List of Past Astronomy Colloquia : 01-Jan-2007 to 01-Jun-2007


Date:   Wednesday 31-January-2007
Speaker:   Dr. Stefan Immler (NASA/GSFC)
Title:  "Living in a Bubble: The Environments of Supernovae"

Supernova (SN) explosions are one of the most energetic phenomena in the present-day universe. During recent years, space-based observatories have allowed us to study previously unexplored wavelength regimes. NASA's Swift observatory is particularly suited to probe the early emission of SNe due to its fast response and large wavelength band coverage, ranging from the optical, UV, and X-ray to the Gamma-ray bands. By studying the interaction of the SN shock with its environment, physical key parameters such as the circumstellar matter density, the mass-loss rate of the progenitor, and temperature of the outgoing and reverse shock can be studied as a function of time. Monitoring of SNe with space-based observatories such as Chandra and XMM-Newton has further allowed us to map the density structure in the environments of SNe out to large radii (>10^20 cm) from the sites of the explosions, to study the transition of a SN into an old supernova remnant, and to probe the mass-loss rates of the progenitor stars over significant timescales (10^4 yrs) in the stellar wind history. In combination, these observations give unprecedented insights into the nature of SN systems and their environments.


Date:   Wednesday 07-February-2007
Speaker:   Dr. Fred Lo (Director, NRAO)
Title:  "TBD"


Date:   Wednesday 14-February-2007
Speaker:   Dr. Thomas Greve (Caltech)
Title:  "Understanding the Gastrophysics of Massive Galaxies -- Lesson Learned from High- and Low-z"

The high abundance of massive, baryon-dominated galaxies at z>2 poses a serious challenge for current models of hierarchical structure formation -- which make the general prediction that the most massive systems form at much later times (z<1) as a result of a gradual build-up (mergers) -- and is suggestive of a very high star formation efficiency in such systems. Currently we do not have a good understanding of the dissipative processes responsible for the rapid condensation of baryons into stars and galaxies, but future FIR/mm facilities such as ALMA/eVLA/Herschel are expected make a big impact in this area.

Here I will adopt the 'local perspective' and discuss the properties of the molecular ISM in local (U)LIRGs -- merger-driven starburst galaxies which are thought to harbor the same extreme conditions as is found in high-z proto-galaxies. In the local Universe a new star formation law -- in the guise of a tight correlation between the IR and HCN 1-0 luminosity -- has been observed to hold in virtually all star forming environments (Gao & Solomon 2004, Wu et al. 2005), although, recently the usefulness of HCN as a tracer of dense gas has been called into question (Gracia-Carpio et al. 2006). Using one of the largest molecular line surveys ever, including multiple transitions of HCN, HCO+ and CS, we have put strong constraints on the dense gas mass and the star formation efficiency in these two systems. We demonstrate that HCN is a robust tracer of dense gas while HCO+ is sub-thermally excited and thus inferior to HCN as a dense gas tracer.


Date:   Friday 16-February-2007 (TODAY)
Speaker:   Dr. Thomas Greve (Caltech)
Title:  "Understanding the Gastrophysics of Massive Galaxies -- Lesson Learned from High- and Low-z"

The high abundance of massive, baryon-dominated galaxies at z>2 poses a serious challenge for current models of hierarchical structure formation -- which make the general prediction that the most massive systems form at much later times (z<1) as a result of a gradual build-up (mergers) -- and is suggestive of a very high star formation efficiency in such systems. Currently we do not have a good understanding of the dissipative processes responsible for the rapid condensation of baryons into stars and galaxies, but future FIR/mm facilities such as ALMA/eVLA/Herschel are expected make a big impact in this area.

Here I will adopt the 'local perspective' and discuss the properties of the molecular ISM in local (U)LIRGs -- merger-driven starburst galaxies which are thought to harbor the same extreme conditions as is found in high-z proto-galaxies. In the local Universe a new star formation law -- in the guise of a tight correlation between the IR and HCN 1-0 luminosity -- has been observed to hold in virtually all star forming environments (Gao & Solomon 2004, Wu et al. 2005), although, recently the usefulness of HCN as a tracer of dense gas has been called into question (Gracia-Carpio et al. 2006). Using one of the largest molecular line surveys ever, including multiple transitions of HCN, HCO+ and CS, we have put strong constraints on the dense gas mass and the star formation efficiency in these two systems. We demonstrate that HCN is a robust tracer of dense gas while HCO+ is sub-thermally excited and thus inferior to HCN as a dense gas tracer.


Date:   Wednesday 21-February-2007
Speaker:   Dr. Lisa Kewley (U. of Hawaii)
Title:  "The Cosmic Star Formation and Metallicity History of Galaxies"

Observing the star formation rate and metallicity since the earliest times in the universe is crucial to understanding galaxy formation and evolution. Current cosmic star formation history studies are plagued by significant discrepancies between star formation rate estimates at different wavelengths. I apply the new generation of stellar population synthesis and photoionization models and show that metallicity is key to resolving the star formation rate discrepancies. Although the cosmic star formation history has been studied extensively, our understanding of the metallicity history of star-forming galaxies is still largely theoretical. I present the first results of our large observational investigation into the metallicity history of star-forming galaxies between 0


Date:   Wednesday 28-February-2007
Speaker:   Dr. Alberto Bolatto (U. of California-Berkeley)
Title:  "The Primitive Universe, Near and Far"

The study of primitive galaxies, whether in the local universe or at cosmological distances, is one of the most active and rapidly evolving fields in present day Astronomy. New and more capable millimeter and submillimeter-wave instruments, such as CARMA, ALMA, and large format bolometer arrays, will revolutionize our understanding of the gas content, the evolutionary state, and the dynamics of these objects. With an eye toward applications at high redshift, I will discuss some of the results and outstanding puzzles presented by nearby low mass systems akin to primeval galaxies. In particular, I will present in detail the relationship between gas and star formation in small galaxies, as evidenced by millimeter-wave interferometry and far infrared Spitzer observations. Finally, I will discuss the very interesting prospects brought about by new radio facilities and technological developments. These advances make long-wavelength astronomy the preferred regime to search and study very distant sources --- with these new facilities it will be possible to find the sources that reionized the universe, and to routinely measure rotation curves and dynamical masses at z~6.


Date:   Wednesday 07-March-2007
Speaker:   Dr. Sheila Kannappan (U. of Texas-Austin)
Title:  "Building Disk Galaxies in a Violent Universe"

Hierarchical models suggest that galaxies form through a series of mergers that create spheroids of stars, while disks of gas and stars grow around these spheroids during the quiet times between galaxy encounters. However, hierarchical simulations generally form large bulges with small disks, while in reality many galaxies are disk-dominated, such as our own Milky Way. Moreover, such disk-dominated galaxies often host central bulges that are not true spheroids, but rather inner disk structures (pseudobulges). These contradictions highlight one of the most fundamental gaps in our understanding of galaxy evolution - modeling disk formation - and challenge the physical basis of the hierarchical paradigm. I will present results that suggest promising strategies for moving forward, and I will describe opportunities for students to participate in this research.


Date:   Wednesday 14-March-2007
Speaker:   Dr. Emeric Le Floc'h (U. of Hawaii)
Title:  "A Spitzer View of Massive Star Formation and AGNs at High Redshift"

Although they are rare objects in the present day Universe, Luminous Infrared (IR) Galaxies played a significant role in the evolution of structures at high redshift. I will discuss how the Spitzer Space Telescope has revolutionized our knowledge on the contribution of this population to the history of star formation and supermassive black hole growth. I will particularly emphasize results on the evolution of IR-luminosity functions up to z~1 as well as the discovery of a population of 2


Date:   Wednesday 21-March-2007
Speaker:   NO COLLOQUIUM SCHEDULED
Title:  "Spring Break"


Date:   Wednesday 28-March-2007
Speaker:   Dr. Edward Churchwell (U. of Wisconsin, Spitzer GLIMPSE PI)
Title:  "Stars, Dust Bubbles, Dark Clouds, and Galactic Structure"

A brief review of some of the major results from the GLIMPSE surveys will be given. In particular, as the title suggests, emphasis will be given to the distribution of stars in the Galactic disk as a tool to determine large scale Galactic structure al a Herschel and Kapteyn. A hallmark of the mid-infrared galaxy is the proliferation of dust bubbles seen in emission and dark clouds seen in silohuette against the IR background. The mid-IR data are revealing interesting new insights into the structure of the IR bubbles and the emission properties of the dust. I will discuss our current understanding of these intriguing objects and show examples of what we believe to be triggered star formation associated with expanding bubbles produced by O and B stars. Examples of IR Dark Clouds will also be shown.


Date:   Wednesday 04-April-2007
Speaker:   Dr. Bill McDonough (U. of Maryland, Dept. of Geology)
Title:  "Accretion of moderately volatile elements in the Solar System"


Date:   Wednesday 11-April-2007
Speaker:   Dr. Michele Trenti (STSci)
Title:  Dynamical Evolution of Star Clusters with Primordial Binaries and Intermediate Mass Black Holes

The evolution of a star cluster is strongly influenced by the presence of primordial binaries and of a central black hole, as dynamical interactions within the core prevents a deep core collapse under these >conditions. I will present the results from a large set of direct N-body simulations of star clusters that include an intermediate mass black hole (IMBH), single and binary stars. I will highlight the structural and dynamical differences for the various cases showing in particular that on a timescale of a few relaxation times the density profile of the star cluster does no longer depend on the details of the initial conditions but only on the efficiency of the energy generation due to gravitational encounters at the center of the system. A large core to half mass radius ratio in old, collisionally relaxed globular clusters appears a promising photometric fingerprint for the presence of an IMBH.


Date:   Wednesday 18-April-2007
Speaker:   Dr. Terry Hurford (NASA/GSFC)
Title:  "The Importance of Tidal Stress on Enceladusa"

Large rifts near the south pole of Enceladus have been observed to be significantly warmer than the surrounding surface and are most likely the source of jets of icy particles observed by Cassini . The ability of Saturn to raise a significant tide on Enceladus is enhanced, since Enceladus surface gravity is small. Moreover, Enceladus eccentricity causes the tide to oscillate in magnitude by 1% as it completes an orbit. Not only does the size of the tide on Enceladus change daily, but it oscillates ~0.5 degrees eastward and westward. The daily change in position and magnitude of the tidal bulge produces stress on the surface of Enceladus that may play a significant role in the eruption of plumes on Enceladus. Moreover, one prominent rift on Enceladus south pole resembles the shape of cycloids on Europa. Tidal tectonic processes may have formed this rift at its current latitude in a similar manner to cycloids on Europa (i.e. that its formation reflects a cracks response to a diurnally varying stress field).


Date:   Wednesday 25-April-2007
Speaker:   Dr. Zoltan Haiman (Columbia U.)
Title:  The Formation of the First Stars and Black Holes and How They Regulated Cosmological Reionization.

The first generation of stars likely formed at very high redshifts, z>20, at locations corresponding to rare peaks of the fluctuating primordial density field. In the absence of any feedback processes, these stars could form in abundance and they, and their accreting black hole (BH) remnants, could significantly reionize the intergalactic medium (IGM). However, the population of the first stars and BHs were likely self-regulating due to their global chemical, radiative, and thermodynamical impact on the IGM, and further star-formation and reionization was delayed as a result. I will argue that star-formation was suppressed at high redshift in low-mass minihalos and that this can be explained by an early H_2-dissociating background, exacerbated by transient photoionization heating of the IGM, and by the strong clustering of the earliest ionizing sources. The low electron scattering optical depth in the three-year WMAP data offers empirical support that the ionizing photon production in the earliest minihalos was indeed suppressed. I will briefly discuss the implications of this result for the hierarchical build-up of supermassive black holes.


Date:   Wednesday 02-May-2007
Speaker:   Dr. Chris Reynolds (UMD)
Title:  The astrophysics of black hole spin


Date:   Wednesday 09-May-2007
Speaker:   Dr. Keivan Stassun (Vanderbilt University)
Title:  "Empirical Constraints on the Formation and Early Evolution of Low-Mass Stars and Brown Dwarfs"

We report on recent progress and future prospects in two broad areas of research centered around the formation of low-mass stars and brown dwarfs. First, we discuss efforts to directly and accurately measure the fundamental physical properties (masses, radii, luminosities) of young low-mass stars and brown dwarfs as critical tests of theoretical star-formation models, including the recent discovery of the first brown-dwarf eclipsing binary. Second, we discuss the broad problem of magnetic field generation and X-ray production in these objects, highlighting results from the recent Chandra Orion Ultradeep Project. Finally, we draw these recent results together to elucidate the physical interrelationships between magnetic field generation and stellar structure during the star-formation process.


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