List of Past Astronomy Colloquia : 01-Jan-2001 to 01-Jun-2001

Date:   Wednesday 31-Jan-01
Speaker:   Dr. Dan Reichart (Caltech)
Title:  Gamma-Ray Bursts: Coming to a Field Near You!

As much as the gamma-ray burst field was revolutionized by the now famous discoveries of the late 1990s, other fields stand to be revolutionized by the burst field in the HETE-2 and Swift eras. First, I review the evidence for a connection between bursts and supernovae. Given that first light is thought to have occurred at a redshift of z ~ 15 - 20, I discuss how bursts will be used to probe these very high redshifts in the very near future. Bursts can be used to probe the star-formation history of the universe, to detect supernovae out to redshifts of z ~ 4 - 5, to probe the metallicity history of the universe, to probe large-scale structure, and to probe the epoch of reionization out to redshifts of z ~ 6 - 7. Next, I present a Cepheid-like distance estimator for the bursts; i.e., I show that the variability of a burst's light curve can be used to estimate its luminosity, and hence its distance. Having applied this estimator to the bulk of the BATSE catalog, I present preliminary redshift and luminosity distributions for the bursts. These distributions hint at the detection of first light. Finally, I present work in which I use scattered light to probe the circumburst environment of GRB 980329. I show that this likely cosmological object reached naked-eye magnitudes before fading away.

Date:   Wednesday 7-Feb-01
Speaker:   Dr. Jane Turner (GSFC)
Title:  X-ray Grating Spectroscopy of Seyfert type Galaxies: The story so far...

High-Resolution X-ray spectra of AGN have only recently become possible using the gratings on Chandra and XMM. Results from the first ~1 year of these two missions have already yielded some exciting insights into the physical conditions of circumnuclear gas in Seyfert type AGN. I will present a brief historical summary of X-ray spectroscopy of AGN prior to these missions. I will then review the new results, discuss their impact on the Big Picture of AGN, and speculate what future observations may hold.

Date:   Wednesday 14-Feb-01
Speaker:   Dr. Chris Reynolds (U Colorado)
Title:  The inner regions of black hole accretion disks

Medium resolution X-ray spectroscopy now allows us to probe the near environment of massive black holes in active galactic nuclei. I will discuss how past, current and future X-ray studies can be used to constrain the physical nature of the inner regions of the accretion flow and the fundamental properties of the central black hole (such as its mass and spin). Motivated by these observational investigations, I will describe recent MHD simulations of the inner regions of black hole accretion disks. It will be shown that phenomena previously in the realm of black hole theorists are now becoming accessible to observational study.

Date:   Wednesday 21-Feb-01
Speaker:   Dr. Paul Nandra (GSFC)
Title:  X-ray signatures of black holes in active galaxies

Compelling evidence for the presence of black holes in the nuclei of active galaxies has been obtained from X-ray observations. Spectroscopy of the iron K emission line has shown evidence for matter orbiting with extremely high velocities, and the characteristic redshift expected due to time dilation in the intense gravitational field. Current and future X-ray data can be used to determine the properties of the black holes, the processes fueling galaxy activity and the evolution of that activity with cosmic time. The importance of these observations in the wider astronomical context will also be discussed.

Date:   Wednesday 28-Feb-01
Speaker:   Dr. Ed Moran (UC Berkeley)
Title:  The Hidden Truth About the Cosmic X-ray Background Radiation

The origin of the cosmic X-ray background radiation (XRB) has puzzled astronomers since its discovery nearly four decades ago. While the debate as to whether the XRB arises from diffuse or discrete sources has been settled, a class of objects with the properties necessary to explain the observed characteristics of the XRB has remained elusive. The brightest extragalactic X-ray sources in the sky -- black-hole powered active galactic nuclei (AGNs) -- have X-ray spectra in the 2 - 20 keV band that are generally far steeper than the spectrum of the XRB itself. Current models, therefore, attribute the majority of the XRB in this energy range to a different population of objects -- AGNs that are largely hidden by dense interstellar gas in their host galaxies. Such objects are expected to have significantly flattened X-ray spectra. In this talk, I discuss the results of optical and X-ray studies designed to provide a critical test of the hidden AGN hypothesis. I also discuss how our efforts to sort out what the XRB is (and what it isn't) can shed light on some important cosmological problems: the origin of the recently discovered infrared background radiation and the star-formation history of the Universe.

Date:   Wednesday 7-Mar-01
Speaker:   Dr. Dara Norman (SUNY/SB)
Title:  Quasar-Galaxy Correlations and the Detection of Magnification Bias

Weak gravitational lensing by clusters and superclusters can change our perception of the distribution of bright background quasars through the phenomenon of magnification bias. It has been suggested that this lensing will lead to an association of moderate redshift quasars (z ~ 1.0-2.0) and foreground galaxies (z ~ 0.2-0.3).

I will explain how gravitational lensing leads to a magnification biasing of distant sources. I will present my own and other work that attempts to detect and measure this biasing through quasar-galaxy correlations. I will discuss what a measure of magnification bias can tell us about cosmological parameters and large-scale structure. Finally, I will present future plans for how we might better measure this phenomenon.

Date:   Wednesday 14-Mar-01
Speaker:   Dr. Lynne Hillenbrand (Caltech)
Title:  Embedded Clusters, Variable Stars, and Planet-Building Disks

Star formation in molecular clouds produces both dense clusters and more isolated stellar products. First I will discuss the typical and extreme properties of the stellar populations found in well-studied molecular clouds close to the sun. These young stars are often variable, both photometrically and spectroscopically. A second part of my talk will describe recent results from a large study of photometric variability at near-infrared wavelengths, and its relation to circumstellar disks in some cases. Planets are formed within the circumstellar disks that near-ubiquitously surround the youngest stars. In the final section of my talk I will discuss our ability at present to place contraints on the timescales for planet formation based on the survival times of these disks.

Date:   Wednesday 21-Mar-01
Speaker:   NO COLLOQUIUM (Spring Break)
Title:  TBA

Date:   Wednesday 28-Mar-01
Speaker:   Dr. Chuck Steidel (Caltech)
Title:  Baryonic Structure in the z=1-4 Universe

In the past several years, there has been considerable progress made in the discovery and observation of galaxies in the high redshift universe. I will try to focus on the question of "what do we do with them?" now that they are observationally accessible and large samples are feasible. I will concentrate on how the observed properties of distant galaxies (including kinematics, star formation rates, clustering properties, chemical abundances, star formation histories, and the connection to the diffuse intergalactic medium) fit in (or not) with theoretical expectations of galaxy and structure formation. Honest opinions about prospects for the future will be expressed.

Date:   Wednesday 4-Apr-01
Speaker:   Dr. Caryl Gronwall (JHU)
Title:  The KPNO International Spectroscopic Survey for Nearby Emission-Line Galaxies

The KPNO International Spectroscopic Survey (KISS) is a new objective-prism survey for extragalactic emission-line objects which reaches significantly fainter flux levels than previous surveys. We are using this H-alpha selected sample to study such topics as: 1) the star formation rate density of the local universe, 2) the spectroscopic properties of the sample, and 3) the spatial distribution of emission-line galaxies.

Date:   Wednesday 11-Apr-01
Speaker:   Dr. Matt Holman (CfA/SAO)
Title:  Evidence for an Extended Scattered Disk

Current nomenclature commonly divides the trans-neptunian region of the solar system into (1) the `Kuiper Belt' and (2) the `scattered disk'. By telescopic tracking, our observing team has established that the orbit of the trans-neptunian object (2000 CR_105) has a perihelion of ~44 AU, and is thus outside the domain controlled by strong gravitational close encounters with Neptune. Because 2000 CR_105 is on a very large, eccentric orbit (with semimajor axis a ~ 216 AU and eccentricity e ~ 0.8 this object must have been placed on this orbit by a gravitational perturbation which is *not* direct gravitational scattering off of any of the giant planets (on their current orbits). This discovery implies that there must be a large population of trans-neptunian objects with large perihelion distances in an `extended scattered disk'. The existence of this object may have profound cosmogonic implications for our understanding of the formation of the outer Solar System. I will discuss some viable scenarios which could have produced the orbit of 2000 CR_105, including long-term diffusive chaos and scattering off of other massive bodies in the outer Solar System.

Date:   Wednesday 18-Apr-01
Speaker:   Dr. Jane Charlton (PSU)
Title:  Star Forming Pockets Outside of Galaxies

A new and mysterious class of object has been revealed by the detection of numerous weak Mg II doublets in quasar absorption line spectra. The properties of these objects will be reviewed. They are not near to luminous galaxies, yet they have metallicities close to the solar value; they are likely to be self-enriched. A significant fraction of the weak MgII absorbers are constrained to be less than 10 parsecs in size, yet they present a large cross section for absorption, indicating that there are more times more of them than there are luminous galaxies. The talk will discuss the possibilities that they are remnants of Population III star clusters or tracers of supernova remnants in a population of "failed dwarf galaxies" expected in cold dark matter structure formation scenarios.

Date:   Wednesday 25-Apr-01
Speaker:   Dr. Steinn Sigurdsson (PSU)
Title:  Planets in clusters?

A large number of extra-solar planets have been discovered. A significant fraction of star formation occurs in clusters, and the formation of planets around stars in clusters can be affected both by the radiative environment and by dynamical processes. I'll discuss some recent results from searches for planets in clusters, and some theoretical issues relevant to formation and detection of planets around cluster stars.

Date:   Wednesday 2-May-01
Speaker:   Dr. Chris McKee (Berkeley)
Title:  The Formation of Stars and Star Clusters

Date:   Wednesday 9-May-01
Speaker:   Dr. Roger Chevalier (U Virginia)
Title:  Pulsar Wind Nebulae

The evolution of pulsar nebulae and their relevance to pulsar properties will be discussed.

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