Astrophysics Colloquium

Colloquium Meetings are held in the Physics and Astronomy Building (PAB) in Room 1-434A from 3:30-4:30 pm every Wednesday of the Academic Year.

Coordinator: Smadar Naoz

Winter 2017 Schedule


Date Speaker Title 

Dan Weisz, UC Berkeley

The Lowest-Mass Galaxies In the Early Universe: Insights from the Local Group

The Local Group is home to ~100 galaxies less massive than the Small Magellanic Cloud (10^8 Msun).  Such low-mass galaxies have become increasingly relevant to a broad range of astrophysics from cosmic reionization to deciphering the nature of dark matter.  Yet, they are simply too faint to be directly detected at any appreciable redshift, compromising our ability to place them into a cosmological context. In this talk, I will describe how observations of resolved stellar populations in Local Group galaxies enable the measurement of detailed star formation histories, which provide the only avenue for tracing the evolution of low-mass galaxies across cosmic time. I will review our current knowledge of low-mass galaxy evolution over 6 decades in stellar mass, with a particular emphasis on the very early Universe.  I will illustrate how local and high-redshift galaxy observations can be used in tandem to improve our understanding of cosmic reionization, and will conclude by discussing prospects for increased synergy between near-field and far-field galaxy studies in the JWST era.


Andy Skemer, UCSC

Characterizing the Coldest Exoplanets

Temperature, rather than mass, is the dominant factor in determining the appearance of gas-giant planets, and the diversity and complexity of worlds both increase at cold temperatures.  The coldest known exoplanets are still much hotter than the gas giant planets in our own Solar System.  Pushing to colder temperatures requires imaging in the thermal infrared (3-5 microns) where self-luminoous gas-giants peak in brightness.  I will present observational studies characterizing the atmospheres of the coldest exoplanets and brown dwarfs, down to a temperature of 250K.  Additionally, I will describe a new instrument that can obtain thermal infrared spectroscopy of directly imaged planets for the first time.

1-25-17 Massimo Stiavelli, STScI

Space Telescope Science Institute, USA

I will review the capabilities of the James Webb Space Telescope to study the era of the first stars, particularly focussing on the study of lensed or unlensed primordial star clusters or dwarf galaxies. I will discuss possible avenues of discovery of pair-instability supernovae using JWST or other telescopes, and how JWST could follow them up. Finally, I will provide a brief status on the mission and on the upcoming opportunities to learn more about JWST and propose for time.


2-1-17 Andrey Kravtsov, U Chicago 

Towards understanding the inefficiency of star formation in galaxies

Understanding how galaxies form is one of the main unsolved problems in astrophysics. One of the long-standing puzzles is the global inefficiency with which galaxies convert baryonic matter available to them into stars. This inefficiency is manifested in 1) the fact that ratio of baryon mass observed within galaxies to the total inferred mass of their host halos is much smaller than the universal baryon fraction and 2) the fact that galaxies convert their observed gas into stars on ~1-2 Gyr time scale (aka depletion time), which is much longer than any dynamical time scale within galaxies. I will review recent progress in galaxy formation simulations due to improvements in treatment of stellar feedback and star formation, which sheds light into the 1st aspect of inefficiency. I will highlight the key role that modelling of star formation and stellar feedback play in setting the basic properties of galaxies, such as stellar mass (and global baryon mass fraction), size, and morphology using specific examples of recent galaxy formation simulations. I will present a new model, in which local star formation efficiency is modelled "on the fly" using a turbulence-based subgrid model based on results of high-resolution simulations of molecular clouds. The model predicts a wide variation of star formation efficiency per free fall time at odds with the usual assumption of constant efficiency. At the same time, our model predicts distribution of star formation rates in broad agreement with observations of both local and resolved extragalactic GMCs. I will show that with realistic implementation of stellar feedback this modelling can reproduce the basic properties of star formation and the Kennicutt-Schmidt relation in galaxies, such as the Milky Way. Finally, using insights from such simulations I will present a simple model explaining why star formation in galaxies is inefficient and depletion times are long.

2-8-17 Fred Rasio, CIERA

Dense Star Clusters as LIGO Source Factories

Theoretical predictions for compact binary mergers from field populations of binary stars are extremely sensitive to the assumptions of stellar evolution, leading, for example, to predicted merger rates for binary black holes that span several orders of magnitude. But in dense stellar environments such as globular clusters, binary black holes form by well-understood gravitational interactions. In this talk I will present an overview of recent theoretical work on the dynamical formation of black hole binaries based on realistic N-body simulations of globular clusters. By calibrating theoretical models against observed Milky Way and extragalactic globular clusters, we find that the mergers of dynamically formed binaries could eventually be detected by Advanced LIGO at a rate of ~ 100 per year, potentially dominating the overall detection rate of gravitational wave sources. Dynamical processes in globular clusters can also form very naturally the more massive black hole binaries like the one that produced GW150914, the first merger signal detected by LIGO.

2-15-17 James Guillochon, CfA

 The Impact of the Debris from Stellar Tidal Disruptions on their Environment

Abstract: When a star is destroyed by tides from a supermassive black hole, the star is stretched into a long, extraordinarily thin debris stream that extends from the star's original periapse (about an AU from the black hole) to distances of tens of parsecs. As the stream extends, it simultaneously cools, clumps, magnetizes, and slams into the ambient medium, delivering a supernova's worth of kinetic energy into the surrounding gas. In my talk, I will describe the predicted observable phenomena arising from this debris, and the impact it can have in the centers of galaxies.
2-22-17 Jennifer Lotz, STScI

 Galaxy Assembly over Cosmic Time

Abstract: Deep HST observations have revealed galaxies fainter than ever seen before, at look-back times when the universe was less than a billion years old. These first dwarf galaxies grow throughout cosmic time via the accretion of gas and dark matter, and via mergers with other galaxies. The detailed structures of galaxies provide direct insight into their most recent assembly events. From large high-resolution imaging surveys, we now have a broad-brush picture of how galaxy shapes and sizes have evolved over the past 10 billion years. But the role of galaxy mergers in galaxy evolution is poorly understood, particularly at early times. More subtle morphological tracers are needed to track the complex processes responsible for the transformation of galaxies. I use new machine learning classifications of galaxy morphology at 0 < z < 3 to identify galaxy mergers and galaxies transitioning to today's Hubble types. Numerical simulations are used to inform the interpretation of these systems. I track the evolution of galaxies as a function these new measures, and discuss the role of mergers in the size growth of galaxies. Finally, I discuss the prospects for studying galaxy assembly in the coming decade.

3-1-17 Emily Rauscher, U. Michigan

The Diversity and Complexity of Exoplanet Atmospheres

We are now in the era of exoplanet characterization.  Over a decade ago the first exoplanet atmosphere was detected and since then we have been gathering compositional and temperature information for the brightest targets, primarily "hot Jupiters".  Recent technical advances are enabling measurements that contain more complex information about exoplanet physical properties; however, that additional complexity also makes interpretation of the data more difficult.  I will discuss the extra boons and challenges that come with these newer measurements, and present my own work on using three-dimensional atmospheric circulation models to guide and interpret observations.  In particular, I will show how we can combine different types of measurements in order to robustly measure, or at least constrain, exoplanet physical properties such as: wind speeds, magnetic field strengths, rotation rates, or obliquities.  As exoplanet missions identify more bright targets for atmospheric characterization, we will be able to apply these techniques to planets beyond hot Jupiters, in our inevitable march toward identifying potentially habitable worlds


 Keren Sharon, U. Michigan

 The Universe, Magnified: The Power of Gravitational Lensing

Abstract: When did the Universe form its first galaxies? What do galaxies look like at the epoch when the Universe formed most of its stars? Some of the answers to those questions (and others) await a new generation of large ground and space based telescopes. In the meanwhile, strong gravitational lensing has become a useful tool to boost the power of present day telescopes, enabling detailed studies of galaxies that are otherwise either too dim or have too small of an angular size on the sky.

3-15-17 Maryam Modjaz NYU

 Stellar Forensics with the Most Powerful Explosions in the Universe

Abstract: Supernovae and Gamma-ray Bursts are exploding stars and constitute the most powerful explosions in the universe. Since they are visible over large cosmological distances, release elements heavier than Helium, and leave behind extreme remnants such as black holes, they are fascinating objects, as well as crucial tools for many areas of astrophysics, including cosmology. However, for many years the fundamental question of which stellar systems give rise to which kinds of explosions has remained outstanding. I will discuss the exciting recent progress that we have made on this question in key areas by publishing and thoroughly analyzing the largest data sets in the world. I will conclude with an outlook on how the most promising venues of research - using the existing and upcoming innovative large time-domain surveys, such as the Large Synoptic Survey Telescope - will shed new light on the diverse deaths of stars.

Colloquium Archive:

Date Speaker Title (mouseover for abstract)

Avery Broderick (Waterloo/Perimeter Institute)

The Current and Future View from the Edge


Tom Greene (NASA Ames)

Characterizing Exoplanets with JWST

10-12-2016 Ciska Kemper (ASIAA)

The production of dust by evolved stars in the Magellanic Clouds and other galaxies

10-19-2016 George Becker (UCR)

The Intergalactic Medium Near Reionization

10-26-2016 Quinn Konopacky (UCSD)

Constraining Planet Formation with Directly Imaged Exoplanets

11-02-2016 Ryan Foley (UCSC)

Continuing the Legacy of Supernova Cosmology

11-09-2016 Rachel Somerville (Rutgers)

The Connection between Quenching and Galaxy Structure

11-16-2016 Chuck Steidel (Caltech)

Reconciling the Stellar and Nebular Spectra of High Redshift Galaxies

11-23-2016 NO COLLOQUIUM Thanksgiving Day Observance
11-30-2016 Sarah Ballard (MIT)

The Grand Planetary Ensemble



Date Speaker Title (mouseover for abstract)

Bryan Gaensler                           (Dunlap Institute)

Radio Polarimetry and Cosmic Magnetism


Joel Primack (UCSC)

New Insights on Galaxy Formation from Observations and Simulations


Kevin Schawinski (ETH)

The galaxy-black hole connection

04-20-2016 Pascal Oesch (Yale)

Galaxy Build-up at Cosmic Dawn: Hubble's Lasting Legacy

04-27-2016 Laura Lopez (OSU)

Observational Assessment of Stellar Feedback in Star-Forming Regions


Nitya Kallivayalil                          (U. of Virginia)

Probing the Dark Halo of the Milky Way

05-11-2016 Scott Gaudi (OSU)

Gravitational Microlensing Surveys for Exoplanets: A Watershed

05-18-2016 Jay Strader (MSU)

Black Holes in Globular Clusters


Rennan Barkana (TAU)

Novel Measurements of Starlight from Cosmic Dawn to the Present

06-01-2016 Daryl Haggard (McGill)

Interpreting Sgr A*'s Most Luminous X-ray Flares



Date Speaker Title (mouseover for abstract)

 No Colloquium

No Colloquium (AAS)


 Dan Stark (U of Arizona)

Galaxies in the Reionization Era


 Alyssa Goodman              (Harvard)

The Intricate Role of Cold Gas and Dust in Galaxy Evolution at Early Cosmic Epochs

01-27-2016  Mark Swain (JPL)

Exoplanet Transit Spectroscopy Surveys:Present and Future

02-03-2016  Avi Shporer (JPL)

Science with orbital phase curves in the space age

02-10-2016  Alice Shapley (UCLA)

The MOSFIRE Deep Evolution Field (MOSDEF) Survey: Insights into the Evolving Physical                                                    Conditions in Star-forming Regions at High Redshift

02-17-2016  No Colloquium


02-24-2016  Sarah Tuttle (UT Austin)

Alt-Instrumentation – From Ground to Space


 David Spergel (Princeton)

From '~' to precision science: Cosmology from 1995 to 2025

03-09-2016  Kevin Bundy (IPMU)

Galaxy Death and the Role of 'Red Geysers'

Date Speaker Title (mouseover for abstract)

 Paul Schechter (MIT)

The measurement of stellar masses in <z> = 0.5 galaxies using the micro-lensing of quasars


 Alycia Weinberger (Carnegie DTM)

Tracing the Formation of Planetary Systems



Veteran's Day

 Courtney Dressing (Harvard/CalTech)

The Frequency and Composition of Small Exoplanets



Thanksgiving Day Observance

 Karin Oberg (Harvard)

The Chemistry of Planet Formation

Date Speaker Title (mouseover for abstract)
04-01-2015  Ji Wang (Yale)

Planet Formation Under Different Environments

04-08-2015  Gwen Rudie (Carnegie/Princeton)

Observing the Baryon Cycle: The Circumgalactic and Interstellar Medium of Galaxies at 2<z<3

04-15-2015  Ann-Marie Madigan (UC Berkeley)

From the Solar System to the Galactic center: unstable disks and infalling clouds

04-22-2015  Nir Shaviv (IAS Princeton/ Hebrew University of Jerusalem)

Cosmic rays and the structure of the Milky Way’s Disk: From the Pamela Anomaly to Paleoclimatology

04-29-2015  Dimitri Mawet (Caltech)

The future of exoplanet imaging and spectroscopy at Keck

05-06-2015  Eric Ford (Penn State)

Characterizing the Distribution of Planetary Architectures with Kepler: The Formation of Systems with Tightly-packed Inner Planets (STIPs)

05-13-2015  Dimitrios Psaltis (U. of Arizona)

Testing General Relativity with the Event Horizon Telescope

05-20-2015  Daniel Stern (JPL)

Surprising New Insights into Quasars from the WISE Satellite

05-27-2015  Drew Newman (Carnegie/Princeton)

Observing the Assembly of Massive Galaxies

06-03-2015  Fabienne Bastien (Penn State)

Convection in Cool Stars, as Revealed through Stellar Brightness Variations