Jump to: navigation, search

Astrophysics Seminars


The CASS Astrophysics Seminar features world-class astrophysicists from around the world speaking on current topics of research. Presentations are aimed at the graduate and post-graduate level, but are open to the general public. CASS seminars take place on Wednesdays from 3:00 - 4:00 p.m. in 383 SERF (Marlar Seminar Room), unless otherwise noted. You can watch a live stream of the talk or prior talks at the CASS Seminar YouTube Channel. The organizers are Prof. Quinn Konopacky and Dr. Alexei Kritsuk.

Upcoming Seminars

Spring 2019

May 22, 2019

 "The Gobbling Monsters within the Hot DOGs"

Chao-Wei Tsai
NAOC, Beijing

 Hot, Dust-Obscured Galaxies, or Hot DOGs, are a class of distant dust-enshrouded galaxies with extremely high luminosity, including several "Extremely Luminous Infrared Galaxies" (ELIRGs) that reach 10^14 L_Sun. Selected by their utmost red colors in WISE bands, their SEDs incorporating WISE, Spitzer, and Herschel photometry indicate that hot dust dominates the bolometric luminosity. The SED analysis suggests a close-to-spherical dust distribution with a range of temperatures. These hyperluminous sources are likely powered by highly obscured active galactic nuclei (AGN), and are unlikely to be lensed. The measured masses of the monstrous black holes within these Hot DOGs using Keck/MOSFIRE reflects that they are accreting at a rate close to the Eddington limit. This hyperluminous, highly obscured population may represent a special evolutionary stage prior to the red quasar and optical quasar phases.

In the case of W2246-0526, the most luminous Hot DOG yet identified, its total luminosity of 3.6 x 10^14 L_Sun at z = 4.601 make it well into the Extremely Luminous Infrared Galaxy (ELIRG, > 10^14 L_Sun) range, and among the few most luminous galaxies known thus far. Using the broad MgII-2799A emission line from Keck/OSIRIS and the blueshift-corrected broad CIV line from Keck/LRIS, we estimate the black hole mass of the obscured AGN to be ~ 4x10^9 M_Sun, and the corresponding Eddington ratio is L_AGN/L_Edd = 2.8. The high Eddington ratio of W2246-0526 may reach the level where the luminosity is saturating due to photon trapping in the accretion flow, and be insensitive to the mass accretion rate. As a result, the black hole mass growth rate could exceed the apparent accretion rate derived from the observed luminosity.

May 29, 2019

 "Solar Eclipses, Solar Transits by Venus and Mercury, and Pluto/KBO Occultations"

Jay Pasachoff
Professor of Astronomy
Williams College

 On August 31, 2017, a total solar eclipse's band of totality swept across the Continental United States from coast to coast for the first time in 99 years. I will show and discuss some of the the images and spectra my team has obtained at the most recent eclipses, including total eclipses in Easter Island (2010), Australia (2012), Gabon (2013), Svalbard (2015), Indonesia (2016), and the United States (2017) as well as comment on annular or partial eclipses observed elsewhere. I will discuss our observational tests underway for the comparison of models of coronal heating. I will also discuss plans for the 2019 and 2020 total eclipses that cross Chile and Argentina.

I will also report on our observations of transits not only of the Sun by the Moon (that is, a solar eclipse), but also across the Sun by Venus and by Mercury. I will discuss ground-based imaging and Total Solar Irradiance space measurements as well as observations of the 2012 transit of Venus with Hubble by reflection off Jupiter and directly with Cassini from Saturn, providing solar-system close-up analogues to exoplanet transits.

I will close with some discussion of our stellar-occultation observations by Pluto and other objects in the outer solar system, and their relation to the recent New New Horizons’ flyby of Ultima Thule, a billion miles beyond Pluto.

My work at solar eclipses has recently been mainly supported by the US National Science Foundation’s Atmospheric and Geospace Sciences Division, and the Committee for Research and Exploration of the National Geographic Society. The solar-system occultation work has been supported by NASA.

June 5, 2019

 "Reaching Beyond our Spectral Grasp of Star and Planet Formation"

Gregory Mace
Research Associate
University of Texas, Austin

 New discoveries generally come from low-resolution observations with weak signals. We are then tasked with significantly increasing the signal and resolution in order to characterize these new objects. Some of the most challenging objects to characterize are young stellar objects in the dusty environments of formation and faint exoplanets close to their host stars. High-resolution infrared spectroscopy allows us to look through the obscuring dust and separate exoplanet and host star spectra. The silicon immersion gratings developed at UT Austin maintain high throughput, while providing a broad spectral grasp, with a fraction of the instrument volume of traditional reflective gratings. I will discuss the application of immersion grating spectrographs (IGRINS, iSHELL, MagNIFIES, GMTNIRS) and the order of magnitude improvement in simultaneous wavelength coverage, throughput, and/or resolution that they enable. Specifically, I will discuss their role in confirming and then characterizing exoplanets, while also tracing the star formation process down to planetary masses. Since these immersion grating instruments are new, or still in development, there is considerable return on efforts to make early use of them to answer a variety of science questions. Additionally, future instrument collaborations for ground, airborne, and space facilities in the 1-10 micron region have the potential to reveal new characteristics at a variety of spectral resolutions.

June 12, 2019

 "Long shot technologies the could enable ground breaking observations

Melville (Mel) Ulmer
Professor of Physics and Astronomy and CIERA
Northwestern University

 I will describe the concept of a unique near IR (NIR) camera and give a progress report. This NIR camera, that will operate at faster than 1 kHz frame rates, is especially useful for extreme adaptive optics observations of exoplanets. “Extreme” comes from the concept of being able to image exoplanets that are extremely faint (fainter by more than 1E6) in comparison to their host stars. Assuming success, and the existence of the TMT, we will be able to provide images (albeit not maps of the continents) of habitable planets. However, we can do even better in space, if we can launch a large enough primary mirror. For example, there is a standing request for a 16m diameter version of the Large Ultra-Violet Optical InfraRed (LUVOIR) telescope whose observations will be able detection of (if they exist) molecule spectral signatures in exoplanet atmospheres associated with life (as we know it). Thus, I will then segue into a description of a novel technology that will enable necessary corrections to a deployed space membrane mirror. For, if simply deployed without post-launch corrections, the mirror figure will not be good enough. The technology is so versatile, it can also be applied to any thin space optic that could benefit from post-launch control, e.g., the deformable mirrors in space coronagraphs. As with the NIR camera, I will give a progress report of my team’s efforts.