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Department of Astronomy
Astronomy  /  Talks & Events  /  Astronomy Colloquium
Astronomy Colloquium

Astronomy Colloquia at Caltech for 2020-2021

Colloquia are held every Wednesday during the academic year at 4pm in the Cahill Hameetman auditorium. Wine and cheese will be served in the Cahill Foyer from 5-5:30pm.

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Date Speaker Talk Title
September 30, 2020 Speaker: Di Li Title: The Commensal Radio Astronomy FAST Survey (CRAFTS)
Institute: Chinese Academy of Science
Host: Vikram Ravi Abstract: The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has released its first call for proposal and will be open to the international community next year. Based on a novel technique of high-cadence CAL injection, we have realized the world's first calibrated commensal survey mode, simultaneously taking data for pulsar search, HI galaxies, HI imaging, and FRBs. I introduce here one of the major survey plans, namely, the Commensal Radio Astronomy FAST Survey (CRAFTS, Li et al. 2018), which has discovered more than 100 new pulsars, including a few dozen MSPs, 5 new FRBs, including one new repeater. I will also briefly describe recent FAST results from CRAFTS and other dedicated programs, including new insights into the characteristic energy of FRBs, the formation process of neutron stars, the evolution of interstellar medium, etc.
October, 2, 2020 Speaker: Danielle Berg Title: Extreme UV Emission: Bridging Galaxy Evolution Across Cosmic Time
Institute: OSU / UT Austin
Host: Chuck Steidel Abstract:In the last few years, our first glimpse of the spectral properties of z∼5−7 galaxies has emerged. Deep UV spectra have revealed prominent high-ionization nebular emission lines (i.e., C IV, He II, C III]) indicating that extreme radiation fields may be characteristic of reionization-era systems. While such strong high-ionization emission lines are atypical of the well-studied z∼0−3 galaxy samples, our recent UV spectral campaigns have revealed several galaxies with analogous emission-line features to reionization-era systems. I will discuss the recent detection of extremely strong UV emission in nearby galaxies and the potential sources of their very hard ionizing radiation fields. Such strong detections of high-ionization emission lines have been linked to the leakage of Lyman continuum (LyC) photons (necessary for reionization) both theoretically and observationally. These extreme UV emission-line dwarf galaxies provide a template for the extreme conditions that are important for reionization, however their features are still poorly understood. In preparation for the coming UV window onto the early universe with the advent of ELTs and JWST, I will introduce the COS Legacy Archival Spectroscopic SurveY - an upcoming large HST program designed to disentangle the stellar and nebular spectral signatures of 45 star-forming galaxies. This program will calibrate new UV diagnostics that will allow us to trace galaxy evolution to the distant universe, unveiling the properties of reionization-era galaxies.
October 14, 2020 Speaker: Keith Hawkins Title: Galactic Archaeology with Gaia and Large Spectroscopic Surveys
Institute: UT Austin One of the key objectives of modern astrophysics is to understand the formation and evolution galaxies. In this regard, the Milky Way is a critical testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical chemical, and age distributions of its stellar populations. Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have and can been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.
Host: Jim Fuller
October 21, 2020 Speaker: Ben Oppenheimer Title: Why Measuring Black Hole Masses is Essential for Understanding the Circumgalactic Medium
Institute: University of Colorado Abstract: Ripped straight from the headlines of the award of this year's Nobel Prize in Physics, I will begin by discussing the potential connection between a galaxy's central supermassive black hole (SMBH) and its gaseous halo, the circumgalactic medium. Cosmological simulations predict a direct connection between these dramatically different scales, which causally arises from the integrated history of SMBH feedback expelling gas beyond the virial radius. While observing this connection is currently challenging, I will discuss the potential of observing hot gaseous halos in X-rays around Milky Way-mass galaxies. I plan to continue discussing Milky Way-mass halos as observed through via UV absorption line signatures, including changes with redshift from z=3 to today.
Host: Chuck Steidel
October 28, 2020 Speaker: Gwen Rudie Title: Feedback and Feeding: The Circumgalactic Medium of Star-Forming Galaxies during Cosmic Noon
Institute: Carnegie Observatories 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 three surveys that use classical IGM absorption line techniques to better understand gas flows and the impact of environment on the life cycle of galaxies. Most of my talk will focus on a new detailed study of metal-enriched absorbing gas in the high-z (2"<"z"<"3) circumgalactic medium (CGM) using data from the Keck Baryonic Structure Survey (KBSS). I will highlight results on the size, kinematics, and thermal properties of circumgalactic gas which provide clear evidence of unbound metal-enriched gas within the halos of "z~2" galaxies and demonstrate the CGM at cosmic noon requires constant heating or replenishment of warm gas. Collectively, these data suggest the CGM provides one of the best testing grounds for models of galaxy-scale outflows. Next, I will briefly show forthcoming results from the Cosmic Ultraviolet Baryon Survey (CUBS) which aims to understand the turn down in the cosmic star formation rate density through study of the CGM at 0.4 "<" z "<" 1.0 using HST/COS. I’ll conclude with a short discussion of a new survey underway at Magellan to map the large-scale environment of galaxies at cosmic noon, the Lyman-alpha Tomography IMACS Survey (LATIS). Collectively, these surveys constrain the nature and sphere of influence of galaxy-scale outflows and intergalactic accretion, as well as the impact of environment on galaxies at cosmic noon and beyond.
Host: Chuck Steidel
November 4, 2020 Speaker: Natasha Batalha Title: Interpreting Exoplanet Atmospheres at the Onset of Next-Generation Telescopes
Institute: NASA Ames Abstract: One of NASA's primary goals is to observationally characterize exoplanet atmospheres, understand the chemical and physical processes of exoplanets and improve the understanding of the origins of exoplanetary systems. Throughout the next decade and beyond, JWST, WFIRST, future mission concepts, and ground based telescopes will work towards achieving these goals by interpreting a diverse set of exoplanet atmosphere observations, ranging from hot gas giants to small temperate rocky worlds. Our understanding and interpretation of this full gamut of spectroscopy data will hinge on our ability to accurately link observations to theoretical models. Therefore, it is imperative that our theoretical models are equipped to tackle these problems. Leading up to this new era in exoplanetary space science, one of our goals has been to ensure that the community is equipped with robust, user-friendly, open-source, theoretical models needed to both plan and execute ground-breaking science. I will first discuss the current landscape of theoretical exoplanet model development. Then, I will discuss our recent developments in cloud, opacity, and spectroscopy models that will work together to enable effective interpretation of exoplanet spectroscopy during this next-generation of observations.
Host: Tiffany Meshkat
November 11, 2020 Speaker: Subo Dong Title: Direct Collision of White Dwarfs as a Possible Major Channel for Type Ia Supernovae
Institute: Peking University 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.
Host: Ilaria Caiasso
November 18, 2020 Speaker: Joss Bland-Hawthorn Title: Galactic seismology: the evolution of the “phase spiral” after the Sagittarius impact
Institute: University of Sydney Abstract: Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars, from white dwarfs to supergiants. The oldest components provide unique insight into how galaxies form and evolve over billions of years. This is a veritable golden age for galactic archaeology with many large surveys now under way to map both chemistry and motions for stars in the Galaxy. Detailed 6D "phase space" information combined with chemistry for millions of stars heralds a new era in how we slice up the Galactic disc. This has already enabled the most remarkable discovery to emerge from ESA's Gaia satellite — the “phase spiral”. This phenomenon, which was not foreseen, is direct evidence of giant waves crossing the disc. We discuss how the phase spiral is generated and what it may tell us about our history. We review the main science goals of Galactic seismology, and look to what the future may hold. These studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and contemporary astrophysics depends on such observations.
Host: Chris Martin
December 2, 2020 Speaker: Ann-Marie Madigan Title: Collective Gravity in the Outer Solar System
Institute: University of Colorado Abstract: There’s something odd going on in our solar system. While the planets move on nearly-circular orbits in a well-defined plane, the minor planets beyond Neptune appear to cluster together in a highly-inclined, eccentric, and tilted structure. Astronomers have invoked the presence of an additional planet (Planet 9) or even a primordial black hole in explanation. In this talk I will show that these theories may be unnecessary. In analogy with spiral arms and bars in galaxies, the collective gravity of individually small but collectively massive bodies can create such structures in the outer solar system.
Host: Tiffany Meshkat
December 9, 2020 Speaker: Rebecca Jensen-Clem Title: Exoplanet Imaging with Extremely Large Telescopes
Institute: UCSC Abstract: In the last thirty years, over 3000 planets have been discovered orbiting nearby stars. This flood of new worlds includes planets unlike any found in our own Solar System, from Jupiter-mass planets with years as short as our day to exotic rocky worlds twice as massive as the Earth. While our understanding of exoplanets' diversity has leapt forward in recent years, fundamental questions remain. For example, what are the dominant planet formation pathways? How do planets acquire their atmospheres? Is there life on other worlds? These questions can only be answered through observations of exoplanets’ spectra, where the characteristic imprints of atoms and molecules making up a planet’s atmosphere are revealed. The most promising method for obtaining spectra of diverse exoplanets is direct imaging: by nulling the light of the parent star with an optical device called a coronagraph, the planet itself can be seen and its light dispersed into a spectrum. So far, only extremely young, massive worlds have been directly imaged, while older, lower mass objects like the Earth remain hidden in the glare of their host stars. In this talk, I will describe two avenues for advancing the state-of-the-art in exoplanet imaging: 1) detecting low-mass exoplanets at Solar System separations with the W. M. Keck Observatory and Thirty Meter Telescope via predictive wavefront control and focal plane wavefront sensing, and 2) characterizing the atmospheres of directly imaged planets with polarimetry -- an untapped method for probing the physics of clouds in the atmospheres of other worlds.
Host: Mansi Kasliwal
January 6, 2021 Speaker: Ellen Zweibel Title: Cosmic Rays in Multiphase Gas
Institute: University of Wisconsin-Madison Abstract: Cosmic rays appear to be universal in galaxies with star formation or nuclear activity. They play a role in the dynamics, energy balance, and stability of the diffuse gas in these systems, and are now included in many state of the art models of galaxy formation and evolution. The exchange of energy and momentum between cosmic rays and thermal gas is largely collisionless, mediated by kinetic scale plasma waves. The properties of these waves depend both on local conditions and the global structure of the thermal gas, introducing complex feedback loops that are only beginning to be probed theoretically and incorporated into models. I will discuss some recent progress in this area, focusing on the role of interstellar clouds.
Host: Jim Fullter
January 20, 2021 Speaker: Stephen Smartt Title:
Institute: Queens University Belfast Abstract:
Host: Mansi Kasliwal
January 27, 2021 Speaker: Erin Kara Title:
Institute: MIT Abstract:
Host: Ilaria Caiazzo
February 3, 2021 Speaker: Francois Combes Title:
Institute: Observatoire de Paris Abstract:
Host: Chris Martin
February 10, 2021 Speaker: Silvia Toonen Title:
Institute: University of Birmingham Abstract:
Host: Jim Fuller
February 17, 2021 Speaker: Tim Bedding Title:
Institute: University of Sydney Abstract:
Host: Jim Fuller
March 10, 2021 Speaker: Risa Wechsler (Biard Lecture) Title:
Institute: Stanford University Abstract:
Host: Mansi Kasliwal