Spring 2024 Colloquium Series

The Spring 2024 Joint Astrophysics Colloquium will be held on Tuesdays at 11:00am in the Peyton Hall Auditorium and will be followed by the Bahcall Lunch at 12:30pm in The Lewis Library.
Please find the spring speaker list below.


Colloquium 2024




Title and Abstract


Jonathan Fortney


New views of planetary and brown dwarf atmospheric physics and chemistry with JWST

JWST has enabled high-fidelity infrared spectroscopy for a wide range of transiting planets, directly imaged planets, and brown dwarfs.  In the past year, for transiting planets, observers have detected several molecules for the first time in an exoplanet.  In this talk I will describe observations of several planets from the MANATEE collaboration, which focuses on Jupiter- and Neptune-class planets in a new temperature regime, below 1000 K, where a range of chemical transitions are expected to occur.  These new observations require a range of new modeling efforts, and in my work I seek to make connections between a diverse range of atmospheres.  These impressive observations enable us to break new ground on previously highly uncertain atmospheric process.  These include the physics of mixing in (isolated) brown dwarfs and in (strongly irradiated) transiting planets, as well as the role of the atmosphere/interior connection, far below the visible atmosphere, in helping to dictate visible atmospheric abundances.


Erin Kara


Black hole accretion in the TDAMM Era

Most of the power from an Active Galactic Nucleus is released close to the black hole, and thus studying accretion at event horizon scales—at the intersection of inflow and outflow—is essential for understanding how much matter accretes and grows the black hole vs. how much matter is ejected, thus effecting the black hole’s large-scale environments. In the past decade, we have had a breakthrough in how we probe the inner accretion flow, through the discovery of X-ray Reverberation Mapping, where X-rays produced close to the black hole reverberate off inflowing gas. By measuring reverberation time delays, we can quantify the effects of strongly curved space time and measure black hole spin, which is key for understanding how efficiently energy can be tapped from the accretion process. In this talk, I will give an overview of this field, and will show how extending these spectral-timing techniques to extreme, transient (and possibly multi-messenger) accretion events like Tidal Disruption Events and Quasi Periodic Eruptions can help us understand the growth and impact of black holes in galactic centers. 


Daniel Scolnic

Duke University

Measurements of the Expansion Rate of the Universe and the Lingering Hubble Tension

The standard model of cosmology has passed every test over the last twenty years.  Yet it remains unsatisfactory, with 95% of the universe being dark components, whose nature we did not understand.  Now, there are possible ‘cracks’ in the model, as recent observations of the local expansion rate of the universe, parameterized by the Hubble constant, do not match predictions using data from the Cosmic Microwave Background and our standard model.  This is the best end-to-end test of our cosmological model, and currently, we do not pass the test.  I will discuss my team’s Pantheon+SH0ES measurements on the local side, and review the numerous crosschecks and tests on our data and analysis.  I will show new data from the James Webb Space Telescope and explain how it strengthens the current tension.  Finally, I will talk about how the community is moving forward, with different probes of the early and late universe, and what new theoretical ideas show the most promise.  


Anna Ho

Cornell University

Finding Relativistic Stellar Explosions as Fast Optical Transients. 

For the last half-century, relativistic outflows accompanying the final collapse of massive stars have predominantly been detected via high-energy emission, as long-duration gamma-ray bursts (GRBs). Yet, it has long been hypothesized that GRBs are the tip of the iceberg of relativistic stellar explosions, because the conditions required to produce and detect a GRB are contrived. I will present results from a search for relativistic stellar explosions using optical time-domain surveys. The emerging zoo includes afterglows at cosmological distances with no detected GRB, supernovae with luminous X-ray and radio emission, and a mysterious class of "fast blue optical transients" with minute-timescale optical flares at supernova-like luminosities. An understanding of the origin of these events and their relation to GRBs will be enabled by upcoming time-domain surveys in other bands, including X-ray, UV, and submillimeter.


Kumiko Kotera


Towards EeV neutrino astronomy with GRAND

We are living exciting times: we are now able to probe the most violent events of the Universe with diverse messengers (cosmic rays, neutrinos, photons and gravitational waves). One challenge to complete the multi-messenger picture resides in the highest energies, as no ultra-high energy neutrinos, with energy > 10^17 eV, have been observed yet. This challenge could be undertaken by the GRAND (Giant Radio Array for Neutrino Detection) project. GRAND is a proposal for a large-scale array of self-triggered radio antennas. It stands out today as a unique experiment which plans to reach ambitious sensitivity and sub-degree angular resolution to launch multi-messenger astronomy at ultra-high-energies. A design has been proposed for the GRAND detector, and the instrumentation is being tested and optimized with 3 small-scale prototypes: GRAND@Nançay, GRAND@Auger, and GRANDProto300. Based on these pathfinders, the GRAND Collaboration is starting to explore improved GRAND technical designs for the next large-scale phase of the project. This will consist in two arrays of 10'000 km2 each, in the Northern and Southern hemispheres to be deployed from 2028. In this talk, we will present the status of the GRAND prototypes, the preliminary designs and simulation results for the next stages, and the rich research program that these will enable.


Mike Brown


Planet Nine from Outer Space

Astronomers have been predicting and searching for planets beyond Neptune for almost 180 years. In all previous cases the predictions were based on bad data, bad physics, or both, and the predictions turned out to be wrong. In 2016, we joined this  inglorious group and declared that orbital alignments of the most distant objects in the solar system demand the presence of a distant giant planet on an eccentric and inclined orbit. I'll describe the mounting evidence for this planet, discuss the counter-proposals, and talk about the ongoing search for what would be the fifth largest planet of our solar system.


 PU Spring Break – No Colloquium



Adam Leroy

Ohio State University

A "Cloud-Scale" View of the Matter Cycle in Galaxies

The gas-star formation-feedback "matter cycle" acts in many ways as the engine of galaxy evolution. Over the last decade, the PHANGS surveys have carried out surveys across the electromagnetic spectrum aimed at resolving galaxies into the fundamental units of this cycle: molecular clouds, HII regions, and star clusters. I will describe some of the key results from these surveys, emphasizing our new, dynamic view of molecular clouds from the PHANGS-ALMA CO survey and the new view of dust revealed by PHANGS-JWST. Based on these surveys, we have our first "big picture" view of the demographics of star-forming molecular clouds, which show a clear link to their larger scale environment. The properties of these clouds, and the contrast between tracers of gas and recent star formation reveal the efficiencies and timescales for star formation, highlighting some tension between the data and popular turbulent models of star formation. The key role of stellar feedback is also visible on multiple scales from these data. Feedback helps maintain the large scale dynamical equilibrium in galaxy disks, rapidly reshapes molecular clouds, and carves pervasive shells and bubbles through the ISM. In all of these areas the revolutionary resolution and sensitivity of JWST through the near and mid-infrared promise to enable the next advances. In this area, I will highlight the PHANGS Cycle 1 and 2 JWST Treasuries, which provide high quality, immediately public near- and mid-IR imaging of a representative sample of z=0 star forming galaxies. There will be pretty pictures of galaxies from across the spectrum!


Evan Kirby

University of Notre Dame

Early r-process enrichment in globular clusters

Stars in nearly all globular clusters show complex relations among the abundances of light elements (up to Na).  Many also show anti-correlations of Mg and Al.  Until now, only one cluster (M15) conclusively showed any star-to-star variation in neutron-capture elements, like Eu.  Using a trick of stellar evolution, I show that these variations are primordial, not caused by external pollution.  Then, I show that M92 also has variations in barium and lanthanide abundances (but not first-peak r-process) among the first generation of stars but not the second.  The evidence points to a rare source of the "main" r-process that happened right at the start of the formation of the cluster.


Jamie Tayar

University of Florida

How well can we estimate stellar ages?

Good estimations of stellar ages are important for a wide range of science cases, from understanding the evolution and habitability of exoplanetary systems to tracking back the cosmic history of our own and other galaxies. These ages, however, are not directly measured, but rather inferred from theoretical models or empirical calibrations using a variety of techniques. I will discuss the current precision and accuracy of isochrone-based, rotation-based, and asteroseismically inferred ages, as well as some of the current challenges of each method. Finally, I will discuss some of the recent and ongoing work in my group to increase the number of stars with well-measured ages, as well as some of the interesting stellar physics puzzles we've run into along the way.


Aparna Venkatesan

University of San Francisco

The Cost of Brightening Night Skies on Ground-Based Astronomy

Dramatic rises in ground-based light pollution in recent years as well
as increasingly congested low-Earth orbits are leading to brightening
night skies worldwide, with consequences reaching far beyond this
decade. I share calculations of the potentially large future rise in
global sky brightness from space objects in low Earth orbit (LEO),
including qualitative and quantitative assessments of how ground-based
astronomical observations may be affected. Debris proliferation is
especially a concern: all log-decades in debris size may contribute
approximately the same amount of night sky radiance, and given the
rising risk of debris-generating events in LEO, this could lead to rapid
rises in global night sky brightness. This will in turn lead to
increased loss of astronomical data and diminished opportunities for
ground-based discoveries as faint astrophysical signals become
increasingly "lost in the noise". This will affect many constituencies
beyond professional astronomy that are reliant on dark and quiet skies,
including Indigenous communities' sky traditions, animal/bird migratory
patterns, amateur astronomy, astrotourism, human/animal circadian
rhythms, and the seasonal and pollination cycles of plants. Globally
coordinated regulatory policies and mitigation strategies are urgently
needed to protect the shared environment and intangible heritage of
space and dark skies for future generations.


Jack Hare


Radiatively-cooled Magnetic Reconnection Experiments

In extreme astrophysical systems, such as black hole coronae and pulsar magnetospheres, the process of magnetic reconnection is significantly modified by strong radiative cooling. This cooling removes internal energy faster than it is injected by the reconnection process, triggering a radiative collapse in which runaway cooling and compression leads to a cold, thin, and dense reconnection layer. The high-energy X-ray radiation released is, in turn, often the only signature of reconnection in these remote astrophysical environments. To study this process, we have conducted a series of radiatively cooled reconnection experiments driven by the Z Machine at Sandia National Laboratories, the world's largest pulsed-power facility. I will present observations of the formation of brightly-emitting and fast-moving hotspots within the reconnection layer, which we interpret as plasmoids formed by the tearing instability. These hotspots emit the majority of the high energy X-rays, localising the radiation signature in space and time.


Tim Bedding

University of Sydney

A Golden Age of Asteroseismology with Kepler and TESS

Asteroseismology uses the natural oscillation modes of stars to study their interiors. The wonderfully precise measurements by NASA's Kepler and TESS missions are ideal data sources for the technique. These space telescopes have been monitoring the brightness of hundreds of thousands of stars, with the main goal of discovering extra-solar planets as they transit their parent stars.  At the same time, observations of stellar oscillations have led to a revolution in asteroseismology. I will discuss some of the key results, including the use of gravity modes to probe the cores of red giant stars, the characterization of stars found to host exoplanets, and the measurement of ages for young stellar associations.


Spitzer Lecturer: Brant Robertson



Fall 2023 Colloquium Series

The Fall 2023 Joint Astrophysics Colloquium will be held on Tuesdays at 11AM and will be followed by the Bahcall Lunch at 12:30.  
Please find the speaker schedule below.




Area of Expertise

Title and Abstract


Ting Li

University of Toronto

MW observation, galactic dynamics, stream detection and characterization

The Power of Milky Way's Stellar Streams Enabled by Multi-Object Spectroscopic Surveys

We are entering an extremely data-rich era in the next decade, with full 6D+chemistry information on dozens of stellar streams, to shape our understanding on the chemo-dynamical evolution of the Milky Way, as well as the nature of the dark matter. In this talk, I will discuss two ongoing spectroscopic programs to study the stellar streams in our Milky Way and highlight a few latest scientific results from these two programs. The Southern Stellar Stream Spectroscopic Survey (S5), started in 2018, is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere. The science results from S5 include a homogeneous study of the kinematic and chemical properties of dozen streams in our Milky Way, the finding of a stream at ~30 kpc possibly perturbed by the dark matter subhalo, the constraints on the mass of the Milky Way and the Large Magellanic Cloud with stellar streams, and the discovery of the fastest hyper velocity stars ejected from Galactic center that can be used to study the shape of the Milky Way halo. The Milky Way Survey of the Dark Energy Spectroscopic Instrument (DESI), on the other hand, is a recently started 5-yr spectroscopic program in the Northern Hemisphere. With just the first year of data collected in 2021-2022, 4 million unique stars have been observed by DESI, including many stars in the streams of the northern sky (e.g. GD-1) and showing some interesting features as well as the streams beyond the Milky Way.


Will Farr

CCA / Stony Brook

Gravitational waves, compact object evolution, gravitational dynamics of exoplanets and dense stellar systems

Title: Cosmology and Fundamental Physics from Stellar Mass Binary Black Holes

Abstract: The first three observing runs of the LIGO, Virgo, and KAGRA gravitational wave detectors produced a wealth of "firsts," including the first observation of multiple "ringdown" modes from a black hole---the remnant of the first-ever binary black hole merger, GW150914.  Just like an excited atom, a recently-formed black hole is expected to emit a characteristic spectrum of gravitational radiation as it settles down to its equilibrium state---in general relativity, a Kerr spacetime.  "Black hole spectroscopy" thus provides the opportunity to directly probe these extreme spacetimes and test fundamental gravitational theories, just as atomic spectroscopy provided an ideal testbed for the new quantum mechanics of the early 20th century.  I will review the 50+ year effort toward black hole spectroscopy, including the recent detection of multiple ringdown modes in GW150914, and describe the near future of this field as LIGO, Virgo, and KAGRA's fourth observing run begins (last month!).  Along the way, I will review the rapidly changing landscape of gravitational wave astronomy, and describe some of the other exciting recent firsts in this new field, particularly the possibility of using binary black hole mergers for intrinsically-calibrated distance measurements to trace cosmic expansion.


Maura McLaughlin



Title: Pulsar Timing Arrays: A New Window on the Gravitational Wave Universe

Abstract: Millisecond pulsars are rapidly rotating neutron stars with phenomenal rotational stability. Pulsar timing arrays world-wide monitor over 100 of these cosmic clocks in order to search for perturbations due to gravitational waves at nanohertz frequencies. The tell-tale sign of a stochastic background of gravitational waves in pulsar timing data is the presence of quadrupolar spatial correlations. Recently, and for the first time ever, pulsar timing array collaborations have found evidence of these spatial correlations in multiple independent pulsar datasets.  The signal is consistent with that expected from an ensemble of supermassive black hole binaries, but could also be attributable to more exotic sources, such as cosmic strings or early universe inflation. I will describe these experiments and the most recent results, concentrating on those from the NANOGrav collaboration, and will discuss the increases in sensitivity expected from the combination of data observed with new and existing telescopes across the globe.


Alessandra Corsi

Texas Tech

Multi-messenger time-domain astronomy, relativistic radio transients, gravitational wave physics

Title: Multi-messenger observations of cosmic collisions and explosions: progenitors, relativistic ejecta, and remnants

Abstract: The births and mergers of neutron stars and black holes, the most exotic objects in the universe, can launch the fastest cosmic jets (gamma-ray bursts; GRBs) and shake the very fabric of space-time with gravitational waves. GW170817, the merger of two neutron stars witnessed through both its gravitational wave siren and its glow at all wavelengths of light, marked the beginning of a golden age in multi-messenger astrophysics. While the GW siren of GW170817 has directly confirmed that at least some short GRBs originate from NS-NS mergers, the smoking gun linking long GRBs to core collapses i.e., the association with stripped-envelope core-collapse supernovae, has been challenged by several electromagnetic (EM) observations. In this context, I will discuss how observations at radio wavelengths can probe the ejecta and environments of compact binary mergers and extreme core collapses and help unveil their progenitors and remnants. I will then highlight opportunities and challenges ahead, as new observational facilities will transform a trickle of multi-messenger discoveries into a flood.


Ralph Schoenrich


Galactic dynamics, Gaia

Title: Spinning into darkness: the slowing bar and Galactic structure


In the inner Milky Way stars trek together in a bar-shaped, rotating overdensity: the central bar, which dominates the inner disc out half-way to the Sun. The rotation velocity/pattern speed of this bar has been intensely debated. The best insight into the bar's motion are gained from its resonant effects on the Milky Way disc: Similar to Jupiter holding in resonant motion Solar System asteroids like Greeks and Trojans, some stars near the Sun are locked in resonant motion, e.g. in corotation resonance circling the Lagrange points on the bar's minor axis. When the bar slows, these resonances move outwards through the Galaxy dragging bound stars along. Along their path they can pick up new stars. The corotation resonance is enhanced by this process, explaining the high contrast of the observed Hercules stream. My group has used this to measure for the first time the slowing of the bar's rotation. We have also resolved long-term issues with bar pattern speed measurements. This also promises new insights into the Milky Way's history. Most importantly, the bar's slowing can only be explained with angular momentum loss to the Milky Way's dark matter halo, thereby proving its inertial mass.


Kevin Schlaufman


Planet formation, stellar populations

Title: The Dynamical Evolution of Exoplanet Systems Over Billions of Years
Abstract: Exoplanet systems are expected to evolve with time as they age. In most cases though, the dynamical evolution of exoplanet systems over billion-year timescales are hard to observe. I'll describe how Galactic kinematics can provide accurate and precise age inferences for exoplanet populations and highlight the impact of those age inferences on models of planet formation and evolution. In particular, I'll show at the population level that (1) plausibly mean-motion resonant multiple-planet systems are among the youngest exoplanet systems and (2) systems with an ultra-short-period (USP) planet are among the oldest exoplanet systems. The former observation implies that convergent disk-driven Type I migration often leaves newly formed planetary systems in mean-motion resonances. The latter observation suggests that USP planets tidally migrate from their locations at the end of the planet formation process to their observed locations over several billion years.


Fall Break



Karen Masters

Haverford College

Galaxy observation, spiral structure, bars

Title: Observing Spiral Arms in Galaxies
Abstract: The iconic spiral arms that decorate the disks of massive galaxies (like our own Milky Way) have been studied observationally and theoretically since they were first recorded 180 years ago, nevertheless the exact details of their nature remains elusive. I will review what is known observationally about spiral arms, including results from the citizen science project Galaxy Zoo ( and the large galaxy survey “MaNGA” (Mapping Nearby Galaxies at Apache Point Observatory - part of the SDSS). I will discuss how these and other observational data are being used to constrain the variety of different physical models which have been proposed to explain spiral arms in galaxies.


Nikhil Padmanabhan


Inflation, dark energy, fuzzy dark matter

Title: Baryon Acoustic Oscillations with Galaxy Surveys: Present State and Some Future Prospects

Abstract: I will weave three separate threads. The first will be to describe recent and ongoing results from the Dark Energy Spectroscopic Survey. I will present the BAO results from the early DESI data, and some of the preparatory work for the Year 1 data. I will then present a new approach to reconstruction based on optimal transport, highlighting some recent results and ideas for future applications. I will end by discussing merging perturbative reconstruction ideas with convolutional neural networks, and a possible new application to primordial non-gaussianity.


Meredith MacGregor


Multi-wavelength observations of debris disks, planetary formation, habitability

Title: How to Form a Habitable Planet

Abstract: Planets form from disks of dust and gas surrounding young stars.  As they grow, these new planets inherit their chemical composition from the surrounding material and then sculpt it through gravitational interactions to form gaps and other asymmetric structures.  In the last decade, the Atacama Large Millimeter/submillimeter Array (ALMA) has revolutionized our ability to study planet formation, allowing us to examine this process in high resolution.  I will present highlights from ongoing work using ALMA and other facilities that explores how planetary systems form and evolve by (1) connecting disk structure to sculpting planets and (2) understanding the impact of stellar flares on planetary habitability.  Together these results provide an exciting foundation to investigate the evolution of planetary systems as a whole through multi-wavelength observations.  In the future, new facilities, specifically in the far-infrared, will help complete our understanding by tracing the chemistry of water and other volatiles critical for life.


Fabian Schmidt



Title: How much (robust) cosmological information can we obtain from galaxy clustering?

Abstract: All large-scale structure cosmologists are faced with the question: how do we robustly extract cosmological information, such as on dark energy, gravity, and inflation, from observed tracers such as galaxies whose astrophysics is extremely complex and incompletely understood? I will describe why guaranteeing this robustness is so difficult, and how a perturbative effective-field-theory (EFT) approach offers such a guarantee when focusing on galaxy clustering on large scales. The natural next question then is: how much cosmological information is left on these large scales if we marginalize over all the free parameters introduced in the EFT? To answer this question, I will introduce our implementation of the EFT on a lattice as an explicit field-level forward model, which can be used both for full Bayesian inference at the field level and for likelihood-free inference based on summary statistics. One crucial advantage of this forward model is the non-perturbative treatment of the displacement from initial positions to observed coordinates, with ramifications for BAO reconstruction and redshift-space distortions.


Masahiro Takada


Cosmology, LSS, weak lensing

Title: Subaru Hyper Suprime-Cam Year 3 Cosmology results: S8 tension?

Abstract: We used more than 25 million galaxies in the Subaru Hyper Suprime-Cam (HSC) shear catalog in the redshift range up to z~1.5 to measure weak lensing distortion effects due to large-scale structures. We used the measured weak lensing signals to perform a blinded cosmology analysis to measure the cosmological parameters of the flat LambdaCDM model. To obtain a robust constraint on the cosmological parameters, we employed an uninformative flat prior on a possible residual systematic error in the mean redshift for  HSC galaxies at z>1. As a result, we were able to measure the “S8” parameter at a 4% accuracy (sigma(S8)~0.04), but the central value exhibits about 2.5sigma tension with the Planck inferred S8 value. Our results indicate a non-zero residual error in the mean source redshift compared to the photometric redshift estimates for the HSC galaxies at z>1. In this talk, I will discuss the HSC cosmology results, and, if time is allowed, present the current status of the upcoming Subaru Prime Focus Spectrograph project, which promises significantly improvement of the HSC cosmology results. 


Janice Lee


ISM, star formation, PHANGS-JWST

Title: Star Formation in Nearby Galaxies: New Insights from 100,000 Star Clusters and Associations

Abstract: Star clusters are a key product of star formation. They trace the densest peaks of the star formation hierarchy, and as (effectively) single-age populations, they have great utility as “clocks” for timing various phases of the star formation cycle. There has been intense interest in characterizing their ensemble properties to gain insight into the physics of star formation, feedback, cluster disruption, survival, and evolution. I will present results from the largest census to-date of star clusters and compact associations from the PHANGS-HST Treasury Survey of 38 nearby spiral galaxies. Exploration of the observational properties of this large sample has revealed the potential of the UBVI color-color diagram as a new diagnostic reference tool for stellar, cluster, and galaxy evolution studies. I will also present initial results based on exquisite new imaging from the PHANGS-JWST Treasury Survey. The sensitivity and resolution of JWST’s infrared capabilities are finally allowing us to step beyond the Local Group, and observe dust embedded star formation on 10-100 parsec scales across environments characterized by physical conditions not found locally. Combined with PHANGS observations across the electromagnetic spectrum that capture all major stages of the star formation cycle, we can follow the progression of star formation – from molecular clouds to embedded and unembedded stellar populations – to provide new constraints on star formation efficiencies and timescales for theoretical models.


Michael Boylan-Kolchin

UT Austin

Galaxy formation

Title: Cosmology and Galaxy Formation at the Crossroads

Abstract: We find ourselves at a pivotal era in the study of cosmology and galaxy formation. The dark energy + cold dark matter (ΛCDM) paradigm is firmly established as the default cosmological model, owing in large part to its incredible success in explaining the large-scale distribution of matter and energy in Universe, yet the nature of both dark energy and dark matter remain deep mysteries. The successes of ΛCDM have also focused attention on its potential shortcomings, which may call for minor adjustments to the theory or presage more substantive revisions to our baseline model of the cosmos. I will discuss some of the areas of potential discord between observations and ΛCDM expectations, including JWST results on high-redshift galaxies and measurements of the expansion rate of the Universe. I will also explore how current and forthcoming facilities either will further solidify ΛCDM as the standard cosmological paradigm or signal a paradigm shift in our basic understanding of the Universe.



Spring 2023 Colloquium Series

The Spring 2023 Joint Astrophysics Colloquium will be held on Tuesdays at 11:00am in the Peyton Hall Auditorium and will be followed by the Bahcall Lunch at 12:30pm in Jadwin Hall.
Please find the spring speaker list below.

Spring Colloquium Poster
Date Name and Institution  Title and Abstract
January 31, 2023 Dave Stevenson
California Institute of Technology
Mixing and Unmixing in Planets

Planets are not layered in general. The strong evidence of layering on Earth arises because the main components (mantle and core) are immiscible. This is a thermodynamic property and nothing to do with gravity. However, immiscible phases can macrosegregate, allowing gravity to place the more dense component at the bottom. Diffusion of the higher molecular weight species is slow and frustrated by convection. However, central concentration of heavy elements can arise in giant planets (including ice giants) because of the way planets are assembled: The densest material is also the material that aggregates first, and hydrogen is accreted on top. In this case, the various constituent parts are miscible (unlike the case of mantle and core on Earth) but separated at birth. In sufficiently massive "SuperEarths", there should be no core and mantle because entropy wins. I will discuss our current understanding of the imperfect separation of core from mantle on Earth and the extent to which a compositional gradient in giant planets may be legacy of formation.
February 7, 2023 Martin Lemoine
Institut d'Astrophysique de Paris
Particle acceleration in astrophysical, magnetized turbulent plasmas

How magnetized turbulent plasmas can accelerate charged particles to high energies represents a long-standing question with far-reaching implications for high-energy and multi-messenger astrophysics. It indeed goes back to the seminal works of Enrico Fermi (1949, 1954) and nowadays, it is commonly invoked to model the generation of non-thermal particle spectra in a broad variety of astrophysical sites, including extreme, relativistic sources. In particular, it has recently been considered as a possible origin for the high-energy neutrinos seen by Ice Cube in the direction of nearby active galactic nuclei. 
Our understanding of particle acceleration in turbulent plasmas has known substantial progress in recent years, mostly spurred by large-scale, kinetic numerical simulations. This talk will address those developments and discuss a theoretical picture to describe the physics at play, based on non-resonant interactions between particles and velocity structures. This model, which can be seen as a modern implementation of the original Fermi scenario, appears supported by recent numerical simulations of turbulence in the semi- and fully-relativistic regime. It also brings to light an interesting connection between the properties of intermittency of the turbulence and the spectrum of accelerated particles. I will discuss those features then conclude with some possible applications and extensions.

February 14, 2023 William H. Matthaeus
University of Delaware
Turbulence in collisionless space plasma: the Big Picture

Turbulence is a near ubiquitous feature of fluids and plasmas in geospace, the heliosphere and in astrophysics and well as in laboratory plasma and in the terrestrial environment. Turbulence links large scale energy reservoirs with small scale dissipation processes. Consequently, it is responsible for many features of space plasmas including heating, charged particle scattering and energy transport across wide ranges of spatial scale and location. This account of the Big Picture of space turbulence starts with the likely energy sources that powers the heating and acceleration of the solar wind. What follows is a progression of cross scale couplings that are described in adaptations of classical turbulence theory. At energy containing scales, von Karman similarity provides a top-down description of energy decay. The von Karman picture is demonstrated using solar wind spacecraft data near earth. At smaller scales a several decade inertial range is present the solar wind. Energy transfer through this cross scale conduit is described using three methods: phenomenology, Yaglom-Kolmogorov third order law, and scale filtering. Transfer through the inertial range generates small scale structures, or intermittency, in which kinetic processes are concentrated. Finally, the kinetic dissipation itself, defined as conversion of fluid scale energy into internal energy, is accomplished though the pressure-work and the pressure strain interaction. These are quantified in some detail due to recent advances in both kinetic plasma simulation, and capable multispacecraft measurements. The cross scale couplings discussed here span scales from the large energy reservoirs to kinetic scale dissipation and are illustrated using spacecraft missions including STEREO, ACE, Helios, Voyager, Parker Solar Probe, Cluster and Magnetosphere Multiscale. The presentation concludes with outstanding questions will be addressed by new missions such as Helioswarm and PUNCH.
February 21, 2023 Christoph Pfrommer
Leibniz-Institute for Astrophysics Potsdam
Cosmic ray feedback and magnetic dynamos in galaxy formation

Understanding the physics of galaxy formation is an outstanding problem in modern astrophysics. Recent cosmological simulations have demonstrated that feedback by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies and to slow down star formation to the small observed rates. However the particular physical processes underlying these feedback processes still remain elusive. In particular, many of these simulations neglected magnetic fields and relativistic particle populations (so-called cosmic rays). Those are known to provide a pressure support comparable to the thermal gas in our Galaxy and couple dynamically and thermally to the gas, which seriously questions their neglect. After introducing the underlying physical concepts, I will present our recent efforts to model cosmic ray physics and magnetic fields in galaxy formation. In particular, I will explain how cosmic rays interact with and propagate through the magnetized plasma in the interstellar and circumgalactic media and how we can observationally test these theoretical considerations using new high-sensitivity MeerKAT observations. I will then demonstrate that cosmic rays play a decisive role in the formation and evolution of spiral galaxies by providing feedback that regulates star formation and drives gas out in galactic winds. Comparing cosmic ray spectra of electrons and protons to observational data and studying the correlation of the far-infrared emission with the gamma-ray and radio emission from galaxies enables us to test the cosmic ray feedback and dynamo models for the growth of galactic magnetic fields. This argues that a complete understanding of galaxy formation necessarily includes these non-thermal components.
February 28, 2023 Kiyoshi Masui
Massachusetts Institute of Technology
A Synoptic View of Fast Radio Bursts with CHIME

For more than a decade, enigmatic extragalactic flashes called fast radio bursts (FRBs) have defied a definitive explanation for their origin. In addition, the unique properties of FRBs make them promising probes of both cosmology and the distribution of gas on intergalactic scales. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is the only radio telescope capable of instantaneously observing hundreds of square degrees with the sensitivity of a 100-meter scale aperture. As a result, its transient search instrument, CHIME/FRB, has detected thousands of FRBs, increasing the known sample by an order of magnitude. I will give an overview of CHIME/FRB's most recent results, where observations of particular sources and statistical analyses of the FRB population are starting to reveal the nature of this mysterious phenomenon. I will then describe an effort to augment CHIME/FRB's capabilities by adding Outrigger telescopes, which will be located across North America and will precisely localize FRB sources using very long baseline interferometry. The resulting large sample of localized FRBs will allow for detailed measurements of the large-scale distribution of baryons in the universe, providing precise constraints on feedback processes in galaxy evolution.
March 7, 2023 Gwen Rudie
Carnegie Observatories
March 14, 2023 Break  
March 21, 2023 Colin Hill
Columbia University

Searching for Cosmological Concordance with New Physics in the Dark
Sector: Hints and Challenges

I will discuss recent and ongoing work focused on attempts to restore concordance amongst cosmological data sets, motivated by discrepancies amongst some inferences of the cosmic expansion rate (H_0) and the matter clustering amplitude (S_8).  I will explain why the most viable models to resolve the H_0 problem invoke new physics at or prior to the last scattering epoch.  Such models include modified recombination scenarios, quasi-accelerating early dark energy (EDE) models (and extensions thereof, featuring EDE-dark matter interactions), or scenarios featuring new light particles with non-trivial interactions. 

I will present constraints on such scenarios derived using data from the Atacama Cosmology Telescope (ACT), the Planck satellite, and large-scale structure surveys.  I will highlight newly obtained constraints on EDE models derived from Lyman-alpha forest data, which severely hinder the ability of this scenario to resolve the H_0 problem.  I will conclude with a look ahead to forthcoming CMB analyses from ACT, which will provide a powerful test of these scenarios in the low-noise, high-resolution regime.

March 28, 2023 Heather Knutson
California Institute of Technology
Sub-Neptune, Super-Earth, or Water World? Exploring the Properties of Small Extrasolar Planets

We currently know of more than 10,000 planets and planet candidates orbiting nearby stars. This population is dominated by planets with masses between 1-10 times that of the Earth, which can be divided into two broad classes (‘sub-Neptunes’ and ‘super-Earths’) depending on whether or not they host a puffy hydrogen-rich envelope. Most of these planets orbit very close to their host stars, and we expect that their atmospheres should be strongly sculpted by mass loss processes. In the first part of my talk I will present new results from ongoing Keck and Palomar surveys utilizing metastable helium to measure present-day mass loss rates for close-in exoplanets. During the second part of my talk I will focus on M dwarf planetary systems, which represent our best opportunity to study rocky exoplanets with high mean molecular weight atmospheres. Studies have suggested that a subset of M dwarf planets may also possess thick water-rich envelopes, but definitive evidence for the existence of ‘water worlds’ has remained elusive. I will discuss our ongoing efforts to constrain the bulk and atmospheric compositions of M dwarf planets using transit timing variations, and evaluate the prospects for directly measuring their surface and atmospheric properties with the James Webb Space Telescope.
April 4, 2023 Nadine Neumayer
Max Planck Institute for Astronomy
The build-up of galactic nuclei: how do black holes get there?

We currently know of more than 10,000 planets and planet candidates orbiting nearby stars. This population is dominated by planets with masses between 1-10 times that of the Earth, which can be divided into two broad classes (‘sub-Neptunes’ and ‘super-Earths’) depending on whether or not they host a puffy hydrogen-rich envelope. Most of these planets orbit very close to their host stars, and we expect that their atmospheres should be strongly sculpted by mass loss processes. In the first part of my talk I will present new results from ongoing Keck and Palomar surveys utilizing metastable helium to measure present-day mass loss rates for close-in exoplanets. During the second part of my talk I will focus on M dwarf planetary systems, which represent our best opportunity to study rocky exoplanets with high mean molecular weight atmospheres. Studies have suggested that a subset of M dwarf planets may also possess thick water-rich envelopes, but definitive evidence for the existence of ‘water worlds’ has remained elusive. I will discuss our ongoing efforts to constrain the bulk and atmospheric compositions of M dwarf planets using transit timing variations, and evaluate the prospects for directly measuring their surface and atmospheric properties with the James Webb Space Telescope.
April 11, 2023 Alexander Ji
University of Chicago
Dwarf Galaxy Archaeology in the Gaia Era

The Milky Way's satellite dwarf galaxies are powerful probes of many important astrophysical processes. Their kinematics provide insights into galactic dynamics and the nature of dark matter, while their chemical compositions preserve a history of early galaxy formation and nucleosynthesis. Recently, all-sky astrometry from the Gaia satellite has enabled the discovery and characterization of dwarf galaxies at different stages of tidal disruption. These new galaxies lie at the extremes of dwarf galaxy parameter space, whether in their structural properties or their star formation histories. In this talk, I will show how spectroscopic observations of the kinematics and chemistry of these newly discovered dwarf galaxies can provide crucial insights into galactic dynamics and nucleosynthesis. I will present a recent kinematic study of the extremely diffuse dwarf galaxy Antlia II that shows it is in the early stages of tidal disruption. The kinematics of Antlia II also clearly show that the Milky Way reference frame has been perturbed by the Large Magellanic Cloud. I will also highlight how two completely disrupted dwarf galaxies contribute to increasingly clear evidence that the rapid neutron-capture process requires both prompt and delayed sources to explain the diverse chemical evolution histories of dwarf galaxies.
April 18, 2023 Avishay Gal-Yam
Weizmann Institute of Science
April 25, 2023 Nia Imara
University of California Santa Cruz


IAS/PU Colloquia Fall 2022

The Fall 2022 Joint Astrophysics Colloquium will be held on Tuesdays at 11AM and will be followed by the Bahcall Lunch at 12:30.  
Please find the speaker schedule below. Videos on IAS website:

Date Name Title
September 6, 2022 Mark Devlin
University of Pennsylvania
Large-Scale Millimeter Wave Surveys

Millimeter-wave surveys of the sky have the potential for yielding a wealth of information about our universe from the first instants of its existence to our own solar system. I will describe how modern measurements of the cosmic microwave background (CMB) are being used to measure how our universe has evolved over cosmic time leading to a better understand the fundamental parameters governing its nature. I will present recent results from the Atacama Cosmology Telescope (ACT) and future plans for the Simons Observatory program.
September 13, 2022 Robin Canup
SWRI, Boulder
Origin of the Moon: New Results and Open Issues

A primary scientific outcome of the Apollo program was the giant impact theory for lunar origin, in which a collision at the end of Earth’s main accretionary phase creates a disk from which the Moon forms.  In the past decade, the nature of a Moon-forming impact has become highly debated, driven by increasingly precise sample analyses that show that the Earth and Moon have essentially identical isotopic compositions across all non-volatile elements. Giant impacts usually produce disks that originate primarily from the impactor (“Theia”) rather than from the target protoearth.  Meteorites that originate from Mars, and nearly all those from parent bodies in the asteroid belt, have quite different isotopic compositions than the Earth.  If Theia had been similarly non-Earth like, and the pre-lunar disk originated primarily from Theia, one would then most naturally expect measurable differences between the Earth and Moon.  Instead, they are indistinguishable across most elements.  Many new scenarios have been proposed to resolve this fundamental dilemma, involving different Theia compositions, protolunar disk evolutions, and/or early Earth-Moon dynamical histories.  I will discuss several of these, as well as key remaining uncertainties and prospects for relevant new constraints from future lunar exploration. 
September 20, 2022 Deirdre Shoemaker
University of Texas at Austin
Brave New World of Numerical Relativity

After decades of preparation, the era of gravitational wave astronomy has begun. The gravitational wave detectors, LIGO and Virgo, have published a catalog of 90 events of coalescing compact objects including black holes and neutron stars. I will present the role that numerical relativity played in the unveiling of the gravitational wave sky and anticipate how it might improve our understanding of gravity as the detectors improve.
September 27, 2022 Kaitlin Kratter
University of Arizona
A Continuous Theory of Stellar System Formation

A satisfying theory for star or planet formation should not consider these processes in isolation. With the power of recent observational surveys, we are well positioned to test detailed theoretical models of multiple-star formation and orbital evolution across diverse galactic environments. Simultaneously we can begin to couple our understanding of the earliest phases of star formation with the onset of planetary system formation. I will review current theoretical and observational progress on multiple star formation, and highlight areas of continued uncertainty. 
October 4, 2022 Alex Schekochihin
University of Oxford
October 11, 2022 Frank van den Bosch
Yale University

New Insight into Cosmology and the Galaxy-Halo Connection from Non-Linear Scales

In our LCDM paradigm, galaxies form and reside in dark matter halos. Establishing the (statistical) relation between galaxies and dark matter halos, the `Galaxy-Halo connection', therefore gives important insight into galaxy formation, and also is a gateway to using the distribution of galaxies to constrain cosmological parameters. After a brief introduction to how clustering and gravitational lensing can be used to constrain the galaxy-halo connection, I show that several independent analyses all point towards a significant tension in cosmological parameters compared to the recent CMB results from the Planck satellite. I discuss the potential impact of assembly bias, and present satellite kinematics as a complementary and competitive method to constrain the galaxy-halo connection. After a brief historical overview of the use of satellite kinematics, I present a novel analysis, and show how it improves our knowledge of the galaxy-halo connection. I end with a re-examination of the cosmological tension, this time using satellite kinematics rather than gravitational lensing.

October 18, 2022 Fall Break- No Colloquium  
October 25, 2022 Jo Bovy
University of Toronto
November 1, 2022 Ben Shappee
IfA, Hawaii
The All-Sky Automated Survey for Supernovae (ASAS-SN)

For the first time, the entire visible sky is being surveyed for the violent, variable, and transient events that shape our universe by the All-Sky Automated Survey for Supernovae (ASAS-SN). Combined, ASAS-SN, Asteroid Terrestrial-impact Last Alert System (ATLAS), The Zwicky Transient Facility (ZTF), the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), Gaia, and the Transiting Exoplanet Survey Satellite (TESS) now monitor the whole sky, at high cadence with a combined total of 32 telescopes distributed at 9 sites around and above the world. Each survey has a different cadence and depth. I will briefly review major ongoing transient surveys, contrasting their capabilities and goals. I will then use a handful of recent discoveries to highlight opportunities that these new capabilities present. I will focus on multi-messenger astronomy (with LIGO and IceCube); high-cadence, high-precision observations (with Kelper, TESS, and POISE); and the remarkable repeating partial tidal disruption event ASASSN-14ko. I will reflect on the challenges the field will face both now and in the LSST-era. Finally, I will end by describing new and future ways we are making ASAS-SN an even more useful tool for the entire community and introduce the Spectral Classification of Astronomical Transients (SCAT) survey on the UH 2.2m telescope, a new survey that will rapidly trigger on, observe, and classify publicly announced transients fully-automatically without the need for human intervention.
November 8, 2022 Dimitrios Psaltis
University of Arizona
November 15, 2022 Samaya Nissanke
University of Amsterdam
First detection of an isolated black hole, and mass measurement of isolated white dwarfs through astrometric microlensing

We recently detected an isolated stellar-mass black hole (BH) unambiguously for the first time, and measured its mass, distance and velocity. We used HST to carry out precise astrometry of the source star of the long-duration, high-magnification microlensing event OGLE-11-462. Combining HST astrometry, OGLE photometry, and spectroscopic measurements, we obtain a lens mass of ~7 solar mass and a distance of ~1.6 kpc. The lens emits no detectable light, which confirms its BH nature. Its proper motion is offset from the mean motion of Galactic-disk stars at similar distances by ~45 km/s, suggesting that the BH received a natal kick from its supernova explosion.

In a reprise of the famous 1919 solar-eclipse experiment that confirmed Einstein's general relativity, the nearby white dwarf Stein 2051b passed very close to a 19th magnitude background star. As it passed in front, Stein 2051B caused a deflection of the background star's image by ~2 milliarcsec, which we observed with HST. This allowed us to determine the mass of Stein 2051 B using this technique of astrometric microlensing for the first time outside the solar system. We have measured masses of 2 WDs through this technique, and our measurements lend support to the white-dwarf evolutionary theory.

November 22, 2022 Dominika Wylezalek
University of Heidelberg
Multiphase and multi-scale AGN feedback processes

AGN feedback is now widely considered to be one of the main drivers in regulating the growth of massive galaxies. In my talk I will describe several efforts in our group to understand the power, reach and impact of AGN feedback processes. We find significant evidence for AGN feedback signatures even in low-luminosity AGN and we are now using molecular gas as a tracer to investigate if and how feedback may impact and quench galaxies at low redshift. At higher redshift, it appears that AGN-driven outflows can indeed suppress star formation in their hosts, consistent with the AGN having a negative impact on galaxy evolution. However, both star formation and quasar activity peak at z ~ 2-3 where AGN are expected to impact the build-up of stellar mass the most and I will present recent efforts in our group to characterise feedback processes in powerful AGN on CGM scales at and near Cosmic Noon. In particular, our team recently discovered a unique population of luminous high-z quasars (ERQs) with extreme outflow properties. At the same time, more and more exotic AGN populations with extreme signatures are being discovered at that redshift. These populations are ideal to obtain a census of the overall mass and energy budget of both outflow and infall/feeding from the CGM, an essential requirement to probe the detailed and full feedback loop. Finally, I will also introduce the JWST ERS Program Q3D which studies the impact of three carefully selected luminous quasars on their hosts. Our program will serve as a pathfinder for JWST science investigations in IFU mode. Depending on JWST's science schedule, I may show some of the very first JWST science results.
November 29, 2022 Lisa Kewley
CfA, Harvard & Smithsonian



Joint IAS Princeton University Astrophysics Colloquium - (not open to the public)

Spring 2024 Colloquium Speakers (not open to the public)


Spring 2022 Colloquium Series

The Spring 2022 Joint Astrophysics Colloquium will be held on Tuesdays  at 11:00am and will be followed by the Bahcall Lunch at 12:30.
Please find the spring speaker list and links to videos below.

Date Name Title
January 25, 2022

Lía Corrales (Virtual)
University of Michigan

New Frontiers of Short Wavelength Exploration: From Astromineralogy to Exoplanet

View presentation here.

Uncovering the physics of galaxy evolution has been a longstanding problem for astronomers.  Physical galaxy properties like the star formation rate, stellar mass, and metallicity can tell us not only how these properties change over generations of galaxies, but they also give us a window into the conditions of the universe at the time of galaxy formation.  Galaxy surveys, which are predominantly used to measure these properties, only observe the brightest galaxies, which are a biased sample.  Line intensity mapping (LIM) observes the aggregate emission from brighter and fainter galaxies over much larger volumes.  The EXCLAIM survey is a pathfinder for this technique, and has the potential to aid in constructing a full census of galaxy emission while minimizing sample variance.  In this talk I will present how we can use LIM surveys to construct a galaxy property census.  Specifically, I will discuss currently planned methods using LIM to uncover galaxy properties like molecular hydrogen density, star formation, and metallicity properties, as well as their limitations.  I will also discuss potential ways we could get around these hurdles using hydrodynamic simulations along with semi-analytic star formation models.  Finally we will discuss how these methods could be applied to EXCLAIM and other upcoming LIM surveys.

February 1, 2022 Anthony Pullen (Virtual)

A Galaxy Property Census with Line Intensity Mapping

View presentation here.

Uncovering the physics of galaxy evolution has been a longstanding problem for astronomers.  Physical galaxy properties like the star formation rate, stellar mass, and metallicity can tell us not only how these properties change over generations of galaxies, but they also give us a window into the conditions of the universe at the time of galaxy formation.  Galaxy surveys, which are predominantly used to measure these properties, only observe the brightest galaxies, which are a biased sample.  Line intensity mapping (LIM) observes the aggregate emission from brighter and fainter galaxies over much larger volumes.  The EXCLAIM survey is a pathfinder for this technique, and has the potential to aid in constructing a full census of galaxy emission while minimizing sample variance.  In this talk I will present how we can use LIM surveys to construct a galaxy property census.  Specifically, I will discuss currently planned methods using LIM to uncover galaxy properties like molecular hydrogen density, star formation, and metallicity properties, as well as their limitations.  I will also discuss potential ways we could get around these hurdles using hydrodynamic simulations along with semi-analytic star formation models.  Finally we will discuss how these methods could be applied to EXCLAIM and other upcoming LIM surveys.

February 8, 2022 Stephon Alexander

The Chiral Universe

View presentation here.

Among a handful of mysteries in the LambdaCDM paradigm of cosmology, I focus on three: Dark Matter, Baryogenesis and the origin of structure. I then provide a pedagogical introduction to Chiral Gravity and show how these three mysteries may be interconnected. I also discuss some observational windows including the inevitability of Superfluid Dark matter.

February 15, 2022 Shirley Ho
Flatiron Institute

Learning Symbolic Equations with Deep Learning

View presentation here.

We develop a general approach to "interpret" what a network has learned by introducing strong inductive biases. In particular, we focus on Graph Neural Networks. The technique works as follows: we first encourage sparse latent representations when we train a GNN in a supervised setting, then we apply symbolic regression to components of the learned model to extract explicit physical relations. The symbolic expressions extracted from the GNN using our technique also generalized to out-of-distribution data better than the GNN itself. Our approach offers alternative directions for interpreting neural networks and discovering novel physical principles from the representations they learn. In particular, we will show examples of recovery of newton's law and masses of solar system bodies with real ephemeris data and recovery of navier-stokes equations with turbulence dataset. We will speculate what one can do with this new tool.

February 22, 2022 Elisabete M. de Gouveia Dal Pino
University of São Paulo
Cosmic Ray Acceleration by Magnetic Reconnection: Probing Extreme Energies and the Origin of Gamma-Ray and Neutrino Emission from Black Holes and Relativistic Jets of Active Galaxies

View presentation here.

Relativistic Jets emanating from Black Holes (BH) of active galaxies are believed to be among the most extreme particle accelerators and very high energy (VHE) emitters in the universe. Only lately, combining theory, numerical simulations and observations, we have started to understand the potential physical processes that prevail in the surrounds of these BHs in order to explain major puzzles like the origin of the VHE gamma-ray flares and ultra-high-energy cosmic rays (UHECRs). In those inner regions, magnetic fields are dynamically dominating and particle acceleration driven by magnetic reconnection is expected. In this talk, I will discuss particle acceleration by magnetic reconnection in turbulent flows and present recent results that combine three-dimensional global magnetohydrodynamic relativistic simulations with the injection of test particles, showing how particles can be stochastically accelerated in the relativistic jets and the accretion flows around BHs by magnetic reconnection up to ultra-high energies. Finally, I will show that these accelerated particles could explain the gamma-ray flares and associated neutrino emission observed in these sources.
March 1, 2022 Suvrath Mahadevan
Penn State
Pathways to Earth-Mass Planets with Precision Spectroscopy

Modern astronomical spectrometers are approaching the exquisite sensitivity to detect the signature of an Earth-mass planet around stars like the Sun. I shall discuss the challenges involved in making these difficult measurements with the Doppler radial velocity technique, and the evolution of the design of these instruments as they seek ever-tighter control of environmental parameters, and increased measurement precision. A suite of new technologies like frequency stabilized laser combs, low drift etalons, and deeper understanding of the detectors is enabling a new level of precision in radial velocity measurements - as well as illustrating new challenges. I will use two such instruments we have built to illustrate some of the underlying physics and measurement challenges. I will then discuss how the stars themselves are the remaining challenge, as magnetically driven processes create ‘stellar activity’ noise that can masquerade as planets and obfuscate their detection. I shall highlight a few paths we are exploring to mitigate this – using our star, the Sun, as a guide, and discuss whether the goal of discovering and characterizing terrestrial mass planets capable of hosting liquid water on their surfaces is now within reach.
March 15, 2022 James Lowenthal
Smith College

Satellite Swarms vs. Astronomy and the Night Sky

View presentation here.

New technology and a lack of regulation are allowing massive swarms, or "constellations", of low-Earth communications satellites such as SpaceX's Starlinks to be launched at relatively low cost, leading to a dramatic rise in the number of satellites at altitudes 300-1200 km already in orbit: over 2000 new satellites in the last 2 years, with more than 100,000 planned by 2030. The large number and the apparent brightness (by reflected sunlight) of these satellites pose serious and possibly catastrophic challenges to ground-based and even space-based astronomy and the appearance of the starry night sky even to casual observers. Radio interference due to intended and unintended emission from LEO satellite swarms further threatens to overwhelm sensitive radio telescopes including those investigating the CMB. There are national and international efforts underway to understand and try to control and respond to these major new challenges; meanwhile, rockets are launching every few days, each with 60+ more satellites, observatories and skywatchers are reporting increasing interference from satellite streaks, and the view of the night sky has already been changed.

March 22, 2022 Eduardo Bañados

The birth of the first massive galaxies and black holes

We are the first generation of human beings able to directly observe and study the cosmic era when the first galaxies and black holes formed. Quasars are among the most luminous sources known and can be studied in detail even during the first billion years of the Universe (at redshifts z>6). I will summarize my team's efforts to search for and characterize the most distant quasars. This has led to the discovery of the largest number of bright quasars at z>6, including the most distant radio-source known at z~7, and the three most distant quasars known at z>7.5. These distant quasars provide important clues about the build-up of the first massive galaxies and black holes, as well as the epoch of reionization. I will review the diverse range of physical properties of these quasars on different scales, including follow-up studies from X-rays to radio wavelengths.

March 29, 2022 Christopher Reynolds
IoA Cambridge
Searching for axion-like particles with X-ray observations of galaxy clusters.

View presentation here.

There has been a surge of interest within the particle-physics and dark matter communities in axions and axion-like particles (ALPs). These particles result from a possible extensions to the Standard Model that may solve the Strong-CP problem, and are further motivated by String Theory. Galaxy clusters are a superb laboratories for exploring ALP physics. Clusters are permeated by a magnetized hot intracluster medium (ICM) that can efficiently interconvert photons and ALPs. Here, I discuss how the transparency (or lack thereof) of the ICM to X-rays becomes a powerful way to constrain ALPs. I show that a careful examination of X-ray observations of cluster-hosted active galactic nuclei (AGN) by the Chandra X-ray Observatory allow the tightest constraints to date on ALP/photon coupling of ALPs in the low-mass regime (ALP masses less than ~peV). I finish by discussing the future prospects of these studies with the next generation X-ray observatories.
April 5, 2022 Sabrina Stierwalt
Occidental College
The Baryon Cycle in Dwarf-Dwarf Mergers: Fueling Hierarchical Assembly

Both cosmological simulations and observations of the ultraviolet luminosity function suggest dwarf galaxies are the dominant population at high redshifts and that the galaxy merger rate per unit volume is dominated by low mass galaxies. However, dwarf-dwarf interactions have not yet been subject to systematic study, even in the nearby universe. I will report on our efforts to do just that: TiNy Titans is the first systematic study of a sample of isolated interacting dwarf galaxies and the mechanisms governing their star formation. How much of what we know about massive galaxy mergers - triggering of starbursts, significant rearranging of gas & dust, AGN growth - occur in the shallower gravitational potential wells of dwarf galaxies? Do we observe these low mass mergers at the rate predicted by cosmological simulations? Do groups of only dwarf galaxies exist? I will also highlight our most recent results investigating the age and mass distributions of the star cluster populations in dwarf mergers as observed with high resolution Hubble imaging.
April 12, 2022 Suvi Gezari
The Wild West of Nuclear Transients

Central massive black holes will reveal themselves in a galaxy when they flicker and flare as they feed on gas and stars. We are conducting a systematic study of nuclear transients in the Zwicky Transient Facility alert stream, and have assembled the largest ever sample of tidal disruption events (TDEs), as well as revealed new extreme populations of flaring and "changing look" AGN. TDEs provide a rare glimpse of dormant massive black holes lurking in the centers of galaxies, and their luminous outbursts of radiation are valuable probes of accretion physics, jet formation, and the circumnuclear environment and stellar population. The growing census of TDE discoveries, with hundreds more on the horizon with the start of the Vera C. Rubin Observatory Legacy Survey of Space and Time, are enabling us to do population studies of TDEs for the first time. I will present exciting new developments in our understanding of the physical conditions driving the light curves, broadband spectral energy distributions, and spectroscopic sub-classes in TDEs, and how they relate to the properties of their host galaxies and the masses of their central supermassive black holes.
April 19, 2022 Hilke Schlichting
April 26, 2022 Andrea Ghez
From a Possibility to a Certainty of a Supermassive Black Hole

Learn about new developments in the study of supermassive black holes. Through the capture and analysis of twenty years of high-resolution imaging, the UCLA Galactic Center Group has moved the case for a supermassive black hole at the center of our galaxy from a possibility to a certainty and provided the best evidence to date for the existence of these truly exotic objects. This was made possible with the first measurements of stellar orbits around a galactic nucleus. Further advances in state-of-the-art of high-resolution imaging technology on the world’s largest telescopes have greatly expanded the power of using stellar orbits to study black holes. Recent observations have revealed an environment around the black hole that is quite unexpected (young stars where there should be none; a lack of old stars where there should be many; and a puzzling new class of objects). Continued measurements of the motions of stars have solved many of the puzzles posed by these perplexing populations of stars. This work is providing insight into how black holes grow and the role that they play in regulating the growth of their host galaxies. Measurements this past year of stellar orbits at the Galactic Center have provided new insight on how gravity works near a supermassive hole, a new and unexplored regime for this fundamental force of nature.


Fall 2021 Colloquium Series

The Fall 2021 Joint Astrophysics Colloquium will be held on Tuesdays at 11AM and will be followed by the Bahcall Lunch at 12:30.  
Please find the Fall speaker schedule at this link and below. Videos on IAS website:

    Date                       Name    Title (link to videos of talks is here.)                                                                                                                                           
Sep  7  11:00am Romain Teyssier Cosmic magnetism from a computational perspective
Sep 14  11:00am Alice Quillen Soft Astronomy
Sep 21  11:00am Amina Helmi New views on the Milky Way
Sep 28  11:00am Uros Seljuk All you need is a Normalizing Flow
Oct  5 11:00am Brett McGuire The PAH Revolution: Cold, Dark Carbon at the Earliest Stages of Star Formation
Oct 12  11:00am Jennifer Lotz Gemini Observatory in the 2020's
Oct 26  11:00am Ellen Zweibel TBA
Nov  2  11:00am Xuening Bai TBA
Nov  9  11:00am Kenta Kiuchi TBA
Nov 16  11:00am Sera Markoff TBA
Nov 23  11:00am Alexandra Amon TBA
Nov 30  11:00am Bethany Ehlmann TBA

2021 Summer Colloquium Series

The 2021 Summer Colloquium Series led by Professor Eliot Quataert begins virtually on
Wed. Jul 21, at 12:00PM, and continues on Tuesdays 7/27 through 8/24 at 12:30PM.
The list of speakers, talk titles and select colloquium recording links appear below.  Zoom link will be sent by email.

Date   Name    Title (Video Link to Talk)
July 21, 2021
Alice Pisani
Unraveling The Universe With Cosmic Voids
Modern surveys provide access to high-quality measurements on large areas of the sky, sampling the galaxy distribution in detail also in the emptiest regions, voids. Void cosmology is becoming an increasingly active sector of galaxy clustering analysis: by measuring void properties, such as density profiles or void number counts, it is possible to constrain cosmological parameters. Cosmic voids are particularly sensitive to the properties of dark energy and neutrinos, and are a powerful tool to test modifications of the laws of general relativity. Studying voids provides a novel perspective to unravel the unsolved mysteries of our Universe.
In this talk I introduce cosmic voids as a tool for cosmology, I present recent results—with a particular focus on the advantages of calibration-free approaches—and I discuss future developments in the field.
July 27, 2021
Kirk Barrow Stanford High-Cadence Synthetic Observations and Neural Networks in the Era of JWST
As astronomers near the commissioning of the extremely large telescopes, the Rubin Observatory, as well as new space-based observatories like the Roman Space Telescope and JWST to peer more deeply into our Universe, our community is challenged to develop a theoretical and modeling framework to characterize and study what will be humanity's greatest astronomical discoveries. My research addresses this need by generating detailed, state-of-the-art synthetic observations from hydrodynamic cosmological simulations. By calculating all the processes that photons undergo as they travel across the Universe from the surface of a distant star to a telescope’s detector, my collaborators and I have been able to disentangle perplexing trends in observed galactic spectra as well as make predictions for what we might unveil in the near future. Topics we have investigated in prior work include massive black hole formation, the first stars and galaxies, and the intricate interplay between nebular emission lines and the escape fraction of ionizing radiation. Looking forward, I propose to create the largest and most detailed database of synthetic observational tools and predictions at a time that will come to define astronomy for generations.
Aug. 3, 2021
Diana Powell
Microphysical Insights into Protoplanetary Disks and Exoplanet Atmospheres
A fundamental understanding of planetary histories and characteristics requires an empirical connection between planet formation and evolved planets—a long-sought goal of astrophysics. This connection is now increasingly possible due to simultaneous revolutions in the observations of protoplanetary disks and exoplanet atmospheres. A key step towards relating these observations of different evolutionary stages is to characterize the composition of material in protoplanetary disks and relate these properties to the atmospheric composition of planets. In this talk, I will discuss initial steps that I have taken towards this goal. I will provide evidence that protoplanetary disks are more than an order of magnitude more massive than previously appreciated, that the detailed properties of clouds shape observations of substellar atmospheres, and that the physics of modeling clouds gives a new understanding of the compositional distribution in protoplanetary disks. I will conclude by briefly discussing avenues for relating planetary properties to the mass inventory in protoplanetary disks in order to develop the observationally-validated framework required to compositionally relate evolved planets to planet formation.
Aug. 10, 2021
Joey Rodriguez
Michigan State
Understanding Planetary Evolution with Eclipsing Disks & Transiting Planets
The success of ground-based transit and RV surveys, and the Kepler/K2 and TESS missions, have shifted the exoplanet field from pure discovery to a combination of discovery, demographic analysis, and detailed characterization, especially for exoplanet atmospheres. We are using data from the TESS, Kepler/K2, and ground-based transit surveys to find keystone planetary systems around bright stars (those well suited for atmospheric observations) that can help address specific questions about planet formation and evolution. We are also studying the birthplaces of planets by searching for occultations of newly formed stars by their protoplanetary disks. These systems provide insight into the conditions required for planet formation. I will describe our recent results from both projects and discuss how we will study these types of objects in future surveys.
Aug. 24, 2021
Vladimir Zhdankin
First-Principles Modeling of Kinetic Turbulence in High-Energy Astrophysical Plasmas
Kinetic plasma physics is an integral, but often neglected, component of many high-energy astrophysical systems. Understanding the dynamics and energization processes in collisionless plasmas is essential for interpreting the observable radiation spectra, luminosities, and variability of systems such as pulsar wind nebulae, accretion flows onto compact objects, and relativistic jets from active galactic nuclei. Kinetic turbulence has long been proposed as a primary process for energizing these collisionless plasmas, but questions remain as to its viability for producing nonthermal particle populations in various physical regimes. Particle-in-cell (PIC) simulations have recently opened this rich topic to detailed, first-principles numerical and theoretical scrutiny. I will overview recent progress on understanding particle energization in relativistic (and trans-relativistic) plasma turbulence, spurred by PIC simulations, including the demonstration of phenomena such as nonthermal particle acceleration, electron-ion thermal decoupling, and intermittent beaming of radiation. These new insights have applications for modeling various high-energy astrophysical systems.

2021 Spring Colloquium Series

The Spring 2021 Joint Astrophysics Colloquium was held virtually on Tuesdays at 11AM and was followed by the virtual Bahcall Lunch at 12:30.

Date   Name    Title (Video Link to Talk) (pvt)
Feb. 2, 2021 Eliot Quataert
The Impact of Cosmic-Rays on Galaxy Formation
Relativistic cosmic-rays created by stellar and black hole feedback may play an important role in many aspects of structure formation.  Cosmic-rays can drive outflows from star forming galaxies and heat diffuse gas in the circumgalactic and intergalactic medium.  In this talk I will describe some of the possible impact(s) of cosmic-rays on galaxy formation, observational probes of their impact, and the
theortical uncertainities in our understanding of the role of cosmic rays in structure formation.                                   
Feb. 9, 2021 Feryal Özel
University of Arizona
Black Hole Physics at the Horizon Scale
Recent observational advances with the Event Horizon Telescope, GRAVITY, and LIGO/VIRGO have opened up new avenues for studying black hole physics at horizon scales. In this talk, I will discuss what we have learned about the spacetimes of astrophysical black holes and how strong-field gravity is imprinted on their images. I will also present how the observations help us model and understand the heating and acceleration of plasmas on horizon scales.
Feb. 16, 2021 Sherry Suyu
Max Planck Institute for Astrophysics
Cosmology with Gravitational Lens Time Delays
Strong gravitational lenses with measured time delays between the multiple images can be used to determine the Hubble constant (H0) that sets the expansion rate of the Universe. An independent determination of H0 is important to ascertain the possible need of new physics beyond the standard cosmological model, given the tension in current H0 measurements. I will describe techniques for measuring H0 from lensing with a realistic account of systematic uncertainties, and present the latest results from a program aimed to measure H0 from lensing. Search is underway to find new lenses in imaging surveys. An exciting discovery of the first strongly lensed supernova offered a rare opportunity to perform a true blind test of our modeling techniques. I will show the bright prospects of gravitational lens time delays as an independent and competitive cosmological probe.
Feb. 23, 2021 Jørgen Christensen-Dalsgaard
Aarhus University
Probing red giants with Kepler
The frequencies of oscillations observed on a stellar surface carry information about the properties of the stellar interior. Asteroseismology, i.e., the unravelling of this information, has made a huge leap thanks to the photometric observations obtained with NASA' Kepler mission, launched in 2009 to search for planets around other stars. In my talk I focus on the study of red-giant stars, showing a broad range of oscillations, probing both the outer parts and the deep core of the stars. Amongst other remarkable results, this has allowed distinguishing stars according to their nuclear energy source and provided detailed information about the properties of internal rotation in these late stages of stellar evolution.
Mar. 2, 2021
Jennifer van Saders
University of Hawaii
Making Sense of Stellar Rotation in Low Mass Stars
Stellar rotation carries a wealth of information about stellar populations. In particular, the technique of gyrochronology was developed to utilize the spin-down of stars as a function of time as an indicator of stellar age. Gyrochronology has the potential to yield precise ages for large samples of stars, providing unprecedented chronological information for studies of the Milky Way and extrasolar planets. However, the technique is in its adolescence: it has been tested and validated under limited scenarios, but its weaknesses and limitations have hitherto been largely unexplored. With time-domain data we can address these gaps: we now have access to datasets of rotation periods for tens of thousands of stars, as well as independent asteroseismic ages and rotation periods for a few hundred old (main sequence) stars.  I will discuss my comparisons of theoretical rotation models to these data, which have yielded unexpected insights into the rotational and magnetic lives of stars (and the Sun!), as well as a better understanding of the power and peril of gyrochronology as a tool. 
Mar. 16, 2021 Brian Nord
University of Chicago / Fermilab
From Galaxies to Faces: Recognizing the Implications of Artificial Intelligence in Astronomy and Society
Artificial Intelligence (AI) refers to a set of techniques that rely primarily on the data itself for the construction of a quantitative model. AI has arguably been in development for three quarters of a century, but there has been a recent resurgence in research and application. This current (third) wave of AI progress is marked by extraordinary results --- for example, in image analysis, language translation, and machine automation. Despite the aforementioned modest definition of AI, its potential to disrupt technologies, economies, and society is often presented as (nearly) unmatched in modern times, due in part to the versatility of the algorithms in modeling a wide variety of data. Similarly, there is great promise for applications across the sciences --- for example, simulations, image classification, and automated experimentation --- which are currently being investigated by researchers across the globe. Along with the significant promise of AI, comes great peril: in societal contexts, the consequences include enhanced surveillance, facial recognition, and automated weaponry. In science contexts, the issues are also significant and in many cases related --- for example, bias, lack of uncertainty quantification, and misuse. To take full advantage of the opportunities for AI to accelerate science and improve society, it's essential that we carefully guide its development.
During this presentation, we will explore modern AI techniques, like neural networks, and review how they are being developed and deployed in astronomy. Then, we’ll discuss ideas for the future usage of AI in science, including technical barriers for long-term application. Finally, we’ll discuss the roles of scientists and academic communities in the development of AI algorithms
Mar. 23, 2021 Julia Roman-Duval
Space Telescope Science Institute
The Nearby Universe  -  A Laboratory to Study the Cosmic Build-up of Dust and Metals in Galaxies
A key component of the baryon cycle in galaxies is the cycle of metals between the gas and the dust phases in the neutral interstellar medium (ISM). How this cycle depends on environment (in particular metallicity and density) has important implications for how accurately we can trace the chemical enrichment of the universe over cosmic times; either by using dust emission in the far-infrared as a tracer of the ISM in galaxies at high and low redshift; or by using spectroscopy of damped Lyman-alpha systems (DLAs) to measure chemical abundances in neutral gas over a wide range of redshifts. Variations of the abundance and properties of dust within and between galaxies furthermore profoundly affects their evolution, given the key role that dust plays in the chemistry, radiative transfer, and thermodynamics of galaxies. In this talk, I will present results from several independent observational efforts to characterize the dust abundance (dust-to-gas and dust-to-metal ratios) in the Magellanic Clouds and other nearby low-metallicity galaxies, using emission-based tracers of the interstellar medium (FIR, HI 21 cm, CO rotational emission) to map their dust and gas content on the one hand; and using UV absorption spectroscopy with Hubble to directly count metals in the gas phase on the other hand. Both types of studies demonstrate significant variations of the dust abundance with density (within galaxies) and metallicity (between galaxies), which have important implications for the sub-grid physics of galaxy evolution. Surprising patterns in the gas-phase abundances were found from the Hubble spectroscopy, and an interesting tension between the emission-based and absorption-based measurements remains unexplained.
Mar. 30, 2021 Anna Watts
University of Amsterdam
A NICER view of neutron stars
NICER, the Neutron Star Interior Composition Explorer, is an X-ray telescope that was installed on the International Space Station in 2017. Its mission is to study the nature of the densest matter in the Universe, found in the cores of neutron stars. NICER uses Pulse Profile Modeling, a technique that exploits relativistic effects on X-rays emitted from the hot magnetic polar caps of millisecond pulsars. The technique also lets us map the hot emitting regions, which form as magnetospheric particles slam into the stellar surface. I will present NICER's current results and ongoing analysis, and discuss the implications for our understanding of ultradense matter, pulsar emission, and stellar magnetic fields.
Apr. 6, 2021 Zhaohuan Zhu
University of Nevada, Las Vegas
Protoplanetary Disk Structure and Young Planet Population
Recent ground based observations at various wavelengths reveal a variety of protoplanetary disk structures. These disk structures may help us to constrain the planet formation process. I will first discuss how to apply the planet-disk interaction theory to substructures in the ALMA DSHARP sample to reveal the potential young planet population. We find that the occurrence rate for >5 M J planets beyond 5-10 au is consistent with direct imaging constraints. Disk substructures allow us to probe a wide-orbit planet population (Neptune to Jupiter mass planets beyond 10 au) that is not accessible to other planet searching techniques. On the other hand, ALMA surveys have suggested that the dust in Class II disks may not be enough to explain the averaged solid mass in exoplanets, under the assumption that the mm disk continuum emission is optically thin. I will question this optically thin scenario and point out that dust scattering can considerably reduce the emission from an optically thick region. This optically thick disk scenario can solve several puzzles in protoplanetary disk observations and can provide enough dust to form exoplanets. Finally, I will talk about my ongoing work on 3-D radiation hydrodynamical simulations of forming Jupiter’s convective envelope.
Apr. 13, 2021 Yamila Miguel
Leiden Observatory
Unveiling the secrets of Jupiter with the Juno mission
With more than 4000 exoplanets found and about 2-dozens of planets with detected atmospheric chemical species, we moved from an era of discovery to a new era of exoplanet characterisation. On the other hand, extremely accurate measurements by Juno and Cassini missions, make this an exceptional time to combine the detail information on the solar system giant planets and the large amount of data from exoplanets to get a better understanding on planetary physics and a better comprehension on planet formation and evolution.
Because our knowledge on the interior structure of the giant planets is linked with the data we obtain from space missions, these last years were crucial for this field: the outstanding accuracy of the gravity data provided by Juno has fundamentally changed our understanding of the interior of Jupiter. It has allowed us to put constrains on the zonal flows, the extent of differential rotation and lead us to find that Jupiter has most likely a dilute core. In this presentation I will review our knowledge on the interior structure of Jupiter and will also show some new results where we find that a non-homogenous envelope is also a constraint set up by the Juno measurements, which is helping us to get closer to unveiling Jupiter’s deep secrets and to reach a better understanding of the giant planets formation history.
Apr. 20, 2021 Ignas Snellen
Leiden Observatory
Exoplanets and the search for extraterrestrial life 
Abstract: Placing the solar system in the context of other planetary 
systems is one of the central objectives driving the study of extrasolar 
planets. One of the most fascinating questions in modern science is 
whether other life-bearing planets exist. In this talk I will review the 
current state of the art of exoplanet research and discuss future ways 
to probe biomarker gases in Earth-like exoplanets that could point to 
biological activity.
Apr. 27, 2021 Alberto Bolatto
University of Maryland
The Inner Workings of Starbursts
Starbursts are a rare phenomenon in the present day universe, but they represent perhaps the most common mode under which stars form and galaxies grow during the z~1-2 peak of cosmic star formation activity. This mode of star formation is not a simple scaling of what happens in typical molecular clouds, but represents a much more efficient manner of converting gas into stars likely through the formation of massive clusters. The resulting feedback in the form of galaxy-scale outflows is, together with AGN, thought to be one of the main forms of regulation of galaxy growth. I will show the analysis of recent, high resolution observations of three of the nearest starbursts: NGC253, NGC4945, and M82.  I will first discuss the properties of their galactic molecular outflows, including our best constraints on the mass and outflow rate, and the properties of the molecular gas. Then I will focus on the results from high-resolution observations which reveal a dozen compact structures with properties corresponding to massive young star clusters and super star clusters (SSCs), most of which are so embedded that are invisible in optical and NIR observations. Finally, I will present the analysis of 0.5-pc resolution observations of NGC253 which reveal feedback and disruption on the scales of these clusters, I will discuss the properties of these "cluster-scale" outflows, and I will compare them to theoretical expectations.

2020 Fall Colloquium Series

The Fall 2020 Joint Astrophysics Colloquium was held virtually on Tuesdays at 11AM and was followed by the Bahcall Lunch at 12:30, also virtual. 
Please find the Fall speaker schedule here.  Videos of the talks can be found on the IAS website:

2020 Summer Colloquium Series

The 2020 Summer Colloquium Series led by Professor Jenny Greene has concluded. The list of speakers, talk titles and select colloquium recording links appear below. 

Date   Name    Talk
Jun 30, 2020 Evan Schneider The Origin of Multiphase Galaxy Outflows
July  7, 2020 Courtney Dressing Exploring Planets Orbiting Nearby Stars
July 14, 2020 Geraldine Cochran Promoting Diversity and Addressing Barriers in Physics
July 21, 2020 Sean Johnson Observations of the gas flows that govern galaxy evolution in absorption and emission at z<1
July 28, 2020 Julie Posselt Equity in Science: Representation, Culture, and the Dynamics of Change in Graduate Education
Aug  4, 2020 Richard Anantua A Glimpse into Horizon-Scale Physics Using Movies and Polarization Maps
Aug 11, 2020 Casey Miller Practicing Equity in Graduate Admission
Aug 25, 2020 Kareem El-Badry Emission-line stars, binary mass transfer, and the search for detached stellar mass black hole

Contributions to and/or sponsorship of any event does not constitute departmental or institutional endorsement of the specific program, speakers or views presented.