Astrophysics Colloquium schedule

Spring 2022

March 30, 2022 - Nikole Lewis (Cornell University)

Revealing the Hidden Secrets of Hot Jupiters


Abstract: 51 Pegasi b, the first exoplanet detected around a sun-like star, was the founding member of the hot Jupiter population that now numbers more than 300. Although often derided for their distinct lack of potential habitability and the havoc they likely wreaked in their systems, hot Jupiters have provided a unique opportunity to explore exoplanet atmospheric physics and chemistry. Here I will discuss recent observations of hot Jupiters with the Hubble and now retired Spitzer space-based telescopes that have revealed unexpected processes at work in these distant worlds. I will also discuss current limitations in our ability to interpret observations of hot Jupiters in order to understand the physics and chemistry that shape their atmospheres. Additionally, I will overview observations of hot Jupiters planned for the recently launched JWST and what we hope to learn by exploring these planets with infrared spectroscopy. Although hot Jupiters have been revealed to possess complex atmospheres, they still remain one of our best opportunities to hone observational techniques and atmospheric theories along the path to answering the questions “How did we get here?” and “Are we alone?”

April 6, 2022, 3:30pm - Marc Kamionkowski (Johns Hopkins University)

The Hubble tension and early dark energy

Abstract: The value of the cosmic expansion rate (the Hubble constant) inferred from observations of supernovae disagree with those inferred from measurements of the cosmic microwave background. Easy explanations for this discrepancy have been elusive, but the past few years attention has turned to the possibility that a modification to early-Universe physics may be required. I will discuss a solution to this "Hubble tension" that involves the introduction of a new component of matter, “early dark energy,” as well as other related ideas.

April 13, 2022 - Katherine R. de Kleer, Caltech

The surface environments of the galilean satellites

Abstract: The galilean satellites of Jupiter provide a laboratory for studying geological activity and its role in surface-interior exchange on planet-scale bodies. The differing degrees of tidal heating in these four satellites play out clearly in the level and style of their activity, from the intense volcanism of Io through the elusive plumes of Europa to the remnants of ancient activity on Ganymede. Callisto orbits outside the orbital resonance, presenting a control case of a long-inactive moon. Active internal processes manifest on the surface in the form of heat and materials delivered from the interior, providing specific detectable signatures that act as windows into the interior. High-resolution multi-wavelength telescope datasets from visible and near-IR (HST/Keck) through millimeter (ALMA) wavelengths sense different depths, from the subsurface up through the tenuous exosphere. This talk will discuss how such datasets are helping us understand the surface environments of these objects and the links between their interiors, surfaces, and atmospheres, and will present some outstanding questions that will be addressed by upcoming missions and observatories.

April 20, 2022 - Bruce Macintosh (Stanford)

Direct Imaging of Extrasolar Planets

Abstract: Direct detection of extrasolar planets - spatially resolving a planet from its host star while blocking, moving, or post-processing the starlight - is a powerful complement to transit, RV, and microlensing approaches. Direct detection is sensitive to planets in wider orbit, and allows spectroscopic characterization of planetary atmosheres. One of the most effective instruments in this regime has been the Gemini Planet Imager (GPI). GPI was a facility instrument combining advanced adaptive optics, a diffraction-controlling coronagraph, and an infrared integral field spectrograph on the Gemini South Telescope. From 2014-2019 we carried out the Gemini Planet Imager Exoplanet Survey (GPIES), which observed 532 young (10-200 Myr) nearby stars. I will summarize the key results of the GPIES program, including constraints on giant-planet distributions and atmospheric properties. We have also extensively characterized GPI’s performance, leading to insights into next-generation systems.

With current technology, direct imaging with GPI or other instruments is sensitive primarily to planets that are significantly younger than, more massive than, and in wider orbits than Jupiter, and such planets are rare. Moving beyond this will require new capabilities. The GPI 2.0 project upgrades the existing instrument with faster adaptive optics, better coronagraph designs, and new spectrograph modes.When deployed on Gemini North, GPI 2.0 will be able to search younger stars in the Taurus and Ophiucus star-forming regions, and be sensitive to Jupiter-like “cold start” planets. I will summarize the science drivers that guided the GPI 2.0 upgrade and the project’s status.

In the even longer run, direct imaging is the best path to characterizing true Earth analogs - planets orbiting in the habitable zone of sunlike stars, beyond the reach of practical transit spectroscopy. Such detection will require a dedicated space mission incorporating an advanced coronagraph. The recent Astro2020 Decadal Survey laid out a vision leading such a mission over the next decades. Finally, I will summarize the path forward for direct imaging leading to that possible Earth.

April 27, 2022 - Kartik Sheth (NASA HQ)

IDEA (Inclusion, Diversity, Equity and Accessibility) to Actions

Abstract: Inclusion and diversity are increasingly being recognized as crucial ingredients for innovation and success. But unlike a mathematical equation, converting good intentions to successful actions is significantly harder in these human-centered challenges. In this presentation, I will describe some of my 2+ decades of work in the areas of inclusion, diversity, equity and accessibility and discuss successes and failures of different approaches.

May 4, 2022 - Katerina Chatziioannou (Caltech)

Understanding neutron stars and dense matter with gravitational wave signals

Abstract: Detections of neutron stars in binaries through gravitational waves offer a novel way to probe the properties of extremely dense matter. In this talk I will describe the properties of the signals we have observed, what they have already taught us, and what we expect to learn in the future. I will also discuss how information from gravitational waves can be combined and compared against other astrophysical and terrestrial probes of neutron star matter to unveil the properties of the most dense material objects that we know of.

Winter 2022

1/12/2022 - No colloquium due to the AAS!

1/19/2022 - Anna Quider (Assistant Vice President for Federal Relations - NIU)

Title: The Federal Science Budget and Policy: A Primer for Astronomers

Abstract: The US federal government touches all aspects of our lives through its $6.8 trillion annual budget, laws, regulations, rules, and policies. Physics and astronomy are no exception. Physics and astronomy research is dependent upon financial support from an alphabet soup of federal agencies: DOD, DOE, NASA, NIST, and NSF, for example. Decisions are made from the White House and Congress down to individual program managers that shape what research gets done, how research is done, and who gets to do it. This talk will introduce federal science policy by examining the current federal science funding and policy landscape and exploring its impact on the fields of physics and astronomy.

Recording of Anna's talk

1/26/2022 - Jack Singal (University of Richmond)

Title: How Bright is the Radio Sky? We Don't Know...yet

Abstract: The actual level of diffuse radio emission on our sky is surprisingly uncertain. Almost all investigations that depend on understanding the radio sky, including cosmic microwave background and 21 cm cosmology foregrounds, ultimately rely on just one measurement - the 408 MHz Haslam map from the 1980s, which traces its absolute calibration to highly uncertain, low resolution blocked aperture measurements from the 1960s and earlier. The question of the absolute level of diffuse radio emission has taken on additional urgency given the claimed high radio synchrotron background level from the ARCADE instrument and other recent measurements. Such a high level of background radio emission would require a new population of incredibly numerous, faint radio sources in the universe, or for our Galaxy to be highly anomalous. This talk will discuss the present situation and the project to make the first ever reliably absolutely calibrated large-scale radio map, using the unique features of the Green Bank Telescope combined with custom instrumentation. This polarization sensitive map of the whole sky North of -47 degrees declination with all Galactic latitudes observed will provide an important new resource for understanding and constraining almost all Galactic and extragalactic phenomena that manifest in, or depend on the understanding of, diffuse radio emission.

Recording of Jack's talk

2/2/2022 - Wen-fai Fong (NU)

Title: Illuminating the Origins of Fast Radio Bursts

Abstract: When we look up at the night sky, we see a static universe. However, observational surveys have revealed that our universe is dynamic with a myriad of transient events. One of the universe's most fascinating and fastest explosive transients to come to light over the past decade are fast radio bursts. While fast radio bursts are seemingly connected to highly-magnetized neutron stars and are among the most prolific transients to occur in nature, the precise origins of fast radio bursts remain uncertain. In this talk, I will discuss this population of transients and our quest to understand their origins, primarily through observational studies of their local and host galaxy environments. I will describe our ongoing campaigns with large ground-based telescopes and HST to build legacy samples of their environments and extract crucial information on their host stellar populations. I will also discuss upcoming upgrades to fast radio burst experiments which will provide a flood of new, well-localized discoveries in the near future.

Recording of Wen-fai's talk

2/9/2022 - Danielle Berg (University of Texas at Austin)

Title: Bridging Galaxy Evolution Across Cosmic Time With the CLASSY Survey

Abstract: Rest-frame far-ultraviolet (FUV) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of JWST and construction of the ELTs will soon usher in a new era, pushing the FUV spectroscopic frontier to z~15-20. The success of these future endeavors hinges on a comprehensive understanding of the massive star populations and interstellar medium (ISM) gas conditions that power the observed FUV spectral features. I will present the COS Legacy Archive Spectroscopic SurveY (CLASSY) Treasury as a powerful and promising solution. CLASSY is a large HST program creating the first high-quality, high-resolution FUV spectral catalog of star-forming galaxies at z~0. The spectra contain a suite of emission and absorption lines that characterize the massive stellar populations that populate metal poor galaxies, the physical properties of large-scale outflows that regulate star formation, and the chemical abundance patterns of the gas and stars. Interestingly, CLASSY is consistent with the z∼0 mass-metallicity relationship, but is offset to higher star-formation rates by roughly 2 dex, similar to z~2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.

2/16/2022 - Maya Fishbach (NU)

Title: Astrophysical Lessons from LIGO-Virgo's Black Holes

Abstract: LIGO and Virgo have observed over 80 gravitational-wave sources to date, including mergers between black holes, neutron stars, and mixed neutron star- black holes. The origin of these merging neutron stars and black holes -- the most extreme objects in our Universe -- remains a mystery, with implications for stars, galaxies and cosmology. I will review the latest LIGO-Virgo discoveries and discuss some recent astrophysical lessons, including mass gaps, evolution with cosmic time, and implications for cosmology. While the latest gravitational-wave observations have answered a number of longstanding questions, they have also unlocked new puzzles. I will conclude by discussing what we can expect to learn from future gravitational-wave and multi-messenger discoveries.

Recording of Maya's talk.

2/23/2022 - Kareem El-Badry (Institute for Theory and Computation in the Harvard/Smithsonian Center for Astrophysics)

Better together: binary stars as probes of star formation and evolution

Abstract: Binary stars are foundational to modern astrophysics. They underpin precision measurements of stellar structure, age, and composition; they provide the most stringent tests of general relativity, they make possible the study of faint and rare objects such as black holes and neutron stars, and they are the progenitors of gravitational wave sources. The components of binaries often interact, dramatically changing their evolution and giving rise to a spectacular zoo of astrophysical phenomenology. To understand stars -- particularly massive stars -- it is necessary to understand binaries. Large-scale stellar surveys such as Gaia, TESS, and SDSS-V are transforming the binary field, making possible both comprehensive population demographics and the discovery of rare objects. I will discuss new insights gleaned from surveys in recent years, including the creation of stripped-envelope stars following binary mass transfer, the formation of equal-mass "twin" binaries in circumbinary disks, and the characterization of planets in binaries. I will focus in particular on the search for dormant stellar-mass black holes in binaries, discussing recent candidates and the path forward to characterizing the detached black hole population.

Recording of Kareem's talk.

3/2/2022 - Kirk Barrow (Stanford/SLAC)

High-Cadence Synthetic Observations and Neural Networks in the Era of JWST

Abstract: 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.

Recording of Kirk's talk.

3/9/2022 - No colloquium

Virtual Astronomy Colloquia are held via Zoom. Meeting information will be sent in email.

Fall 2021

10/6/2021 - Peter Behroozi (University of Arizona)

Title: Trinity: Unveiling the Connection between Halos, Galaxies, and Supermassive Black Holes

Abstract: We present a new empirical model, Trinity, that observationally constrains the joint relationship between supermassive black holes (SMBHs), galaxies, and dark matter halos from z=0 to z=7. Trinity is unique in its ability to systematically combine constraints from a wide variety of SMBH and galaxy observations, allowing the resulting inferences to be free of theoretical assumptions that have been present in past models. We can hence recover the average growth and merger histories of all detectable SMBHs, describe the evolution of the SMBH—galaxy relationship through time, and even constrain the cosmic evolution of physical SMBH properties such as the radiative efficiency. We also provide a natural solution to the origin of the high accretion rates necessary to grow the largest SMBHs observed at z>6.

Recorded talk

10/13/2021 - Sukanya Chakrabarti (RIT)

Title: Towards precision measurements in Galactic dynamics

Abstract: For more than a century now, astronomers have used kinematic analysis, i.e., modeling the positions and velocities of stars, to estimate the accelerations of stars that live within the gravitational potential of the Milky Way. Recent observations have revealed that our Galaxy had a highly dynamic history. For time-dependent potentials (like that of our Galaxy), there are discrepancies between the true acceleration and that derived from kinematic estimates. I will review two independent methods that we have developed to directly measure the accelerations of stars in the Milky Way using extremely precise time-series observations. I will first review our work on measuring accelerations from high precision RV observations conducted over decade-long baselines. In particular, I will discuss theoretical expectations of the vertical acceleration profile that are motivated by Gaia observations. I will then talk about our analysis of pulsar timing observations, from which we were able to measure Galactic accelerations for the first time, and from these measurements, derive fundamental Galactic parameters, including the Oort limit, the local dark matter density, and the oblateness of the Galactic potential. I will end by talking about prospects for measuring dark matter sub-structure with our ongoing ESPRESSO observations, and the possibility for developing an “acceleration ladder”.

Recorded talk

10/27/2021 - Chuck Horowitz (Indiana University)

Title: Supernovae Ignited by Nuclear Fission

Abstract: Type 1A supernovae (SN Ia) are giant stellar explosions that provide important distance indicators in cosmology. Presently there is tension between Hubble constant values determined from SN and from other means. SN Ia are thought to involve white dwarf stars, but it is unclear how they explode. We propose a new mechanism involving a natural nuclear fission explosion. White dwarfs cool and eventually crystalize. Our molecular dynamics simulations find that the first solids to form, as the star cools, are greatly enriched in actinides such as uranium. This is because actinides have the highest nuclear charge. These solids may support a fission chain reaction that, in turn, could ignite carbon fusion and explode the star. This physics parallels that in terrestrial nuclear weapons.

Recorded talk

11/3/2021 - Claude-Andre Faucher-Giguere (Northwestern University)

Title: Simulating galaxy formation with FIRE: some results on the role of feedback and of the circumgalactic medium

Abstract: Galaxies are remarkably diverse in their properties, ranging from irregular to disky to elliptical in morphology, and from blue to red in color. At the same time, when analyzed systematically, galaxy populations exhibit striking regularities, with clear trends with mass and redshift. How does this "regular complexity" emerge from the hot Big Bang? I will present results from the FIRE simulations which shed some light onto the processes that shape galaxies. The FIRE zoom-in simulations resolve the multiphase interstellar medium of galaxies and model several different feedback processes (including Type II/Ia supernovae, stellar winds, and radiation) while including the cosmological environment. I will highlight predictions for the formation of disk galaxies, the "burstiness" of star formation, and galactic winds. I will also summarize recent results on a phase transition experienced by circumgalactic gas as halos grow (the virialization of the CGM), and discuss how the interplay between feedback on small scales and the physics of halo gas on larger scales may drive important aspects of galaxy evolution.

Recorded talk

11/10/2021 - Ruth Angus (American Museum of Natural History)

Title: Measuring the Ages and Rotation Rates of Cool Stars with TESS and Other Photometric Surveys

Abstract: It has been 50 years since the discovery that stellar rotation periods can be used to date stars, however, converting a rotation period to an age via 'gyrochronology' is still far from simple. In fact, each time a new batch of data becomes available, the relationship between stellar rotation period and age is revealed to be even more complex than previously thought. Light curves from the TESS mission provide an opportunity to measure the rotation rates of millions of stars in the Solar neighborhood, which could help to refine gyrochronology models. However, although TESS provides incredible sky-coverage, its light curves are relatively short, just 27 days long, and it proves difficult to measure rotation periods longer than around 14 days for any star with TESS, even those with a year of near-continuous monitoring. Measuring rotation periods with TESS and measuring ages from those rotation periods are both non-trivial tasks, but a catalog of TESS rotation periods and ages would be a rich repository for scientific opportunity. In the near future, the Vera C. Rubin observatory will provide an overwhelming number of light curves for stars across the Galaxy. Can we refine our age-dating and period-measuring techniques in time to fully leverage the incredible data set of LSST? In this talk, I will discuss ongoing efforts to turn photometric survey data into an archive of stellar rotation periods and ages which could reveal the evolutionary history of our Galaxy, its stars, and the planets that orbit them.

Recorded talk

11/17/2021 - Vikki Meadows (University of Washington)

Title: Do The Clouds of Venus Contain Phosphine?

Abstract: In late 2020, the observation of a 266.94 GHz feature in the Venus spectrum was attributed to phosphine (PH3) in the Venus clouds, suggesting unexpected geological, chemical or even biological processes. Since then, the planetary science and astronomy communities have engaged in a number of important scientific tests of the Venus PH3 hypothesis. These include re-analyzing the original discovery data, searching for corroborating features elsewhere in the Venus spectrum, and exploring alternative explanations for the detection and interpretation of the 266.94 GHz line. Many of these steps can be considered part of a larger scientific framework for biosignature assessment. In this talk I will review the discovery papers, and describe a comprehensive research effort to independently analyze both the data and the phosphine interpretation for the 266.94 GHz line. Our work suggests an alternative hypothesis—that typical amounts of Venus SO2 can fully explain both the detection and non-detection of absorption lines in the discovery data. This study further emphasizes the importance of understanding planetary context when searching for, and interpreting, potential signs of life beyond the Earth.

Recorded talk

11/24/2021 - Thanksgiving (no colloquium)

12/1/2021 - Jamie Law-Smith (UCSC)

Title: Interactions between black holes, stars, and galaxies

Abstract: A physical understanding of the high energy interactions between black holes and stars, coupled with the context of their galactic birthplaces, will allow us to use these systems as tools to better understand black holes at all masses, the lives and deaths of stars, and the dynamical mechanisms operating in galaxies. In this talk, I will discuss one particular interaction: the tidal disruption of a star by a supermassive black hole. I will present a library of tidal disruption event simulations and will show that it can be reduced to a single relationship. I will present the chemical structure of the disks formed after tidal disruption, which is important for understanding the spectra of these events. I will also connect these AU-scale events to kpc-scale galaxy physics: I will present a systematic study of tidal disruption event host galaxies in the context of the local galaxy population, and in particular our finding that they are highly centrally concentrated. We expect ~50,000 tidal disruption events detected with Rubin over 10 years, allowing us to use these events as unprecedented probes of supermassive black hole demographics, nuclear stellar populations, the physics of super-Eddington accretion, and dynamical mechanisms operating in galactic centers.

Recorded talk

Astrophysics Colloquium archived talks