Astronomy Colloquia
Astronomy and Astrophysics Colloquia - Usually Wednesdays, 3 PM, BSLC 001, unless otherwise specified. Reception starts at 4 PM in TAAC 71; persons with a disability who believe they may need assistance, please call the departmental secretary in advance at 773-702-8203 or email See also the list of KICP Wednesday Colloquia which alternate with the Astronomy and Astrophysics Colloquia.

Current & Future Astronomy Colloquia
DateTalk TitleSpeaker
April 1, 2015TBAKendrick Smith, Perimeter Institute
April 15, 2015Probing the Nature of InflationDaniel Green, University of Toronto
April 29, 2015TBAAndrew MacFadyen, New York University
May 13, 2015TBAPiero Madau, University of California Observatories
May 27, 2015TBAEdo Berger, Harvard University
June 10, 2015TBAJosh Winn, MIT

Past Astronomy Colloquia
DateTalk TitleSpeaker
March 11, 2015Studies of Star-Forming Galaxies in the Reionization EraRichard Ellis, Caltech
February 25, 2015Galaxies in the Reionzation EraDaniel Stark, University of Arizona
February 11, 2015Triumphs and tribulations of near-field cosmology with wide-field surveys: a biased perspectiveBeth Willman, Haverford College
January 28, 2015Origins and Demographics of Super-Earth and Sub-Neptune Sized PlanetsLeslie Rogers, Caltech
January 21, 2015The Circumgalactic and Interstellar Medium of Star-Forming Galaxies at 2<z<3Gwen Rudie, Observatories of the Carnegie Institution for Science
January 14, 2015Frontiers in Cosmology and Radio Astronomy: 21cm cosmology as a probe of reionization and beyondAdrian Liu, University of California, Berkeley
January 7, 2015Probing structure formation beyond LCDMNeal Dalal, University of Illinois, Urbana-Champaign

Probing structure formation beyond LCDM
January 7, 2015 | BSLC 001 | 3:00 PM | Host: Hsiao-Wen Chen
Neal Dalal, University of Illinois, Urbana-Champaign

Cosmological structure formation has long been recognized as a sensitive probe of fundamental physics, especially physics beyond the Standard Model, and recent years have seen tremendous progress in our understanding of structure formation, both from the observational and theoretical sides. In this talk, I will describe some of my group's work on this subject. First, I will discuss a novel method we have developed for numerically simulating nonlinear structure formation in cosmologies where traditional N-body simulations produce large errors. I'll present preliminary results of our simulations for cosmologies with massive neutrinos, and I will describe a new potential signature of neutrino mass in large-scale structure. Finally, I will describe how upcoming ALMA observations of sources from the South Pole Telescope will vastly improve our knowledge of small-scale cosmic structure, thereby constraining the physics of inflation and dark matter.

Frontiers in Cosmology and Radio Astronomy: 21cm cosmology as a probe of reionization and beyond
January 14, 2015 | BSLC 001 | 3:00 PM | Host: Hsiao-Wen Chen
Adrian Liu, University of California, Berkeley

In recent years, 21cm cosmology has emerged as an exciting new way to map our Universe. By using the 21cm hyperfine transition as a tracer of neutral hydrogen, one is sensitive not only to the large scale distribution of matter, but also to the astrophysical conditions of the high-redshift intergalactic medium (IGM). The redshifted 21cm line is therefore particularly well-suited for understanding the as-yet unobserved Epoch of Reionization (EoR), a key part of our cosmic history when the first luminous objects were formed and systematically ionized the IGM.

In this talk, I will highlight recent progress in 21cm cosmology, including recent observations from the Precision Array for Probing the Epoch of Reionization (PAPER). These observations disfavor “cold reionization” scenarios, where early luminous sources did little to reheat the IGM. Along the way, I will discuss novel techniques that have been developed for moving beyond technical hurdles (such as foreground contamination) to a first detection of the cosmological 21cm signal. I will conclude by introducing the Hydrogen Epoch of Reionization Array (HERA), a recently commenced experiment that promises to make high signal-to-noise measurements of the power spectrum of 21cm emission. This will not only provide new and direct observational constraints on the EoR, but will also benefit other cosmological probes by reducing uncertainties on a key epoch of cosmic history, thus transforming 21cm cosmology from a promising theoretical idea to a practical way to probe our Universe.

The Circumgalactic and Interstellar Medium of Star-Forming Galaxies at 2<z<3
January 21, 2015 | BSLC 001 | 3:00 PM | Host: Hsiao-Wen Chen
Gwen Rudie, Observatories of the Carnegie Institution for Science

The exchange of baryons between galaxies and their surrounding intergalactic medium (IGM) is a crucial but poorly-constrained aspect of galaxy formation and evolution. I will present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey designed to explore both the physical properties of high-redshift galaxies and their connection with the surrounding intergalactic baryons. The KBSS is optimized to trace the cosmic peak of star formation (z~2-3), combining high-resolution spectra of 15 hyperluminous QSOs with densely-sampled galaxy redshift surveys surrounding each QSO sightline. I will characterize the physical properties of the gas within the circumgalactic medium (CGM) through measurements of the spatial distribution, column densities, and kinematics of ~6000 HI absorbers surrounding ~900 foreground star-forming galaxies that lie within 50 kpc to 3 Mpc of a QSO sightline. This analysis provides clear evidence of gas inflow and outflow as well as accretion shocks or hot outflows from these forming galaxies. My ongoing study of metallic absorbers in these fields will provide detailed information about the enrichment patterns and overall abundance of metals as a function of distance and velocity, providing a high-fidelity probe of the nature and sphere of influence of galaxy-scale outflows at high-z. I will also discuss KBSS-MOSFIRE, a rest-frame optical spectroscopic survey of more than 800 galaxies in these same QSO fields. These data provide new insight into the physical properties of HII regions at high redshift which show remarkable differences in their ionization and excitation conditions compared to low-redshift star-forming regions. These results have significant implications for both diagnostics of the chemical abundances of high-z galaxies as well as our understanding of massive stars during the peak of cosmic star formation.

Origins and Demographics of Super-Earth and Sub-Neptune Sized Planets
January 28, 2015 | BSLC 001 | 3:00 PM | Host: Hsiao-Wen Chen
Leslie Rogers, Caltech

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. The nature of planets in this regime is not known; terrestrial super-Earths, mini-Neptunes with hydrogen-helium gas layers, and water-worlds with several tens of percent water by mass are all a-priori plausible compositions. Disentangling the contributions from each of these scenarios to the population of observed planets is a critical missing link in our understanding of planet formation, evolution, and interior structure. I will review individual highlights from the diverse complement of sub-Neptune-size planets discovered to date, and present statistical analyses constraining the nature and origins of short-period rocky planets. With the suite of space-based exoplanet transit surveys on the horizon (K2, TESS, CHEOPS and PLATO) and continuing development of ground-based spectrographs (e.g., MAROON-X and G-CLEF), the pace of exoplanet discovery and characterization is poised to continue accelerating. I will conclude by describing pathways forward to identify bulk composition trends in the growing census of known exoplanets and to connect these composition trends back to distinct planet formation pathways.

Triumphs and tribulations of near-field cosmology with wide-field surveys: a biased perspective
February 11, 2015 | BSLC 001 | 3:00 PM | Host: Hsiao-Wen Chen
Beth Willman, Haverford College

Over the last decade, wide-field surveys have revolutionized our view of the Milky Way’s stellar halo and dwarf galaxy population. Much of this observational progress has been motivated by a series of apparent “crises” for our cosmological model: the missing satellites problem, too big to fail, and the apparent planar distribution of dwarf satellite galaxies. These challenges have effectively functioned as flashlights pointing us to interesting galaxy formation physics. I will highlight related observational progress in our understanding of galaxy formation using near-field observations. I will then focus on the limiting impacts of observational bias and ways that current and future surveys will be used to tackle these biases. In particular, I will present new predictions for the number of Milky Way dwarf galaxies expected to be discovered in DES and LSST, RR Lyrae stars as a tool to discover dwarf galaxies in previously unexplored territory, and the use of M giant stars to map the Milky Way’s halo beyond its virial radius.

Image credit: J. Bullock, M. Geha, R. Powell

Galaxies in the Reionzation Era
February 25, 2015 | BSLC 001 | 3:00 PM | Host: Hsiao-Wen Chen
Daniel Stark, University of Arizona

Deep infrared images from the Hubble and Spitzer Space Telescopes have recently pushed the cosmic frontier back to just 500 million years after the Big Bang, delivering the first reliable census of galaxies in what is likely the heart of the reionization era. I will discuss implications of these results for the build-up of stellar mass in early galaxies. I will then present the latest results of a large ground-based spectroscopic program aimed at using the Lyman-alpha emission line as a probe of the ionization state of the IGM at z>7. The results indicate that Lyman-alpha is strongly attenuated in galaxies at z=7-8, as would be expected if the IGM is still partially neutral. Finally, I will introduce a new approach to the spectroscopic study of galaxies at z>7. I will demonstrate that various metal lines in the rest-UV are much stronger in early galaxies than we expected. I will discuss a large new observational campaign aimed at targeting these lines in the brightest known gravitationally-lensed galaxies with the goal of providing the first constraints on the metallicity, ionizing spectrum, and stellar populations of galaxies in the reionization era.

Studies of Star-Forming Galaxies in the Reionization Era
March 11, 2015 | BSLC 001 | 3:00 PM | Host: Hsiao-Wen Chen
Richard Ellis, Caltech

Deep exposures with the Hubble Space Telescope (HST) have provided the primary evidence that star-forming galaxies were present in the first billion years of cosmic history. Sometime during this early period the intergalactic medium transitioned from a neutral gas to one that is fully ionized. How did this 'cosmic reionization' occur and were star-forming galaxies responsible? Recent imaging of deep fields with HST's Wide Field Camera 3 in conjunction with Spitzer photometry and Keck spectroscopy has provided important new insight into understanding when reionization occurred and the role of early galaxies in the process. Gravitational lensing by foreground clusters is providing complementary evidence. I will review this rapid progress in our understanding of what could be considered the last missing piece in our overall picture of cosmic history and discuss the remaining challenges ahead of future facilities such as TMT, GMT and JWST.

April 1, 2015 | BSLC 115 | 3:00 PM | Host: Wayne Hu
Kendrick Smith, Perimeter Institute


Probing the Nature of Inflation
April 15, 2015 | BSLC 001 | 3:00 PM | Host: Wayne Hu
Daniel Green, University of Toronto

The idea that the early universe included an era of accelerated expansion (Inflation) was proposed to explain very qualitative features of the first cosmological observations. Since then, our observations have improved dramatically and have lead to high precision agreement with the predictions of the first models of inflation, slow-roll inflation. At the same time, there has been significant growth in the number of mechanisms for inflation, many of which are qualitatively distinct from slow-roll.
Nevertheless, most of these ideas are also consistent with current data. In this talk, I will first review inflation and its current observational status. I will then discuss the important theoretical targets for the future and the prospects for achieving them.

April 29, 2015 | BSLC 001 | 3:00 PM | Host: Wayne Hu
Andrew MacFadyen, New York University


May 13, 2015 | BSLC 001 | 3:00 PM | Host: Wayne Hu
Piero Madau, University of California Observatories


May 27, 2015 | BSLC 001 | 3:00 PM | Host: Wayne Hu
Edo Berger, Harvard University

June 10, 2015 | BSLC 001 | 3:00 PM | Host: Wayne Hu
Josh Winn, MIT