation feedback is thought to play a crucial role in destroying natal clouds and quenching star formation, our theoretical understanding of how it actually occurs in turbulent, magnetized GMCs remains incomplete. In this talk, I will report recent progress we made in modeling destruction of GMCs by UV radiation feedback. I will first overview observational constraints on cloud-scale star formation and cloud lifetimes and predictions made by analytic models of cloud disruption. I will then present the results from our radiation (magneto)hydrodynamic simulations of star-forming GMCs in which the propagation of UV radiation is modeled via ray-tracing method. Our parameter study explores how star formation efficiency, destruction timescale, and escape of radiation vary in a wide range of star-forming environments. I will briefly discuss observational implications of our results.
Date: Nov 19
Speaker: Ben Horowitz
Title: HyPhy: Mapping Dark Matter to Hydrodynamics with Posterior Inference
Abstract: There has been an explosion of recent work using machine learning techniques for various tasks in parameter inference and mock data generation. However, comparatively little work has explored error estimation within this framework, either from uncertainty in the underlying mapping or errors introduced by limitations in the model. In this talk, I will discuss ongoing work to quantify these errors in the context of a domain to domain translation task; mapping from dark matter only simulations to full hydrodynamical outputs. Using a fully convolutional conditional variational autoencoder, we are able to infer realistic looking hydrodynamical posteriors which emulate known cosmological summary statistics.
Date: Dec 3
Speaker: Matthias Raives
Title: The Core-Collapse Supernova Critical Condition and the Birth of Millisecond Proto-Magnetar Winds
Abstract: The mechanism powering core-collapse supernovae remains uncertain. I will discuss aspects of the critical condition for explosion, focusing on the model problem of spherical accretion onto a standing accretion shock. My recent work explores the importance of turbulence in setting the explosion condition and in explaining the qualitatively different outcomes between one-dimensional and multi-dimensional models. I will then transition to a discussion of the first few seconds after explosion, during the "proto-neutron star" cooling epoch, when a neutrino-driven wind emerges from the cooling neutron star into the overlying massive stellar progenitor. I will present results from new two-dimensional and three-dimensional MHD calculations of magnetocentrifugal thermal winds and connect them to the birth of highly magnetic, rapidly rotating neutron stars ("proto-magnetars"), which have been invoked in models of gamma-ray bursts and super-luminous supernovae. I will mention some additional potential applications of these results to winds from normal stars and irradiated Hot Jupiters.
Date: Dec 10
Speaker: Arun Kannawadi
Title: Cosmic shear surveys - Present & Future
Abstract: In this talk, I will give an overview on how gravitational lensing probes the evolution of the large-scale structure of the Universe and allows us to constrain cosmological parameters. I will present the constraints from the state-of-the-art lensing surveys (KiDS, DES, HSC) and highlight the so-called S8 tension between lensing and CMB parameters. I will also cover how none of the known systematic effects, nor simple extensions to the standard cosmological model are unable to bridge these differences. Finally, I will conclude with how the Stage-IV surveys in the near-future might be able to throw more light on this discrepancy and provide some insight into the nature of dark matter and dark energy.
Date: Jan 14
Speaker: Dan Tanaru
Title: Galaxy Structure and Dynamics in the 2020s
Abstract: Over the next decade, numerous ground- and space-based instruments will provide unprecedented deep imaging and spectroscopy over the entire sky. This will be a tremendous opportunity for quantifying the structure and dynamics of low-redshift galaxies by measuring their fundamental physical properties like mass, energy and momentum. I will discuss several aspects of the technical challenges in making robust measurements of this type - modelling the morphology of galaxies using imaging from the Subaru Hyper-Suprime Cam and Rubin Observatory, and self-consistent dynamical modelling using integral field spectroscopy from the SAMI Galaxy Survey (Taranu et al. 2017) - as well as the prospects for using space-based imaging, (spatially) unresolved spectroscopy and 21cm HI data.