List of Past Astronomy Colloquia : 01-Sep-2008 to 31-Dec-2008

Date:   Wednesday 03-Sep-2008
Speaker:   Prof. Dr. Andreas Eckart (University of Cologne)
Title:  "Coordinated Multi-Wavelength Observations of the Super-Massive Black Hole at the Center of the Milky Way"

Date:   Wednesday 10-Sep-2008
Speaker:   No Colloquium


Date:   Wednesday 17-Sep-2008
Speaker:   Dr. Richard Crutcher (University of Illinois)
Title:  "Testing Star Formation Theory with Magnetic Field Observations"

The two extreme-case models of what drives star formation are: (1) magnetic fields are strong enough to support clouds against gravitational collapse, and evolution proceeds by ambipolar diffusion building up mass but not magnetic flux in cloud cores, and (2) magnetic fields are sufficiently weak that the evolution of the ISM and of molecular clouds is governed by turbulence. In this talk, I report the results of recent OH and CN spectral-line Zeeman observations and of Bayesian analysis studies of Zeeman data that test these models in order to determine observationally which drives the star formation process. The result appears to be a definitive answer to the question about which of the two drivers dominates the star formation process.

Date:   Wednesday 24-Sep-2008
Speaker:   Prof. Neil Nagar (University of Concepcion)
Title:  "A Multicomponent Source Model for UltraHigh Energy Cosmic Rays: Nearby Powerful Radiogalaxies, Magnetars, and Something Else?"

The origin of UltraHigh Energy Cosmic Rays (UHECRs), light particles with energies greater than about 50 Exa Electron Volts, is an enduring mystery. Thanks to their extreme energies UHECRs are minimally deflected by Galactic and extragalactic magnetic fields. Their arrival directions therefore indicate their source directions to within a few degrees. More than 60 UHECRs have been detected and characterized by AGASA and the Pierre Auger Observatory. Using 27 UHECRs, the Pierre Auger Collaboration has claimed a correlation between UHECRs and nearby active galactic nuclei (AGN). Other groups have used the same data to explore correlations with subclasses of AGNs and with magnetars.

Using the AGASA and Pierre Auger Observatory UHECRs we have claimed the first direct evidence that nearby extended radiogalaxies are the most likely source of at least one third of the UHECRs detected by the Pierre Auger Observatory. We will present the evidence for an origin of UHECRs in nearby extended radiogalaxies and its implications, including UHECR production, magnetic fields and IGM heating. Competing results, and the origins of the remaining UHECRs will also be explored, especially in relation to magnetars and transient events like gamma ray bursts. The restricted space for more exotic explanations like dark matter anhilation or primordial black hole evaporation will also be mentioned.

Date:   Wednesday 01-Oct-2008
Speaker:   Prof. Dr. Ralf Dettmar (Ruhr University Bochum)
Title:  "Gaseous Halos of Spiral Galaxies and the Interstellar Disk-Halo Connection"

Multi-wavelength studies of edge-on disk galaxies show the presence of the various phases of the interstellar medium (ISM) in galactic halos. This includes the ionized gas, the hot medium, as well as the cosmic ray component coupled to the large scale magnetic field. The properties of these ISM components in halos are discussed in the framework of the disk-halo interaction, the large scale circulation of matter in a supernova driven ISM.

Date:   Wednesday 08-Oct-2008
Speaker:   Dr. Patrick Michel (Observatoire Cote d'Azur)
Title:  "Origin and Disruption Mechanisms of Asteroids with Implications for their Physical Structure"

I will present our current understanding of the origin and evolution of Near-Earth Objects, with a focus on numerical simulations of catastrophic disruption. I will discuss how we model this evolution and explain what we have learned from these models. I will also present simulations of rotational fragmentation, which we have found is probably at the origin of many asteroid binaries, and discuss the consequences of these processes on the physical properties of asteroids (by comparison with ground-based and space-based observations). As time permits, I will say a few words on the sample return mission Marco Polo under study at ESA.

Date:   Wednesday 15-Oct-2008
Speaker:   Prof. Nahum Arav (Virginia Tech)
Title:  "Measuring kinetic luminosity of quasar outflows: results from VLT observations, and implications to AGN feedback"


Date:   Wednesday 22-Oct-2008
Speaker:   Prof. Harlan Spence (Boston University)
Title:  "On Interplanetary Medium Structure and Explosive Solar Transients: Probing Space and Time With Galactic Cosmic Rays and Solar Energetic Particles"

While much is known about the global plasma and magnetic structure of the interplanetary medium (IPM), and about the processes which produce the solar wind and which spawn explosive solar transients, fundamental questions about the structure and dynamics of the solar corona and the heliosphere persist. In this talk, I highlight several uses of solar energetic particles (SEP) and galactic cosmic rays (GCR) for probing such questions in new ways, ranging from observations of the most extreme events to observations of the everyday, most probable state. Starting from the most extreme, chemical imprints of pre-space-age SEPs in arctic ice shed light on the frequency and size of historically significant coronal mass ejections (CMEs) and the processes associated with these rarest explosive transients which accelerate solar particles to energies of up to ~1 GeV. During more commonly-occurring CMEs, recent multipoint space-based GCR measurements reveal heretofore unresolved structure in classic Forbush decreases and help to identify associated coherent magnetic structures that may be responsible. During non-transient intervals, short time-scale variations in these same multi-point GCR observations imply the presence of some modulating meso-scale structure in the IPM even during unremarkable periods. Recent in situ (WIND/STEREO) and remote-sensing (STEREO) observations confirm that the solar wind is indeed richly structured, not only owing to explosive transients producing the largest coherent size scales, but also through processes near the Sun which continuously and routinely inject medium to small scale structure into the solar wind. Finally, I discuss implications of the transient extremes and IPM structuring and outline some of the new questions raised by these observations.

Date:   Wednesday 29-Oct-2008
Speaker:   Prof. Jessica Rosenberg (George Mason University)
Title:  "Taking a Census of Baryons in the Local Universe: The Sloan and ALFALFA Surveys"

I will discuss a project to combine two large overlapping surveys that probe the galaxy population at optical and 21 cm wavelengths in order to study the baryons in galaxies. The nature and distribution of the baryons in galaxies can test predictions of the Lambda-CDM model while also providing a probe of the astrophysical processes that affect the state of the baryons. Until now it has been nearly impossible to study both the gas and stars in a statistically significant sample of galaxies within in a single volume of space. Such a study is critical because it dramatically reduces the bias in the samples studied as well as accounting for two of the major components of the baryons. With the advent of large area surveys in both optical and radio 21 cm, studying the stars and gas simultaneously is finally possible. This project is in its preliminary stages. I will discuss our plans for and preliminary results from studying the overlap between ALFALFA, an ongoing blind HI survey at Arecibo, and the Sloan Digital Sky Survey (>~ 4000 deg^2), for this purpose.

Date:   Wednesday 05-Nov-2008
Speaker:   Prof. Andrey Kravtsov (University of Chicago)
Title:  "Cosmological Simulations of Clusters of Galaxies: Status, Problems, Challenges"

I will describe high-resolution self-consistent cosmological simulations of clusters forming in the concordance Cold Dark Matter model with vacuum energy. The resolution of the simulations is sufficiently high to resolve formation and evolution of cluster galaxies and their impact on cluster gas. We use these simulations to study the effects of galaxy formation on the global properties of clusters, such as the shape of cluster dark matter halo and its density profile, the baryon fractions, gas density and temperature profiles. I will present comparisons of simulations results with the recent X-ray Chandra, Sunyaev-Zeldovich, and optical observations of clusters with highlights of both successes and problems of the models. I will show that despite complexities of their formation and uncertainties in their modeling, clusters of galaxies both in observations and numerical simulations are remarkably regular and consistent outside of their core region (~5% of the virial radius), which holds great promise for their use as cosmological probes. I will briefly describe the current status of cosmological constraints with clusters.

Date:   Wednesday 12-Nov-2008
Speaker:   Prof. Robert Benjamin (University of Wisconsin, Whitewater)
Title:  "A New GLIMPSE of the Milky Way's Stellar Structure"

The mid-infrared regime (3-5 microns) is the ideal part of the spectrum to study the stellar content of the inner Galaxy. At these wavelengths, dust extinction is (almost) insignificant, emission from stars is still bright, and the diffuse emission from dust observed at longer wavelengths is very low. I will share recent results from the Spitzer Space Telescope GLIMPSE survey which show evidence for the Galactic bars (more than you may have known about) and spiral arms (fewer than you might have thought). I will also summarize what I consider to be the major unsolved questions regarding the structure of the Galactic disk.

Date:   Wednesday 19-Nov-2008
Speaker:   Prof. Kara Hoffman (University of Maryland)
Title:  "South Pole Neutrino Telescopes"

Date:   Wednesday 26-Nov-2008
Speaker:   No Colloquium
Title:  "Thanksgiving"

Date:   Wednesday 03-Dec-2008
Speaker:   Dr. John Carr (Naval Research Laboratory)
Title:  "Water and Organic Molecules in Protoplanetary Disks"


Date:   Wednesday 10-Dec-2008
Speaker:   Prof. Houjun Mo (University of Massachusetts)
Title:  "The Connection Between Galaxies and Dark Matter Halos"

In the current paradigm of structure formation, it is now possible to study how galaxies of different properties reside in different dark matter halos. Using galaxies in the SDSS, we can now study systematically how galaxies of different stellar masses, colors, star formation rates, etc are distributed in halos of different masses, and in different halo-centric distances. I will present a review of the results obtained in this area, and discuss how they constrain galaxy formation and evolution in the cosmic density field.

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