Astronomy Colloquia

January 14, 2009 | 15:30, RI 480 | Host: Paolo Privitera
Braneworld Black Holes
Ruth Gregory, Durham University
Note: Refreshments served at 3:15 pm
Braneworlds are a fascinating way of hiding extra dimensions by confining ourselves to live on a brane. One particular model (Randall-Sundrum) has a link to string theory via living in anti de Sitter space. I'll describe how the ads/cft correspondence has been used to claim that a braneworld black hole would tell us how Hawking radiation back reacts on spacetime, thus solving one of the outstanding problems of quantum gravity - the ultimate fate of an evaporating black hole. I'll review evidence for this
conjecture, ending with some recent work that shows it may be problematic.

February 4, 2009 | 15:30, RI 480 | Host: Paolo Privitera
Expectations, and Probes of Intergalactic Magnetic Fields
Phil Kronberg, Los Alamos Laboratory
Note: Refreshments served at 3:15 pm
It is likely that intergalactic magnetic fields are naturally seeded by (1) “normal” stellar processes in galaxies, and (2) by central galactic black holes. There may also be (3) a primordial component from the pre-galactic Universe, but this is probably overwhelmed by the first two processes except possibly in cosmic voids. I discuss some methods for probing intergalactic fields, and some recent results. Some are new and tentative, and serve to focus on what better data of the same kinds could be obtained with present instruments. I present a brief overview of what we know about galactic and extragalactic magnetic fields from the local universe from “here” up to z ~ 3. This includes some global a priori calculations of IGM field strengths based on known facts of galactic magnetic energy outflows. I briefly include ideas on of where some distributed UHECR acceleration sites may be found.

February 11, 2009 | 15:30, RI 480 | Host: Paolo Privitera
Atypical Thermonuclear Supernovae from Tidally Crushed White Dwarfs
Enrico Ramirez, University of California, Santa Cruz
Note: Refreshments served at 3:15 pm
Suggestive evidence has accumulated that intermediate mass black holes (IMBH) exist in dwarf galactic nuclei and some globular clusters. As stars diffuse in the cluster, some will inevitable wander sufficiently close to the hole that they suffer tidal disruption. An attractive feature of the IMBH hypothesis is its potential to disrupt not only solar-type stars but also compact white dwarf stars. Attention is given to the fate of white dwarfs that approach the hole close enough to be disrupted and compressed to such extent that explosive nuclear burning may be triggered. Consistent modeling of the gas dynamics together with the nuclear reactions allows for a realistic determination of the explosive energy release. Although the explosion will increase the mass fraction escaping on hyperbolic orbits, a good fraction of the debris remains to be swallowed by the hole, causing a bright soft X-ray flare lasting for about a year. Such transient signatures, if detected, would be a compelling testimony for the presence of a moderately mass black hole.

March 4, 2009 | 15:30, RI 480 | Host: Paolo Privitera
Ice Fishing for Cosmic Neutrinos
James Beatty, Ohio State University
Note: Refreshments served at 3:15 pm
Cosmic rays at GZK energies produce neutrinos as a consequence of their interactions with the cosmic microwave background radiation. When these neutrinos interact in the Antarctic ice sheet, the resulting showers produce coherent Cherenkov radiation at radio frequencies via the Askaryan mechanism. A balloon instrument, the Antarctic Impulsive transient Antenna (ANITA), has flown over Antarctica twice, most recently during the past austral summer. Results from the first flight and information about the second flight will be presented.

April 1, 2009 | 15:30, RI 480 | Host: Scott Dodelson
A Dustier Universe Than You'd Like
Ryan Scranton, UC, Davis
Note: Refreshments served at 3:15 pm
Using galaxies and quasars from the Sloan Digital Sky Survey, we have extended previous work measuring the weak lensing magnification. We can now compare our measurements directly to those made using galaxy-galaxy shear lensing and find that the results agree remarkably well. Our new technique also allows us to make the first measurements of the extended dust halos associated with the galaxies in our sample, detecting a level of intergalactic dust roughly twice that expected from theoretical estimates. At a mean redshift of z ~ 0.35, we find that the dust halos of galaxies extend to Mpc scales, following a power-law density distribution and exhibiting a reddening slope equivalent to that seen in the LMC. From this we infer a smooth halo component to the dust that cannot be accounted for by radially averaging satellite galaxies and find a dust mass density for the universe roughly twice that from previous estimates. Finally, we present some preliminary estimates for the impact of this dustier universe on current and future supernova cosmology measurements.

April 15, 2009 | 15:30, RI 480 | Host: Scott Dodelson
Constraining Dark Energy: First Results from the SDSS-II Supernova Survey
Josh Frieman, FNAL/University of Chicago
Note: Refreshments served at 3:15 pm
A decade ago, two teams studying distant type Ia supernovae discovered that the expansion of the Universe is speeding up. Since then, supernova surveys from the ground and from space have brought major improvements in the quality and quantity of SN Ia data, confirming the discovery of cosmic acceleration. This talk will focus on early cosmological results from the Sloan Digital Sky Survey-II Supernova Survey, which operated for 9 months in 2005-7, discovering and measuring light curves and spectra for over 500 SNe Ia. I will discuss the observational challenges to determining supernova distances for cosmology and how those challenges are being addressed. These results inform the prospects for using supernovae to obtain improved cosmology measurements from planned and proposed surveys in the future.

April 29, 2009 | 15:30, RI 480 | Host: Scott Dodelson
Empirical Constraints on the Formation and Evolution of Low-Mass Stars and Brown Dwarfs: A Data-Intensive Approach
Keivan Stassun, Vanderbilt University
Note: Refreshments served at 3:15 pm
Recent and ongoing large surveys, both from the ground and from space, are enabling new data-intensive approaches to a variety of problems in stellar astrophysics. This talk describes three such projects, each serving as a vignette of a different but complementary mode of data-intensive research into low-mass star formation and evolution. The X10000 Project takes a panchromatic, time-domain approach to study the structures of young stellar coronae in order to understand the role of extreme coronal mass ejections in the angular momentum evolution of young stars. SLoWPoKES takes an ensemble, data-mining approach to extract from the Sloan Digital Sky Survey the largest sample of wide low-mass binaries ever assembled, which can be used to constrain binary formation theory and for refining the fundamental mass-age-activity-rotation-metallicity relations for low-mass stars. The EB Factory project takes a time-domain, data-mining approach to identify rare, but astrophysically very interesting, case studies from among the large numbers of eclipsing binaries being harvested by surveys for transiting exoplanets. We will highlight recent discoveries from this work, and will draw these results together to elucidate the physical interrelationships between stellar rotation, magnetic field generation, and stellar structure during the star-formation process.

May 13, 2009 | 15:30, RI 480 | Host: Scott Dodelson
Astronomy from Dome A, Antarctica
Lifan Wang, Texas A&M University
Note: Refreshments served at 3:15 pm
The Antarctic plateau provides many exciting possibilities for astronomical observations. The next decade may see a significant astronomical buildup on the Antarctic Plateau. Dome A and Dome C are currently the two most promising sites. These high points on the plateau have unique properties for astronomical observations. Two of these arise from the extreme cold: the column density of water vapor is lower than at any other site, thus opening unique windows at infrared and submillimeter wavelengths; and the ambient temperature, and thus the thermal background emission of telescope mirrors, is lower than at any other site. Two more advantages arise from the unique character of the atmospheric turbulence: the atmospheric boundary layer is extremely thin, only tens of meters, which opens the possibility of wide field, high resolution imaging by either adaptive correction of the thin ground layer or by raising the telescope above the boundary layer; the wind speeds at all levels of the atmosphere are low, which is highly favorable for adaptive correction. It will likely be possible to form diFFraction limited images over a good fraction of the sky down to visible light wavelengths three times HST resolution for an 8 m telescope. Dome A, being the highest and coldest point in Antarctica, is especially promising based on the results of recent theoretical models and site surveys. More comprehensive site monitoring should be planned for the next decade. Based on existing data, the site has certain areas of astronomical observations can already be planned with little risks. Wide field near-IR imaging, for example, relies critically on the thermal background and the low temperature at Dome A makes it an ideal site. Another key area is likely to be exoplanet imaging and spectroscopy in the L-band, where the combination of super-diFFraction limited AO correction and the very low thermal background will enable very high contrast imaging at very close inner working angle, for example 0.15 arcsec for an 8 m telescope.

May 27, 2009 | 15:30, RI 480 | Host: Scott Dodelson
Lessons from the Nearby Universe about Star Formation in Galaxies
Alberto Bolatto, University of Maryland
Note: Refreshments served at 3:15 pm
The study of the relation between gas and star formation in galaxies is a matter of great current interest, and a crucial piece of information in our understanding of galaxy evolution. In order to determine how primordial density fluctuations become observable structures in the present day universe, it is crucial to characterize the processes that drive star formation on galactic scales. In turn, I will argue, this requires considering the creation of the self-gravitating cold molecular phase of the interstellar medium: Giant Molecular Clouds (GMCs).

In this talk I will present multi-scale measurements of the relation between gas and star formation in galaxies (the "star formation law"), and discuss our current knowledge of this relation. I will also discuss our new results on dust formation by supernovae, frequently assumed to be the major dust creation pathway in the early
universe. The properties of GMCs, the major reservoirs of star-forming gas, play a key role in setting the initial conditions for the formation of stars. I will discuss the results from a comprehensive study of the resolved GMC properties in a number of extragalactic systems, including both normal and dwarf galaxies, and I will contrast the results of the virial and high-resolution far-infrared studies and what they tell use about molecular clouds in primitive galaxies and the relation between gas and the formation of stars.