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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 4:00 - 5:00 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. Shelley Wright and Prof. George Fuller.

Upcoming Seminars

Fall 2017

September 19, 2017

NOTE: Special Talk
on Sexual Harassment
Tuesday @ 1:00 in the Qualcomm Room, Jacobs Hall - 1st floor (EBU1)

Joan Schmelz
Arecibo Observatory Deputy Director
Universities Space Research Association (USRA)

 With the issue of sexual harassment repeatedly in the news, one hopes that student groups, academic departments, and university admini- strators are discussing what can be done to eliminate this plague from our community. There are fundamental flaws in the current system where the pressure for righting these wrongs is often placed on the shoulders of young women who can be in the most vulnerable stages of their careers, and where such harassing behavior can remain an “open secret” for years. We have to change the system – to train those with privilege to become not only allies but advocates who will add their voices and prestige to fight for right, to create a “safe space” where anyone facing sexual harassment can get help and advice, and to shine a light on the harassers who still operate in the shadows. Universities and their senior staff can do more to deter, detect, and effectively address inappropriate behavior. This talk gives advice and suggestions on how to change the system and is intended for students, faculty, and administrators.

This event is sponsored by the Division of Physical Sciences and the Office of the Vice Chancellor for Equity, Diversity, and Inclusion at UCSD.

September 20, 2017

 "History, Science & Discovery at the Arecibo Observatory"

Joan Schmelz
Arecibo Observatory Deputy Director
Universities Space Research Association (USRA)

 The Arecibo Observatory celebrated its 50th anniversary in 2013 and continues to do cutting-edge research in radio astronomy, planetary radar, and atmospheric science. Early discoveries include the 33-ms period of the pulsar in the Crab Nebula supernova remnant and the 59-day rotation rate of the planet Mercury. Later discoveries include the first binary pulsar, the first radar maps of Venus, the first megamaser galaxy, the first millisecond pulsar, and the first extrasolar planet. Arecibo’s legacy of great discoveries continues unbroken to the this day with the new detection of the first repeating Fast Radio Burst, which rules out an entire class of models requiring catastrophic explosions. Arecibo’s planetary radar observations provides information on the orbit and physical properties of the Near-Earth Asteroid, Bennu, paving the way for NASA’s OSIRIS-REx mission, which will travel Bennu and return a sample to Earth in 2023. The atmospheric radar has recently discovered the unexpected large amount of helium in the upper reaches of the ionosphere, forcing us to revise our understanding of the coupling between the ionosphere and the plasmasphere. High priority science investigations now include using pulsars to search for gravitational waves, characterizing near-Earth objects that threaten civilization, and heating the ionosphere to perform controlled plasma experiments. This talk will discuss 50 years of science at the Arecibo Observatory, from the early construction to the latest discoveries.

Bio: Joan Schmelz works for Universities Space Research Association (USRA) and currently serves as the deputy director of the Arecibo Observatory in Puerto Rico. She was honored in 2015 as one of Nature’s top ten people who made a difference in science for her work fighting sexual harassment. She is a former program officer for the National Science Foundation's Division of Astronomical Sciences and the former chair of the American Astronomical Society's Committee on the Status of Women in Astronomy. She was a professor at University of Memphis for over 20 years where she recently resigned her tenured position. During that time, she was a regular visitor to the Harvard-Smithsonian Center for Astrophysics, where she worked with multiple colleagues developing observational constraints to test models of solar coronal heating. Schmelz has published papers on a variety of astronomical topics including stars, galaxies, interstellar matter, the cosmic microwave background, and the Sun using data from ground- and space-based telescopes at every band of the electromagnetic spectrum (except gamma rays). She also writes regular posts for the Women in Astronomy BlogSpot on topics such as unconscious bias, stereotype threat, and the gender gap.

September 27, 2017

 "Formation of Close-in Super-Earths, Super-puffs and Ultra-Short Period Planets"

Eve Lee
Sherman Fairchild Postdoctoral Scholar in Theoretical Physics

 The riddle posed by super-Earths is that they are not Jupiters: their core masses are large enough to trigger runaway gas accretion, yet somehow super-Earths accreted atmospheres that weigh only a few percent of their total mass. In this talk, I will demonstrate that this puzzle is solved if super-Earths formed late, in environments akin to the inner cavities of transitional disks. Super-puffs present the inverse problem of being too voluminous for their small masses. I will show that super-puffs most easily acquire their thick atmospheres as dust-free, rapidly cooling worlds outside 1 AU, and then migrate in just after super-Earths appear. Super-Earths and Earth-sized planets around FGKM dwarfs are evenly distributed in log orbital period down to ~10 days, but dwindle in number at shorter periods. I will demonstrate that both the break at ~10 days and the slope of the occurrence rate down to ~1 day can be reproduced if planets form in disks that are truncated by their host star magnetospheres at co-rotation. Planets can be brought from disk edges to ultra-short (<1 day) periods by asynchronous equilibrium tides raised on their stars.

October 4, 2017

 "Secondary astrophysical production of anti-deuteron and anti-Helium3
cosmic ray"

Ryosuke Sato
Postdoctoral Fellow
Weizmann Institute

 Cosmic-ray anti-deuterium and anti-helium have long been suggested as probes of dark matter, as their secondary astrophysical production was thought extremely scarce.

But how does one actually predict the secondary flux? Anti-nuclei are dominantly produced in pp collisions, where laboratory cross section data is lacking.

We make a new attempt at tackling this problem by appealing to a scaling law of nuclear coalescence with the physical volume of the hadronic emission region. The same volume is probed by Hanbury Brown-Twiss (HBT) two-particle correlations.

We demonstrate the consistency of the scaling law with systems ranging from central and off-axis AA collisions to pA collisions, spanning 3 orders of magnitude in coalescence yield. Extending the volume scaling to the pp system, HBT data allows us to make a new estimate of coalescence, that we test against preliminary ALICE pp data.

For anti-helium the resulting cross section is 1-2 orders of magnitude higher than earlier estimates. The astrophysical secondary flux of anti-helium could be within reach of a five-year exposure of AMS02.

October 11, 2017

Anna Nierenberg
UC Chancellor Postdoc
UC Irvine

October 18, 2017

 "Outflows in active galactic nuclei"

Sibasish Laha
Postdoctoral Scholar

 The last couple of decades have seen the most crucial developments in the understanding of AGN winds. This can be attributed mostly to the advent of great observatories like ALMA (Molecular outflows), Hubble space Telescope (UV outflows), XMM-Newton and Chandra (X-ray outflows). Coupled with advancement in theories, our understanding about AGN outflows in different wavelength bands (Radio, Infra-red, Optical, UV and X-rays) has never been better, yet there are many outstanding questions which we still need to answer. We present here the results from a comprehensive study of the warm absorbers (WA) in X-ray in a flux limited complete sample of Seyfert galaxies (WAX-I, Laha et. al. 2014, MNRAS 441, 2613), using high resolution XMM-Newton data. We found that the WA clouds are present in around 65% of the sources. We also found a gap in the ionization parameter distribution of the WA, pointing to thermal instability. We have found evidences of WA being radiatively driven and they originate from the dusty torus (WAX-II, Laha et al. 2016, MNRAS 457, 3896L). The dust opacity can also play a leading role in driving these clouds. These WA clouds can sometimes give “effective feedback” to the host galaxies. In another extensive sample study of AGN exhibiting molecular outflows (to be submitted), we find that the AGN plays the most important role in driving these large kpc scale outflows. However, we are still uncertain how the AGN interacts with these large scale molecular clouds.

October 25, 2017

 "What matter(s) around galaxies? Shining a bright light on the cold phase of the
Circum Galactic Medium"

Sebastiano Cantalupo
Assistant Professor
Institute for Astronomy, ETH Zurich

 I will present the results of a new program to directly detect and study high-redshift cosmic gas in emission using bright quasars and galaxies as external "sources of illumination’. By looking in emission rather than in absorption, this program provides new and unique information on the morphology and physical properties of the cold phase of the Circum Galactic Medium (CGM) on both large and small scales. In particular, I will show results from ultra-deep narrow-band imaging and recent integral-field-spectroscopy as a part of the MUSE Guaranteed Time of Observation program that revealed numerous giant Lyman-alpha emitting filaments extending up to several hundred kpc around quasars and bright galaxies. I will discuss how the unexpectedly high luminosities of these systems, together with the constraints from Helium and metal extended emission, represent a challenge for our current understanding of cosmological structure formation. In particular, I will show that current observations suggest that a large amount of “cold" and dense gaseous “clumps" should be present around high-redshift galaxies and I will present our first attempts to understand the origin and nature of these structures in the Early Universe. At the same time, current galaxy formation models lack an efficient mechanism to prevent too much cooling of the CGM onto galaxies at later epochs and rely on very strong “ejective" feedback. In the second part of the talk, I will show how the interaction between high-energy radiation from star-forming galaxies and the CGM - still ignored by almost all galaxy formation models - provides a natural way to prevent excessive CGM cooling onto galaxies (“preventive” feedback). Finally, I will mention recent COS observations that provide support for the importance of this “preventive” feedback mechanism and, at the same time, can give vital constraints on the SED of star-forming galaxies in the FUV range.

November 1, 2017

Jarred Roberts
Postdoctoral Scholar

November 8, 2017

Cameron Hummels
NSF Postdoc Fellow

November 15, 2017

 "Clouds in the Galactic Sky - The Gaseous Halos of Galaxies from the synergy of Observation and Theory"

Cameron Liang
Graduate Student, Department of Astronomy and Astrophysics
University of Chicago

 Undoubtedly, the Earth's atmosphere is an integral part of its ecosystem. Everyday weather and long-term climate of the atmosphere are directly linked to activities on the surface of the Earth and vice versa. Gaseous halos, known as the circumgalactic medium (CGM), are the equivalent atmosphere of galaxies. The galactic climate arises from infalling gas from intergalactic space, enriched materials launched from the interstellar medium and more. The CGM is one of the largest gas reservoirs with complex baryonic cycles. It is paramount to improve our understanding of the CGM to achieve a complete picture of galaxy formation and evolution.

In this talk, I will first focus on the observational efforts to place empirical constraints on the spatial extent and the metallicity of the CGM. I will then present some theoretical work on the baryonic cycles in cosmological zoom-in simulations and show that the CGM provides orthogonal constraints to star formation and feedback processes. Finally, I will present a new high-resolution (< 1pc) simulation study to model the CGM more systematically with radiative cooling, thermal conduction, and magnetic fields.

November 29, 2017

 "Cosmology Results from the Dark Energy Survey Year 1"

Elisabeth Krause
Research Scientist

 This talk presents cosmology constraints from a combined analysis of galaxy clustering and weak gravitational lensing, using 1321 deg2 of griz imaging data from the first year of the Dark Energy Survey (DES Y1). The analysis combines (i) the cosmic shear correlation function of 26 million source galaxies in four redshift bins, (ii) the galaxy angular autocorrelation function of 650,000 luminous red galaxies in five redshift bins, and (iii) the galaxy-shear cross-correlation of luminous red galaxy positions and source galaxy shears. These three measurements yield consistent cosmological results, and provide constraints on the amplitude of density fluctuations (S8 = 0.794+0.029-0.027) and dark energy equation of state (w = -0.80+0.20-0.22) that are competitive with those from Planck cosmic microwave background measurements.

I will describe the validation of measurements and modeling from catalogs to cosmology, and highlight cosmology constraints from the combination of DES Y1 with external data sets.

Based on DES Collaboration 2017 (1708.01530) and supporting papers.

December 6, 2017

 "The Breakthrough Listen Search for Advanced Life Beyond Earth”

Andrew Siemion
Director, Berkeley SETI Research Center
UC Berkeley

 The Breakthrough Listen Initiative is an ambitious effort to conduct the most comprehensive and sensitive search for advanced extraterrestrial life in humanity’s history. Breakthrough Listen has secured approximately 20% of the time on two of the largest radio telescopes in the world, the 64m Parkes Telescope in NSW, Australia and the 100m Green Bank Telescope at Green Bank Observatory in West Virginia, along with 36 nights per year on the 2.4m Automated Planet Finder at Lick Observatory. Breakthrough Listen has also entered into an agreement with the National Astronomical Observatory of China to collaborate on the development of search techniques, software and observing procedures for the 500m FAST Telescope, and the Jodrell Bank Observatory / University of Manchester to work together in a similar fashion toward developing SETI capabilities on the 76m Lovell Telescope and the MERLIN network.

Breakthrough Listen observations at the APF employ the Levy Spectrometer to conduct ``spectroscopic optical SETI’’ observations, searching for artificially narrow spectral lines that are known only to arise from technological sources (lasers). At the GBT and Parkes, Breakthrough Listen is deploying state-of-the-art digital backends capable of searching for a wide variety of signals indicative of a technological source, across many GHz of instantaneous bandwidth. As of this writing, Breakthrough Listen has deployed a 6 GHz system at the Green Bank Telescope and a 5 GHz system at the Parkes Telescope.

The current Breakthrough Listen target list includes a spectral-type complete sample of nearby stars, 100 nearby galaxies spread over all morphological types, a complete survey of the galactic plane and exotic objects and targets of opportunity (e.g. KIC 8462852, FRB121102). The Breakthrough Listen team is currently exploring opportunities to engage in commensal SETI programs with the SKA and its precursors. These potential extensions to the Breakthrough Listen program would allow significant expansion of the Breakthrough Listen target list and would lay the groundwork for extremely high sensitivity observations with the full SKA. These latter observations would be the first SETI ever conducted that would be sensitive to Earth-level leakage radiation from nearby stars.

Here I will review the Breakthrough Listen program, current observational capabilities and latest results.

Winter 2018

January 17, 2018

Jérémy Chastenet
Postdoctoral Scholar

January 24, 2018

Victor Robles
Postdoctoral Researcher
UC Irvine

Spring 2018

April 25, 2018

Jorge Pineda
Research Scientist

May 9, 2018

Jessica Werk
Assistant Professor
University of Washington

May 16, 2018

Ruth Murray-Clay
UC Santa Cruz