Astrophysics Science Division Colloquium Series
Schedule: Fall 2014

Astrophysics Science Division Colloquium Series
Schedule: Fall 2014

Recent schedules:

  2014, Spring  
2013, Fall 2013, Spring 2013, Summer
2012, Fall 2012, Spring  
2011, Fall 2011, Spring  
2010, Fall 2010, Spring  

ASD Colloquia are Tuesdays at 3:45 pm (Meet the Speaker at 3:30 pm)
in Bldg 34, Room W150 unless otherwise noted.


Aug 26 John ZuHone (GSFC) - The Physics of Gas Sloshing in the Cores of Galaxy Clusters


Sep 9 Emily Levesque (Colorado) - Discovery of a Thorne-Zytkow Object Candidate in the Small Magellanic Cloud
Sep 16 Kartik Sheth (NRAO) - Reconstructing the Mass Assembly of Galaxy Disks over the last 12 Billion Years with ALMA, HST and Spitzer
Host: Amber Straughn
Sep 23 Mario Livio (STScI) - Brilliant Blunders
Sep 30 Steve Boggs (Berkeley) - The Compton Spectrometer and Imager ULDB Science Program


Oct 7 David Merritt (RIT) - Relativistic statistical mechanics of black-hole nuclei
Oct 21 Volker Bromm (UT Austin) - The First Stars and Galaxies: Run-up to the JWST
Oct 28 Iossif Papadakis (Crete) - The energy dependence of the AGN X-ray variability


Nov 4 David Kaplan (UWM) - Finding Radio Transients with the Murchison Widefield Array and Beyond
Nov 18 -
Nov 25 Ori Fox (Berkeley/GSFC) - The Impact of Binary Stars on Our Understanding of Supernova Progenitors


Dec 2 Jacqueline Hodge (NRAO) -
Dec 9 Mariska Kriek (Berkeley) -
Dec 16 Arash Bodaghee (GCSU) - Cold Exoplanets and Eccentric Rings

The Physics of Gas Sloshing in the Cores of Galaxy Clusters

John ZuHone


Tuesday, August 26, 2014


Many X-ray observations of relaxed galaxy clusters reveal the presence of sharp, spiral-shaped discontinuities in the surface brightness of the X-ray emitting gas. Spectral analysis of these features shows that the colder gas is on the brighter side, hence they have been dubbed "cold fronts." These features arise naturally in simulations from the cool-core gas "sloshing" in the gravitational potential. Their sharpness and stability has important implications for the microphysics of the ICM. The sloshing motions may have other effects, such as contributing to the heating of the cluster core and the acceleration of relativistic particles. I will present simulations of gas sloshing in clusters, explain their formation and evolution, and discuss the implications for the physics of the ICM, including fresh results on heat conduction, viscosity, and radio mini-halos.

Discovery of a Thorne-Zytkow Object Candidate in the Small Magellanic Cloud

Emily Levesque


Tuesday, September 9, 2014


Thorne-Zytkow objects (TZOs) are a theoretical class of star in which a compact neutron star is surrounded by a large, diffuse envelope. Supergiant TZOs are predicted to be almost identical in appearance to red supergiants (RSGs), with their very red colors and cool temperatures placing them at the Hayashi limit on the H-R diagram. The best features that can be used at present to distinguish TZOs from the general RSG population are the unusually strong heavy-element lines present in their spectra. These elements are the unique products of the star's fully convective envelope linking the photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. As part of a recent high-resolution spectroscopic search, my collaborators and I have discovered a TZO candidate in the Small Magellanic Cloud. It is the first star to display the distinctive chemical profile of anomalous element enhancements thought to be unique to TZOs. The positive detection of a TZO would provide the first direct evidence for a completely new model of stellar interiors, a theoretically predicted fate for massive binary systems, and never-before-seen nucleosynthesis processes that would offer a new channel for heavy-element production in our universe.

Reconstructing the Mass Assembly of Galaxy Disks over the last 12 Billion Years with ALMA, HST and Spitzer

Kartik Sheth


Tuesday, September 16, 2014


Bars are a key signpost in the evolutionary history of a disk galaxy. When a disk is sufficiently massive, dynamically cold and rotationally supported, and sufficient time has elapsed for the baryonic matter to exchange energy and angular momentum with the dark matter halo or the outer disk, the formation of a bar is inevitable. Therefore understanding the evolution of the bar fraction as a function of the host galaxy properties and as a function of redshift provides important clues to the evolutionary history of galaxies. I will present the latest results on local bars from the Spitzer Survey of Stellar Structure in Galaxies (S4G) and discuss the observations for the declining bar fraction with redshift from the COSMOS survey. A plausible reason for the decline in the bar fraction may be that galaxy disks were too dynamically hot to host bars at higher redshift which we have investigated using the DEEP2 / AEGIS data. Together these data are beginning to provide a coherent and consistent picture for the assembly history of disks on the Hubble sequence. The star formation in these disks is also now being understood with the latest results from ALMA. I will show the latest results on the cosmological evolution of the molecular gas content in a mass-selected sample of galaxies at three epochs, z=2.2, z=1, and z=0.3 and discuss planned Cycle 1 observations of the molecular gas environment in the prototypical barred spiral NGC 1097.

Brilliant Blunders

Mario Livio


Tuesday, September 23, 2014


Even the greatest scientists have made some serious blunders. "Brilliant Blunders" concerns the evolution of life on Earth, of the Earth itself, of stars, and of the universe as a whole. In this talk, I shall concentrate on and analyze major errors committed by such luminaries as Charles Darwin, Linus Pauling, and Albert Einstein. I will also scrutinize the various types of blunders and attempt to identify their causes. Most importantly, however, I'll argue that blunders are not only inevitable, but rather part and parcel of progress in science and other creative enterprises.

The Compton Spectrometer and Imager ULDB Science Program

Steve Boggs


Tuesday, September 30, 2014


The Compton Spectrometer and Imager (COSI) is a ULDB-borne soft gamma-ray telescope (0.2-5 MeV) designed to probe the origins of Galactic positrons, uncover sites of nucleosynthesis in the Galaxy, and perform pioneering studies of gamma-ray polarization in a number of source classes. COSI uses a compact Compton telescope design, resulting from a decade of development under NASA's ROSES program - a modern take on techniques successfully pioneered by COMPTEL on CGRO. COSI performs groundbreaking science by combining improvements in sensitivity, spectral resolution, and sky coverage. The COSI instrument and flight systems have been designed for flight on NASA's 18 MCF superpressure balloon (SPB). We are now beginning a series science flights to fulfill the COSI science goals, starting with a SPB flight in 2014 from Antarctica. In this talk, I will present the redesign of the COSI instrument and payload, as well as the overall flight program and science goals of our ULDB science flight program.

Relativistic statistical mechanics of black-hole nuclei

David Merritt


Tuesday, October 7, 2014


Most galaxies are believed to contain supermassive black holes (SBHs) at their centers. While the SBHs are sometimes associated with gas, they are always associated with stars. The long-term evolution of these nuclear star clusters is driven by gravitational encounters between the stars, in a gravitational field that is dominated by the SBH. The problem is made more interesting by the fact that even small, relativistic corrections to the equations of motion can radically change the collective behavior, by destroying correlations that would otherwise drive the evolution. I will discuss recent work on this problem using N-body, analytic and Fokker-Planck methods. Applications to the Galactic center, and to the production of EMRIs (extreme-mass-ratio inspirals), will be discussed.

The First Stars and Galaxies: Run-up to the JWST

Volker Bromm

UT Austin

Tuesday, October 21, 2014


How and when did the cosmic dark ages end? I present simulations of the formation of the first stars and galaxies, discuss their feedback on the intergalactic medium, and describe ways to probe their signature with next generation facilities. I will identify the key processes and outline the major remaining uncertainties.

The energy dependence of the AGN X-ray variability

Iossif Papadakis


Tuesday, October 28, 2014


Active Galactic Nuclei emit copious amount of X-rays. I will review the spectral and timing observational properties of the AGN X-ray emission, putting emphasis on the advances we have made the last two decades in our understanding of the X-ray variability phenomenology in these systems. In general, the X-ray variations in AGN show a "red-noise" behavior, with no indication of periodic modulation. Nevertheless, characteristic time scales have been detected the last 10 years in the power-spectra of a few X-ray bright AGN. They correspond to frequencies at which the slope of the X-ray power spectrum changes from -1 (at lower frequencies) to a steeper slope of about -2 at higher frequencies. These time scales depend on the mass of the central black, and (maybe) on accretion rate as well. I will present results from a recent work to investigate whether these time scales depend on energy as well, using archival data of X-ray bright AGN, which have been observed for more than ~0.5 Msec (each) by XMM-Newton. I will discuss the implications of these results on the scaling of these time scales with BH mass, as well on ideas that have been put forward to explain the X-ray variability properties of AGN. I will also present results regarding the dependence of the power spectrum amplitude and slope on energy, and I will discuss briefly their implications on our understanding of the X-ray variability properties of these objects.

Finding Radio Transients with the Murchison Widefield Array and Beyond

David Kaplan


Tuesday, November 4, 2014


Explorations of the radio sky in the time-domain are an exciting frontier in astrophysics, and one where new observational capabilities will open up new windows on the universe. As one of a new generation of widefield, low-frequency radio telescopes, the Murchison Widefield Array has enormous potential to conduct blind searches for radio transients. We are working to develop a common framework with the Australian Square Kilometer Array Pathfinder (ASKAP) Variables and Slow Transients (VAST) survey to allow real-time transient detection and characterization. I will discuss the expected types of sources that we hope to discover with the full array, explore some of our initial results, and highlight plans for the next generation of facilities.

The Impact of Binary Stars on Our Understanding of Supernova Progenitors

Ori Fox


Tuesday, November 25, 2014


Despite the robust empirical supernova (SN) classification scheme in place, the underlying progenitor systems remain ambiguous for many subclasses. The most straightforward constraint relies on a detection of the progenitor star in high-resolution pre-explosion images. Such a direct identification is typically not feasible, however, even with modern telescopes such as Hubble. Instead, astronomers are forced to rely on supernova "forensics." I will begin the talk with a review of the limited number of direct progenitor detections already made, followed by a discussion of the indirect methods for constraining supernova progenitors that have never been seen. Although progenitor discussions have historically considered mostly single star systems, I will focus a significant portion of the discussion on the impact binary stars may have on our understanding of these results. I will conclude with a discussion about potential contributions from WFIRST and JWST.

Cold Exoplanets and Eccentric Rings

Arash Bodaghee


Tuesday, December 16, 2014


The only way to find true Neptune analogs around other stars and study their orbits is to interpret images of debris disks, extrasolar analogs of the Kuiper belt. Crisp new HST and ALMA images of debris disks around nearby stars like Fomalhaut show narrow rings offset from the star. Surely these eccentric rings are dynamical signposts of exo-Neptunes, hidden or not so hidden, sculpting the rings. But new models of debris disks are changing our understanding of the exoplanet-debris disk connection. I'll describe SMACK, a new kind of debris disk model that tracks planetesimal collisions in 3D, and how these collisions confute our usual dynamical intuition about eccentric rings. Then I'll consider the possibility that many of these systems contain hidden amounts of gas. The "photoelectric instability" can sculpt rings in such disks with no need for planets at all.

Jeremy Schnittman
NASA Logo, National Aeronautics and Space Administration