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.
The Physics of Gas Sloshing in the Cores of Galaxy ClustersJohn ZuHoneGSFCTuesday, August 26, 2014AbstractMany 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 CloudEmily LevesqueColoradoTuesday, September 9, 2014AbstractThorne-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 SpitzerKartik ShethNRAOTuesday, September 16, 2014AbstractBars 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 BlundersMario LivioSTScITuesday, September 23, 2014AbstractEven 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 ProgramSteve BoggsBerkeleyTuesday, September 30, 2014AbstractThe 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 nucleiDavid MerrittRITTuesday, October 7, 2014AbstractMost 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 JWSTVolker BrommUT AustinTuesday, October 21, 2014AbstractHow 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 variabilityIossif PapadakisCreteTuesday, October 28, 2014AbstractActive 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 BeyondDavid KaplanUWMTuesday, November 4, 2014AbstractExplorations 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 ProgenitorsOri FoxBerkeley/GSFCTuesday, November 25, 2014AbstractDespite 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. |
The gas-star formation connection at high-redshiftJacqueline HodgeNRAOTuesday, December 2, 2014AbstractWhile the cosmic star formation rate density has now been constrained out to impressively high redshifts, equally important for our overall understanding of the Universe as we observe it today is determining not just when, but how these high-redshift galaxies formed their stars. This can be achieved through detailed studies that 1) map the distribution and intensity of the star formation within individual high-redshift galaxies, and 2) connect this star formation back to the cold molecular gas reservoirs that fuel the bursts. Submillimeter galaxies (SMGs) are a key population in such efforts, as they are gas-rich galaxies which host the most intense episodes of star formation in the known Universe. I will present results from a case study of the z~4 SMG GN20, where the unparalleled Very Large Array (VLA) imaging has allowed us to map the cold gas reservoir on scales of ~1 kpc just 1.6 Gyr after the big bang. Our recently acquired rest-frame FIR data with the Plateau de Bure Interferometer resolves the obscured star formation on the same scales, allowing us to study the gas-star formation connection in this unlensed z~4 galaxy. I will present our efforts with the Atacama Large Millimeter Array on the ALESS project, extending our studies of high-redshift star formation to a larger sample of SMGs. Finally, I will discuss our ongoing VLA campaign to constrain the unbiased cold gas history of the Universe through the first large scale blind survey of cold molecular gas at z>~2. |
Reconstructing the Formation Histories of Massive GalaxiesMariska KriekBerkeleyTuesday, December 9, 2014AbstractIn past years, large and deep photometric and spectroscopic surveys have significantly advanced our understanding of galaxy growth, from the most active time in the universe (z~2) to the present day. In particular, the evolution in stellar mass, star formation rate, and structure of complete galaxy samples have provided independent and complementary insights into their formation histories. In addition, detailed studies of the properties of distant galaxies have lead to a better apprehension of the physical processes which govern galaxy growth. Nonetheless, many outstanding questions remain. In this talk I will give an overview of our current picture of galaxy growth in the past 11 billion years, discuss current challenges and outstanding questions, and introduce new and ongoing efforts to further unravel the formation histories of massive galaxies. |
The Norma Arm: a breeding ground for High-Mass X-ray binariesArash BodagheeGCSUTuesday, December 16, 2014AbstractHard X-ray surveys of the Milky Way have revealed a population of around 200 high-mass X-ray binaries (HMXBs). These are systems in which a compact object (usually a neutron star, but sometimes a black hole) accretes material shed by a massive stellar companion. Given that the donor star is short-lived, the distribution of HMXBs offers a way to trace the recent star-formation history of not only our Galaxy, but other nearby galaxies as well. In the Milky Way, the Norma Arm features the highest density of HMXBs, which is not surprising given that this region is also host to active stellar nurseries. In this talk, I will present results from X-ray surveys of the Norma Arm performed by INTEGRAL, Chandra, and most recently by NuSTAR. I will describe how the 1100 new X-ray sources detected by Chandra were separated into different candidate source classes based on their observed X-ray and infrared properties. This enabled the most promising HMXB candidates to be selected for optical and infrared follow-up observations. A few of them appeared in a NuSTAR map of the Norma Arm providing broader X-ray coverage. The multi-wavelength description of a typical HMXB will be shown, demonstrating how such objects serve as laboratories for studying the physics of matter under extreme gravitational and electromagnetic fields. Finally, the spatial distribution of HMXBs in the Milky Way will help point towards constraints on the evolutionary timescales of massive stars and the compact objects they become. |
Jeremy Schnittman