Astrophysics Colloquium schedule


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. .

2/2/2022 - Wen-fai Fong (NU)
2/9/2022 - Danielle Berg (University of Texas at Austin)
2/16/2022 - Maya Fishbach (NU)
2/23/2022 - Kareem El-Badry (Institute for Theory and Computation in the Harvard/Smithsonian Center for Astrophysics)
3/2/2022 - Kirk Barrow (Stanford/SLAC)
3/9/2022 - David Hogg (NYU)
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: 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



 Archives below

Summer 2021


3/31/2021 - Jose Cernicharo (Grupo de Astrofísica Molecular, Instituto de Física Fundamental, CSIC)

Title: TThe chemical complexity of interstellar and C-rich circumstellar clouds: A comparison of the chemistry in TMC-1 and IRC+10216

Abstract: I will present the most recent results of the deep line surveys of TMC-1 and IRC+10216, performed with the YEBES 40m and IRAM 30m radio telescopes, and with the ALMA interferometer. These surveys cover the frequency ranges 31-50, 70-116, 120-180, and 200-350 GHz.  In the case of TMC-1 our data show several hundred U-lines in the 31-50 GHz range that certainly arise from several tens of new species, probably hydrocarbon radicals. For IRC+10216 the ALMA data show a forest of more than 1500 narrow U-lines arising from the dust formation zone for which we have not found any obvious carrier candidate. Although the chemistry of a circumstellar envelope (IRC+10216) could be, a priori, unrelated to that of a protostellar dark core (TMC-1), we have found that the comparative study of their chemical composition can provide key information on the role of ion-neutral and radical-radical reactions. Time plays a critical role in the growth of cyclical molecules in circumstellar envelopes. However, in interstellar clouds radical-radical reactions permit the development of a rich chemistry leading to the formation of pure hydrocarbon rings, including the first non-functionalized PAH, indene (c-C9H8).

Spring 2021


3/31/2021 - Sarah Millholland (Princeton)

Title: Tidal Sculpting of Short-Period Exoplanets

Abstract: Multiple-planet systems composed of close-in super-Earth/sub-Neptune-sized planets are ubiquitous, representing a dominant outcome of planet formation. This population exhibits predictable hallmarks of architectural regularity and uniformity, such as low eccentricities and inclinations, similar orbital spacings, and intra-system correlations in planetary masses and radii. On top of this first-order structure, however, these systems also exhibit surprising anomalies that require explanation. Examples include (1) ultra-short period planets, whose extremely-irradiated orbits have been separated off from the rest of their systems; (2) planets piled up wide of mean-motion resonances; and (3) a subset of Neptune-sized planets that show signs of radius inflation. In this talk, I will propose that tidal dynamics can account for these specific anomalies and more. Specifically, I will discuss the critical role of enhanced tidal dissipation due to non-zero planetary axial tilts (obliquities), which arise by way of prevalent dynamical resonances. I will highlight strategies for testing these tidal theories and observing obliquities directly in the future.

4/7/2021 - Jennifer Burt (JPL)

Title: Chasing TESS planet candidates with bright futures

AbstractNASA's TESS mission has produced more than 2000 planet candidates to date, and the TESS community is working to achieve the mission's level one science requirement of delivering masses for 50 new TOI planets by Summer 2021. With such a large starting pool, the decision of what candidates to follow up requires the collaboration of numerous ground based facilities. I will discuss the flow down from TESS Object of Interest (TOI) to confirmed planet through the lens of two recent confirmations. The first, TOI-824 b, is a hot, sub-Neptune planet orbiting a K4V star that sits in a crowded section of the night sky. Follow up efforts revealed an incorrect initial TESS radius estimate due to biased background estimates, demonstrating the importance of higher resolution ground based photometric follow up. The second, TOI-1231 b, is a temperate, Neptune-sized planet orbiting a nearby M3V star. Exhibiting only one transit per sector of TESS monitoring, the planet is one of the coolest small planets detected in the primary mission. Models of atmospheric observations suggest that it will be possible to detect spectral features in an atmosphere similar to that of K2-18 b enabling the first comparative planetology in the 250-350 Kelvin temperature range. Both of these new planets offer exciting atmospheric follow up possibilities and move TESS two steps closer to accomplishing its level one science goal.

Recorded talk


4/14/2021 - Alan McConnachie (University of Victoria / NRC Herzberg Institute of Astrophysics)

Title: At the limits of Gaia: the dynamics of the outer Galaxy and the Local Group seen with DR2 and EDR3

AbstractIt might be a cliché to say that Gaia is revolutionising our understanding of the structure of the Milky Way and its constituent stellar populations, but it is true and cannot be overstated. Most of this revolution is occurring within 10\,kpc from the Sun; beyond this, Gaia parallax uncertainties are of order 100%. However, Gaia proper motions can remain accurate to an interesting level for much more distant objects. We have undertaken an extensive wide field imaging campaign of the northern skies, concentrating on obtaining exquisite u-band data for thousands of square degrees, approximately 3 magnitudes deeper than SDSS. Combined with other bands, these data are excellent for obtaining photometric parallaxes for a wide range of stellar populations. I will show how these data, combined with Gaia proper motions, are allowing for the discovery of new streams and substructures in the very outer parts of the Galaxy (10 – 100,kpc distant), including a feature associated with the globular cluster NGC5466 that promises to be a highly useful galactic potentiometer. I will also discuss how precise proper motions for the entire dwarf galaxy population of the Milky Way (50 – 400kpc distant) are allowing us to examine the association, or otherwise, of groups of globular clusters and dwarf galaxies that point to group accretion events. Finally, I will present new Gaia proper motions for distant, isolated, dwarf galaxies (~500kpc distant), and examine their orbital evolution within the Local Group gravitational potential.

4/21/2021 - Ivo Labbe (Swinburne University of Technology)

Title: Rise and Fall of the First Galaxies in the era of JWST

Abstract: The formation of the first galaxies is a most exciting and elusive frontier. A lasting legacy of the Hubble and Spitzer space telescopes is the discovery and characterization of galaxies to redshift z∼11, looking back 97% of the time to the Big Bang. I will review our current sketch of galaxy build-up at cosmic dawn, highlighting fundamental questions that remain unanswered: when did the first galaxies form, what is their role in reionizing cosmic Hydrogen, and how do they evolve into the galaxies we observe at later times? The launch of the James Webb Space Telescope later this year will be an absolute game changer. JWST will deliver ultra-sensitive imaging and spectroscopy at wavelengths previously inaccessible. I will look ahead and discuss several initiatives in Webb's first year that address key outstanding issues, including our program UNCOVER, which aims to explore the so-called "Dark Ages" (10 < z < 20), the period after Recombination when the first stars and galaxies formed.

4/28/2021 - Lori Lubin (UC-Davis)

Title: Understanding Cluster Formation and Galaxy Evolution ... ORELSE


Abstract: The Observations of Redshift Evolution in Large Scale Environments (ORELSE) Survey is a systematic photometric and spectroscopic search for structure on scales > 10 Mpc around 18 known clusters at 0.6 < z < 1.3.  The survey covers 5 square degrees, all targeted at high-density regions, making it comparable in area and spectral coverage to historic extragalactic field surveys.  The goal of the survey is to study galaxy evolution across all scales -- from dense cluster cores to infall/intermediate-density regions to the field. In this talk, I describe the survey design, the galaxy sample, and our novel environmental metrics. I present some recent results on using the large galaxy sample to create a quantifiable cluster catalog, measure galaxy properties as a function of stellar mass, environment, and redshift, and constrain the nature of the active galaxy population. Finally, I describe our latest survey C3VO, which combines ORELSE with the higher-redshift survey VUDS to chart cluster formation and its effect on member galaxies over the last 12 billion years.

Recorded talk

5/5/2021 - Sean Johnson (University of Michigan)

Title: Observations of the gas flows that govern galaxy evolution in absorption and emission at z<1


Abstract: Galaxy growth is regulated by gas accretion from the intergalactic medium (IGM) and feedback from supernova and AGN-driven winds. The low-density halo gas around galaxies known as the circum-galactic medium (CGM) is at the nexus of these baryon cycles and represents both a dominant reservoir for fueling future growth and a fossil record of past feedback. Advancing our physical understanding of galaxy evolution requires observations of the gas cycles between the interstellar medium (ISM), CGM, and IGM. I will review three recent, and ongoing surveys that provide unique insights into the gas flows around galaxies through the combination of multi-wavelength datasets from Hubble, Magellan, and VLT. These surveys demonstrate that: (1)  Metal-enriched gas around low-redshift galaxies is predominantly confined to galaxy halos -- even in the shallow-potential wells of low-mass star-forming dwarfs. (2) Galaxy interactions play a key role in distributing gas from the ISM to the CGM while also likely triggering black hole growth. And (3) the putative hot wind component of AGN outflows is dynamically unimportant in a prototypical galactic-scale AGN superwind. Each of these results represents a surprising first glance from surveys that will expand dramatically over the next few years.

Recorded talk

5/12/2021 - Guido Roberts-Borsani (UCLA)

Title: Age-dating galaxies in the Reionization era and pinpointing the timing of Cosmic Dawn

Abstract: One of the major quests of modern extragalactic astronomy is to pinpoint the birth of the first stars and galaxies. To this end, NIR surveys with Hubble and ground-based telescopes do not probe far enough into the NIR to directly detect the very first objects - such a feat will have to await the NIR capabilities of JWST. However, recent results using the Spitzer Space Telescope have revealed a promising alternative: observations of bright, z>9 sources for which especially red photometric colours reveal the presence of a so-called 'Balmer break', indicative of mature (>100 Myrs) stellar populations that can greatly aid in age-dating the earliest galaxies. At lower redshifts, however, the cause of such red Spitzer/IRAC colours is not so clear cut: large degeneracies arise due to the contamination of the Spitzer/IRAC filters by strong nebular emission lines from young stars. Thus, distinguishing between the two is imperative to constrain the star formation histories of bright, Reionization-era galaxies. In this talk I will detail our recent efforts to constrain the star formation histories of z>7 galaxies using a combination of detailed photometric modelling as well as a ground-based spectroscopic campaign (with Keck, the VLT, ALMA and Gemini) and demonstrate how the arrival of JWST will unambiguously determine the ages of the first galaxies.

Recorded talk

5/26/2021 - Emily Rice (City University of New York)

Science of All Stripes: 
Creating an Eclectic Academic Career and Broadening Inclusivity

The landscape of academic science has changed significantly in recent decades and is poised to change even more in the near future. We can leverage these cultural changes to create an environment that is both inclusive to more people and effective in preparing students (science majors and non-majors alike) for a wider variety of careers and more broadly defined success. I’ll share my own path to science, including graduate school at UCLA, and a variety of science projects I have been involved in along the way to becoming tenured faculty at the City University of New York, including: planetarium shows, parody music videos, media appearances, Astronomy on Tap public outreach events, STARtorialist science fashion blog and shop, the AstroCom NYC research mentorship program, and the BDNYC  brown dwarf research group. The implicit mission that connects these eclectic projects is to expand support for, participation in, and even the definition of science.

Recorded talk


Winter 2021

1/6/2021 -  Leslie Rogers (U. Chicago) 
Title:  Journey to the Center of the Super-Earth

Abstract: Sub-Neptune, super-Earth size exoplanets are a new planet class. Though absent from the Solar System, they are found by microlensing, radial velocity, and transit surveys to be common around distant stars. In this talk, I'll review both recent developments and outstanding puzzles in our understanding of the nature and origin of these enigmatic planets.

1/20/2021 -  Ilya Mandel (Monash University)
Title:  The promise of gravitational-wave astrophysics

Abstract: The first detections of gravitational waves from compact-object mergers have opened up new opportunities and challenges in astrophysics. I will survey the plausible formation scenarios for merging compact-object binaries.  I will then describe ongoing efforts to extract the astrophysical evolution of massive stellar binaries from observations of gravitational waves emitted during mergers of the stellar remnants.

Recorded talk.

1/27/2021 -  Charlotte Mason (Harvard/CfA)
Title:  Constraining Reionization with Lyman Alpha Emission

Abstract: The reionization of hydrogen in the Universe's first billion years was likely started by photons from the first galaxies. We will probably never observe these galaxies directly, but their properties can be inferred by measuring the timeline and morphology of reionization. I will describe how we can use galaxies at our current observational frontiers to measure the reionization process. In particular, both the strength and line shape of Lyman alpha (Lyα) emission can probe the intergalactic medium (IGM), but modelling physics from pc to Gpc scales is required. I will describe how we can constrain the timeline of reionization and properties of ionizing bubbles by comparing observations of both Lyman-break and Lyman-alpha selected galaxies to models and simulations using statistical inference methods. I will present measurements which favour a late and relatively rapid reionization, and place these in the context of high redshift galaxy formation.

Recorded talk.

2/3/2021 - Subo Dong (Peking University) 
Title:  Direct Collision of White Dwarfs as a Possible Major Channel for Type Ia Supernovae

Abstract: The explosion mechanism of Type Ia supernovae (SN Ia) is unknown. The continuity in the properties of SNe Ia across the luminosity function suggests a single dominant channel to explain the population. We argue that direct collisions of white dwarfs may be promising as such a major channel. I will present results of our efforts to test the collision model.

2/10/2021 - Natalie Hinkel (Southwest Research Institute) 
Title:  The Star-Planet Connection: Using Stellar Elemental Abundances to Understand Exoplanets
Abstract: The future for exoplanet science is dependent not only on successful missions such as TESS, WFIRST, and JWST, but also on building bridges to the geology, planetary science, and data science communities. It is by using the resources and experiences from these other disciplines that we can uncover more subtle trends within exoplanetary data and establish a holistic connection between stars and planets. As part of my interdisciplinary research, I study the patterns in stellar abundances using the Hypatia Catalog, the largest elemental abundance dataset for stars near to the Sun. Because stars and planets are formed at the same time, meaningful connections can be made between the chemical properties of stars and their orbiting planets. I will discuss how stellar abundances may be used to determine planetary structure and mineralogy, which impacts the crustal composition, tectonic processes, and other planetary geochemical cycles which directly influence the overall habitability. Using more data driven techniques, I will describe a Netflix-esque recommendation algorithm to determine which stars in the solar neighborhood are likely to host to-date undetected giant exoplanets, based on the abundances of specific elements within stars. Finally, I will outline the steps needed in the near future to truly define planetary habitability per the physical and chemical relationship between stars and planets.

2/17/2021 - Masami Ouchi  (University of Tokyo)
Title: Co-evolution of Galaxies and the Inter-Galactic Medium over z~0-10 Being Uncovered with Large Telescopes Including Subaru, ALMA, and SKA 
Abstract: In a widely accepted physical picture of galaxy formation, galaxies grow by inflow of the inter-galactic medium (IGM) via filaments of the large-scale structures. The process of inflow triggers star formation that makes massive stars providing metal-rich gas in the inter-stellar medium (ISM). The thermal and kinetic energy of massive stars and supernovae produce gas outflow and UV radiation via the circum-galactic medium (CGM), which proceed the IGM metal enrichment and cosmic reionization, respectively. To understand the process of galaxy formation over the galaxy (ISM+CGM) to the cosmology (IGM) scales by observations, one needs to investigate a combination of gas distribution, metal abundance, and dynamics/ionization states, in contrast with many of previous observational studies.
This talk reviews our recent results of galaxies (ISM+CGM) and IGM at z~0-10 observed with various large telescopes including Subaru and ALMA, and discuss the interplay of matter/radiation exchanges between galaxies and the IGM over the cosmic history. I touch future prospects, introducing the next generation studies of the galaxy-IGM interplay at the epoch of reionization by a combination of the forthcoming optical/near-infrared and 21cm SKA radio observations.

3/4/2021 - Karl Glazebrook (Swinburne University of Technology)
Title: Monsters in the Deep: Massive quiescent galaxies at z~4 and above
Abstract: The last few years has seen the spectroscopic confirmation of significant populations of massive quiescent galaxies at redshifts of 4 and above, providing a significant challenge to models of early galaxy assembly. This has raised a number of important questions such as (i) are they truly quiescent? Has star formation been obscured? (ii) are their stellar masses correct? Can we measure dynamical masses? (iii) how much tension is there with simulations? (iv) where are their ancestors? This topic is to get exciting new data as new surveys from the ground and space will allow us to find the earliest massive galaxies at even higher redshift and spectroscopy of these from JWST will tell us the nature of the first stellar populations.

3/10/2021 - Brad Hansen and Ben Zuckerman (UCLA)

Title: Fight or Flight? Exploring some options available to long-lived technological civilizations in the Milky Way

Abstract: We will give two 25 min talks related to the astrophysical  constraints on the lifetime of a technological civilization in the Milky Way, in the face of stellar evolution of its  original host star.

The first talk will examine strategies to mitigate the high cost of migration from one star to another by waiting for the closest approach of another star (the "flight" option). The second talk will examine the constraints on the frequency of civilizations that attempt to survive by adjusting to life in orbit around a white dwarf (the "fight" option).


Fall 2020

10/7/2020 - Tim Brandt (UCSB) 
Title:  Discovering, Weighing, and Characterizing Exoplanets and Brown Dwarfs

Abstract: I will present a combination of three observational techniques--astrometry, radial velocity, and imaging--to discover, weigh, and characterize massive exoplanets and brown dwarfs. While thousands of planets are known, only a few have both measured masses (from radial velocity and astrometry) and atmospheric properties (inferred from spectra). Advances in adaptive optics and infrared instrumentation now enable us to see young exoplanets millions of times fainter than their host stars. Despite huge gains in sensitivity, however, high-contrast imaging surveys remain plagued by a lack of discoveries. I have calibrated a huge data set of stellar reflex motions; it can identify unseen planets and brown dwarfs by the gravitational tugs they exert on their host stars, and enable us to measure their masses and orbits. With masses, orbits, and spectra of a growing sample of planets and brown dwarfs, we can finally test models of substellar formation and evolution.

10/14/2020 - Rachel Bezanson (University of Pittsburgh)
Title:  The Formation of Massive Galaxies: deep, high-redshift spectroscopy from the LEGA-C and SQUIGGLE Surveys and Beyond

Abstract:  Massive galaxies reside in the densest and most evolved regions of the Universe, yet we are only beginning to understand their formation history. Today massive galaxies are red and dead ellipticals with little ongoing star formation or organized rotation; naturally they were expected to be relics of a much earlier formation epoch. In this talk I will describe the complex evolutionary history of massive galaxies that has emerged over the last decade, discussing the structural and kinematic evolution of massive galaxies during and after they stopped forming stars (“quenched”) and eventually transformed from rotationally supported disks into kinematically hot ellipticals. I will describe ongoing efforts to better understand this metamorphosis at intermediate redshifts, highlighting results from the ultra-deep LEGA-C spectroscopic survey of ~3000 massive galaxies at z~0.8 and the focused multi-wavelength SQUIGGLE survey of post-starburst galaxies at z~0.6 caught immediately following their cosmic shutdown. I will show that early quiescent galaxies, retained significant rotational support (~twice as much as local ellipticals), implying that the mechanisms responsible for shutting down star formation do not also have to destroy ordered motion. Furthermore, I will describe first results from the SQUIGGLE survey, including the ALMA discovery of vast reservoirs  (~1010 Msun) of cold molecular hydrogen remaining in young post-starburst galaxies caught a few 100 Myrs after quenching their primary episodes of star formation. This enigmatic result suggests that the quenching process does not have to fully heat or deplete the gas to halt star formation. Finally, I will discuss prospects for extending spectroscopic studies of galaxies at cosmic noon - the peak of massive galaxy formation and shutdown - with JWST and eventually 30-m class telescopes. 

10/21/2020  - Mark Morris (UCLA)
Title: New Floodlights on the Galactic Wind
Abstract:  While there has long been a limited amount of morphological and dynamical evidence for an outflow from the central regions of our Galaxy, the last decade has seen an explosive growth in the evidence for a powerful Galactic wind on all scales, starting with the discovery of the gamma-ray Fermi Bubbles. While the lower boundaries of the Fermi Bubbles are nested within a hot plasma seen in soft X-rays and in the form of a “microwave haze,” the connection of these large-scale features to activity in the Galactic center was not evident until recent X-ray and radio continuum observations revealed the "Galactic center chimneys” extending a few hundred parsecs out from the center and connecting to the bases of the Fermi Bubbles. In this talk I will describe what we have learned about these Galactic exhaust channels using a multi-wavelength approach. I will also describe the recent discovery of a population of relatively dense atomic and molecular clouds that have apparently been accelerated out of the Galaxy’s central molecular zone up to hundreds of km/s by the much faster hot plasma constituting the bulk of the Galactic wind.

Recorded talk.


10/28/2020 - Ana Bonaca (CfA/ITC)
Title:  Uncovering the nature of dark matter with stellar streams in the Milky Way
Abstract:  Stars orbiting in the halo of our galaxy, the Milky Way, are a window into the distribution of dark matter. Tidally disrupting star clusters are especially valuable tracers, because in pristine conditions they produce thin stellar streams of nearly uniform density. I will present maps of stellar streams based on the latest photometric and astrometric data that reveal variations in the width and density of streams -- typical signatures of dynamical perturbation. Dynamical modeling of a perturbed stream GD-1 suggests it recently had a close encounter with a massive and dense perturber, while precise radial velocities constrain the perturber's orbit and present-day location. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. Data being delivered by big ground-based surveys of this decade (e.g., DESI, LSST) will enable such studies in hundreds of stellar streams. I will discuss how the upcoming observations can be used to measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Milky Way halo.


11/4/2020 - Arpita Roy (STScI)
Abstract: The study of exoplanets, arguably more than any other field of astrophysics, has grown in direct consonance with new instrumentation. Currently in build+commissioning phases are planet hunting Doppler spectrographs aiming at 10-30cm/s RV precision in the optical in quest for Earth analogs, and <1m/s in the near-infrared in pursuit of M dwarf planets. These massive instruments leverage a range of technological advances, from high-homogeneity illumination delivery setups, to sophisticated wavelength calibration, ultra stable environmental control, and precision data analysis. Building these instruments is a storied journey of failure and success, innovation and disappointment. In this talk we will delve into the experience and lessons that come from technologically pushing the field forward, as well as the complexity and sophistication of these systems that will enable ground-breaking new science.

11/11/2020 - Veterans Day holiday - no Colloquium

11/18/2020 - Chris Fassnacht (UCD)
Title:  Investigating the Nature of Dark Matter with Gravitational Lenses
Abstract: The nature of dark matter is one of the major questions facing not only astrophysics, but also high-energy physics.  Our "standard model" of cosmology, Lambda CDM, provides an excellent match to observations of the Universe on large scales, but there are some indications that the model may not adequately reproduce observations on galaxy scales and smaller.  As a result, alternative dark matter models have been proposed to explain the discrepancies.  A powerful way to test these non-CDM models is to determine the statistics of low-mass halos, since some of the models (e.g., warm dark matter) predict fewer halos at low masses than CDM.  In the interesting mass range where the predictions from WDM diverge strongly from those of CDM, halos are expected to host very few to no stars, making their detection difficult.  Strong gravitational lensing provides an excellent complement to Local Group observations, especially because it can detect purely dark halos at even cosmological distances.  In this talk I will discuss two methods of using gravitational lenses to detect low-mass halos and present some recent results, as well as giving some thoughts about future efforts to use these techniques.

11/25/2020 - Thanksgiving holiday - no Colloquium


12/2/2020 - Jorge G. Moreno (Pomona College)
Title:  Spatially Resolved Galaxy Interactions
Abstract: For decades, late-stage galaxy mergers have been recognized as naturally occurring events within the hierarchical LCDM paradigm, capable of triggering starburst and quasars. Early-stage mergers (interactions), albeit not as dramatic as their late-stage counterparts, are believed to shape galaxies in gentler and more long-lasting ways: by enhancing star formation, suppressing gas metallicity, igniting AGNs, augmenting H2 fuel, etc. But more importantly, their cumulative effect may ultimately stimulate the transformation of spirals into lenticulars in dense environments. The focus of this talk is to address the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy merger simulations (stellar mass ratio = 2.5:1), which employs the "Feedback In Realistic Environments-" model (FIRE-2). This framework resolves star formation, feedback processes, and the multi-phase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (HI) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally-concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR-enhancement in the central kiloparsec is fuel-driven (55% for the secondary, 71% for the primary) -- whilst central SFR-suppression is efficiency-driven (91% for the secondary, 97% for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially-resolved (integral-field spectroscopic) galaxy surveys -- capable of examining vast and diverse samples of interacting systems -- coupled with multi-wavelength campaigns aimed to capture their internal ISM structure.

12/9/2020 - Sara Seager (MIT)
Title:  The Search for Signs of Life Beyond Earth by Way of Atmospheric Biosignature Gases
Abstract: For thousands of years, inspired by the star-filled dark night sky, people have wondered what lies beyond Earth. Today, the search for signs of life is a key factor in modern-day planetary exploration, both for in situ exploration of our own Solar System’s planets and moons and for telescope remote sensing of exoplanets orbiting nearby stars. We aim to detect a gas in a planetary atmosphere that might be attributed to life. A suitable “biosignature gas” must: be able to accumulate in an atmosphere against atmospheric radicals and other sinks; have strong atmospheric spectral features; and have limited abiological false positives. The study of biosignature gases is thus intertwined with chemistry. Life on Earth produces thousands of gases. Which gases might be potential biosignatures in an as yet unknown range of possibly exotic exoplanetary environments? New computer simulations and next generation telescopes soon coming on line make us the first generation with the capability to search for signs of life on what should be a growing number of suitable exoplanets.  We might be surprised by finding biosignature gases on planets closer to home in our own Solar System, such as phosphine on Venus, motivating new space missions for in situ exploration.


Astrophysics Colloquium Archive