Department of Astronomy

Abstracts for Spring 2011 Colloquia (most recent update: February 18, 2011)

Jan. 18, 2011	Raffaella Schneider, INAF/Osservaorio Astrofisico di Roma (Tinsley Scholar) "Dust in the Early Universe" In the last few years mm and submm observations of high redshift quasars and galaxies have provided a powerful way of probing the very existence and properties of dust within 1 Gyr of the Big Bang. It is generally believed that at these cosmic times dust could have only condensed in the explosive ejecta of supernovae since intermediate-to-low mass stars did not have time to evolve off the main sequence into the dust-condensation stage. To account for the dust masses observed at z > 6, about 1 Msun of dust per supernova needs to be produced, in agreement with the condensation efficiencies predicted by supernova dust nucleation models. However, the largest dust masses measured so far in supernova ejecta are about two orders of magnitude smaller. Conversely, indications for supernova dust beyond z ~ 6 have recently come from observations of dust reddening of the host galaxies of a z = 6.2 quasars and z = 6.3 gamma-ray burst. In this talk, we will review the recent advancements made in this controversial subject. We will present the results of recent models for dust production in supernova ejecta and intermediate mass stars and its subsequent evolution in the interstellar medium of high redshift galaxies. We will also discuss the implications of an early epoch of dust formation for the cosmic transition between Population III and Population II stars.
Jan. 25, 2011	Jonathan Braithwaite, University of Bonn (Tinsley Scholar) "Magnetic relaxation in ICM bubbles and the magnetic flux problem in star formation" In various astrophysical situations a volume of magnetised plasma is first stirred by turbulent motions and then allowed to relax without outside influence. For instance, various types of radiative star, which are observed to contain static magnetic equilibria, have previously undergone convection. I look at the principal factors in the initial turbulent state which determine the properties of the eventual equilibrium, presenting also simulations of the relaxation of magnetic field in bubbles observed as x-ray cavities in the intergalactic medium. I make predictions concerning the nature of the magnetic field in such bubbles and how it depends on the nature of the AGN jets/outflows which inflate them. Finally, I look at a possible solution to the so-called "magnetic flux problem" in star formation, where a protostar must somehow lose almost all of the magnetic flux present in its parent cloud.
Feb. 1, 2011	Peter Goldreich, California Institute of Technology "Reading the Record of Ancient Impacts" Debris from asteroids and comets continually bombards the Earth. Impacts by big bodies launches ejecta on trajectories that transports it thousands of kilometers from the inpact site. These events are recorded in discrete layers of sub-millimeter size spherules in sea floor sediments. Larger, rarer, and interestingly shaped objects are found on land. I will describe the physical processes that occur in energetic impacts. Emphasis will be given to the mechanisms that determine the sizes and shapes of the ejecta and the cooling of the fireball. An analogy with structure formation in the expanding universe will be exploited, and a possible relation with the formation of chondrules will be explored.
Feb. 8, 2011	Kim-Vy Tran, Texas A&M University and University of Zurich "Star Formation in Galaxy Clusters Over the Past 10 Billion Years" Understanding how galaxies form and evolve in clusters continues to be a fundamental question in astronomy. The ages and assembly histories of galaxies in rich clusters test both stellar population models and hierarchical formation scenarios. Is star formation in cluster galaxies simply accelerated relative to their counterparts in the lower density field, or do cluster galaxies assemble their stars in a fundamentally different manner? To answer this question, I review results from our Spitzer/MIPS Infra-Red Cluster Survey (SMIRCS; 0 < z < 1) and present first results for one of the most distant clusters yet discovered at a look-back time of nearly 10 billion years (z = 1.62).
Feb. 11-12, 2011	McDonald Observatory Board of Visitors Meeting (Austin)
Feb. 12, 2010 Saturday 1-2 PM ACE 2.302	Daniel Jaffe, University of Texas at Austin (19th Annual Great Lecture in Astronomy) "Exploring Newly Discovered Worlds with the Giant Magellan Telescope" Less than two decades ago, mankind knew about only one planetary system: our own. Since the discovery of the first extrasolar planets in 1995, a furious effort by the astronomical community has led to the discovery of hundreds of extrasolar planets. Many of these systems look very different from our solar system, having massive planets close to the parent stars while ours are far from the Sun, and/or planets in highly eccentric orbits, where most orbits in the Solar System are quite circular. The high contrast between the brightness of the parent stars and the faintness of the planets makes it very hard to obtain concrete information about the planets themselves: How and when do they form? How massive and how dense are they? How hot are they? Do they have atmospheres and what are these atmospheres made of? A new generation of large ground-based telescopes, including the Giant Magellan Telescope , in which UT Austin and Texas A&M are partners, will afford us the first direct measurements that can address all of these questions for exoplanets approaching the size and mass of planets like the Earth but residing in star systems many light years away. GMT in particular, with its superb high-contrast imaging and its Innovative capabilities for infrared spectroscopy, will be able to move us from an era of detection to one of real exploration. I will discuss the exciting future of extrasolar-planet studies and how it builds on the solid foundation of the current exoplanet research that forms an important part of the current program in Texas.
Feb. 15, 2010	George F. R. Ellis, University of Cape Town "Is Inhomogeneity Important in Cosmology?" The real universe is comprised of a cosmic web of structures, with vast walls and voids on larger scales and mostly empty space on smaller scales. Is this significant for cosmology, or can we ignore it and used the standard linearized Robertson-Walker models with impunity? There are three ways inhomogeneities can affect the values we assign cosmological parameters on the basis of astronomical observations. Firstly there may be back-reaction from small scale inhomogeneities to large scale dynamics; this almost certainly occurs, but is probably not important, although there are some caveats to that statement. Secondly the observational properties of a universe with major voids can be significantly different from those of a smoothed out model: this has the potential to cause significant re-evaluation of the interpretation of the supernova observations. Thirdly a large-scale underdensity, with our Galaxy somewhere near the centre, can mimic the apparent acceleration of the universe, indeed the observations can be explained by inhomogeneous models with no dark energy present. These models are philosophically unpopular, but philosophy will have to give way to observational tests that can check whether the Copernican Principle is indeed satisfied or not. Recent claims that these models have been observationally disproved are overstated: the required perturbation calculation have not yet been properly deployed.
Feb. 16, 2010 Wednesday 3:00 pm	Charles Steidel, California Institute of Technology (Tinsley Visiting Professor) "Exploration of the Circum-Galactic Medium at High Redshifts" During the peak epoch of galaxy formation, the intergalactic medium is both the source of gas fueling star formation in forming galaxies, as well as the waste dump for the products of star formation and black hole accretion that are not retained by galaxies. By studying the "Circum-Galactic Medium", the region within a few hundred physical kpc of forming galaxies, one can begin to constrain the flow of baryons into and out of galaxies. At present, there is a puzzling discrepancy between observations and theoretical expectations whose resolution may be key to unraveling the aspects of galaxy formation that are least well-understood.
Feb. 22, 2010	Jessica Lu, California Institute of Technology "Clarifying our View of Star Formation in Extreme Environments with Adaptive Optics" The formation of stars is a fundamental astrophysical process; and yet we still debate whether it varies with environment. Milky Way young star clusters range in mass over four orders of magnitude; but, the best-studied star forming regions, such as Taurus and Orion, represent only a small range of initial conditions at the low mass end. Young star clusters with masses greater than 10,000 solar masses are promising targets for determining whether the initial mass function (IMF) that results from the star formation process is universal or depends on environment. Such clusters are challenging observational targets as they require high spatial resolution at infrared wavelengths and are heavily contaminated by field stars. I present results from a Keck adaptive optics study of several massive young star clusters in the Milky Way, including around the supermassive black hole at the Galactic Center. Precise IMFs are constructed by using high-precision astrometry and spectroscopy to distinguish individual cluster stars. I will discuss whether the measured IMFs differ for massive clusters at a range of Galactocentric radii and how they compare to the "universal" IMF established locally.
Mar. 1, 2010	Robert Kraft, University of California/Lick Observatory "Chemical Abundances in the Oldest Galactic Stars: Globular Clusters vs. the Halo Field" This is a review of the quest to determine the chemical abundances of the oldest stars in the Galaxy, the members of the halo field and globular cluster populations. From forty years of spectroscopic studies, it appears that cluster and field stars are not chemically identical but differ significantly especially in the behavior of O, Na, Mg and Al. We explore both deep mixing and primordial abundance variations as explanations for these differences, and find that both are needed. The unusual abundance ratios of the Mg isotopes found in some clusters points to the importance of a primordial "pollution" scenario involving the output of intermediate mass AGB (asymptotic mass branch) stars. We discuss briefly scenarios for the formation of the Galactic halo.
Mar. 8, 2010	Jeffrey A. Newman, University of Pittsburgh "Title: TBA" Abstract: to come.
Mar. 15, 2011	Spring Break: 14-18 March: No talk scheduled.
Mar. 22, 2011	Lars Bildsten, UCSB/Kavli Institute for Theoretical Physics (Tinsley Visiting Professor) "Diverse Energy Sources for Supernovae" The theoretical community is beginning to appreciate (and predict) the potential diversity of explosive outcomes from stellar evolution while the supernovae surveys are finding new kinds of supernovae. I will speak about two such new supernovae. The first are ultraluminous core collapse supernovae with radiated energies approaching 10^51 ergs. I will present our recent work that explains these events with late-time energy deposition from rapidly rotating, highly magnetized neutron stars: magnetars. I will close with our theoretical work on helium shell detonations on accreting white dwarfs that predict a new class of supernovae; called ".Ia's". The first such candidate may well have been found by the Palomar Transient Factory.
Mar. 24, 2011	Lars Bildsten, UCSB/Kavli Institute for Theoretical Physics (Tinsley Visiting Professor) "Exploding Stars!" Stars explode once every second in the Universe, often becoming brighter than their home galaxies. Though most remain undiscovered by astronomers, recently enhanced capabilities to scan the skies now detect over 10 per day. This has revealed new modes of explosions, some much brighter than we expect, and some much fainter. After describing the common outcomes, I will focus on the exciting new discoveries and their novel theoretical interpretations.
Mar. 29, 2011	Pawan Kumar, University of Texas at Austin "Recent Advances in our Understanding of Enigmatic Gamma-ray Bursts" After a brief summary of key observational properties of gamma-ray bursts an overview of the recent progress in our understanding of these explosions will be provided. Data obtained by the Swift satellite has answered several long standing questions, and also added new puzzles. Swift has shed some light on the workings of the "central engine" in these bursts, and last year it discovered a burst at a redshift of 8.25 (when the universe was 625 million years old) which is one of the most distant objects we know today (detection of a galaxy at z~10 was reported in January this year). The Fermi satellite (launched in June 2008) has provided excellent data covering a very broad energy band --- 8 keV to ~300 GeV --- that has clarified some aspects of these bursts, and also added a few new mysteries.
Apr. 5, 2011	Dimitar D. Sasselov, Harvard-Smithsonian Center for Astrophysics "Title: TBA" Abstract: to come.
Apr. 12, 2011	Caryl Gronwall, Pennsylvania State University "The Physical Properties of Lyman-alpha Emitters from z=2 to 3" Lyman-alpha emitters (LAEs) are low-mass, nearly dust-free objects that represent galaxies "in the act" of formation. We believe that LAEs at redshifts of 2 to 3 are the progenitors of present-day L* galaxies. We have used the Mosaic camera of the CTIO 4-m telescope to conduct a deep, narrow-band survey of Ly-alpha Emitting Galaxies (LAEs) in the Extended Chandra Deep Field-South over the redshift ranges 3.08 < z < 3.15 and 2.04 < z < 2.08. Our survey covers 0.28 square degrees down to a limiting line flux of about 1.5E-17 ergs/cm2/s at z=3.1. Our LAE sample now consists of several hundred candidate line-emitters. By combining our narrow-band data with archival broadband photometry from MUSYC (the Multiwavelength Survey by Yale-Chile), we have been able to determine the photometric properties of these samples including their luminosity functions, equivalent widths, colors, star-formation rates, and dust properties of these objects as well as the evolution of these properties from z=3 to 2. We have also measured the stellar populations using the combined optical and infrared photometry from archival Spitzer data as well as their morphological properties using Hubble Space Telescope imaging. I will also discuss future prospects for studying Ly-alpha emitters using HETDEX.
Apr. 19, 2011	Aristotle Socrates, Institute for Advanced Study, Princeton "Title: TBA" Abstract: to come.
Apr. 26, 2011	Ethan Vishniac, McMaster University, Ontario, Canada "Title: TBA" Abstract: to come.
May 3, 2011	Speaker, Affiliation TBD "Title: TBA" Abstract: to come.

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Please report any errors or omissions to: G. Orris, email: argus@astro.as.utexas.edu