List of Past Astronomy Colloquia : 01-Jan-2004 to 01-Jun-2004


Date:   Wednesday 4-Feb-2004
Speaker:   Dr. D. Proga (JILA, U. Colorado)
Title:  "How outflows reveal and assist accretion onto black holes"


Date:   Wednesday 11-Feb-2004
Speaker:   Dr. N. Turner (JPL)
Title:  "Accretion Power in Active Galactic Nuclei"

Active galactic nuclei are among the most luminous objects known. They are a key problem in astrophysics because they link galaxy formation to the growth of supermassive black holes, and because they are observed to cosmological look-back times. Unfortunately, we do not know how the gravitational energy released during accretion is converted to the photons reaching our detectors. I will discuss direct numerical radiation-MHD calculations, showing how the energy release may occur in the case of black holes accreting at 1% to 100% of the Eddington limit.


Date:   Wednesday 18-Feb-2004
Speaker:   Dr. C. Fragile (LLNL)
Title:  "Jet-Induced Star Formation: A possible "positive feedback" mechanism from supermassive black holes"

Accumulating observational evidence from a number of radio galaxies suggests an association between their jets and regions of active star formation. The standard conjecture is that shocks generated by the jet propagate through an inhomogeneous medium and trigger the collapse of overdense clouds, which then become active star-forming regions. Although this model provides a useful picture, the complex nature of this problem, including nonlinear effects from higher order coupling between hydrodynamics, cooling, magnetic fields, and gravity suggest that a numerical investigation is crucial. In this talk, I will briefly review some of the most compelling observational data and then present results of recent hydrodynamic and magnetohydrodynamic simulations of radiative shock-cloud interactions. These will show that for moderate cloud densities and shock Mach numbers, cooling instabilities provide a highly efficient collapse mechanism. I will then argue that this "positive feedback" may be important in explaining why the most massive black holes are found in the most massive galaxies.


Date:   Monday 23-Feb-2004
Speaker:   Dr. M. Ricotti (Cambridge)
Title:  "The cosmological impact of the first galaxies"

Primordial galaxies host the first sources of light emitted into a previously dark universe. Using cosmological simulations that include the effects of radiation and supernova explosions, I show how the formation of primordial galaxies is self-regulated on cosmological scales. Recently the WMAP satellite has detected a very early epoch of reionisation of the intergalactic medium. I show that this result can be explained under a scenario in which X-rays from the first black holes partially ionised the intergalactic medium long before stellar ionisation occurred. I discuss observations that can be used to test this scenario and assess the cosmological impact of the first galaxies and black holes.


Date:   Monday 01-Mar-2004
Speaker:   Dr. F. Miniati (MPA, Garching)
Title:  "Shock Waves and Cosmic Rays in Galaxy Clusters"

About one third of massive clusters of galaxies exhibit Mpc scale diffuse radio emission revealing the presence of relativistic electrons and magnetic fields in their media. Despite the fact that these phenomena were first observed more than 30 years ago, the origin of the emitting particles is still unknown. After reviewing the observational facts, I will explore the possibility that the relativistic particles are accelerated at large scale shock waves, associated with the process of cosmic structure formation. I will present studies based on fully cosmological simulations that, in addition, follow the processes relevant for the evolution of relativistic electrons and protons (namely: acceleration, propagation and energy losses). I compute non-thermal radiation spectra from radio to gamma-rays produced by the simulated CR populations through various emission mechanisms and compare it with existing measurements. In addition to being able to reproduce the main features of clusters diffuse radio emission, we find that cosmic ray proton pressure may affect the dynamics of the intracluster medium; the next generation of gamma-ray telescopes should be able test this possibility as well as reveal unprecedented information about cosmic accretion shocks.


Date:   Wednesday 03-Mar-2004
Speaker:   Dr. V. Bromm (Harvard CfA)
Title:  "The First Sources of Light"

How and when did the cosmic dark ages end? I present simulations of the formation of the first stars and quasars, discuss their feedback on the IGM, and describe ways to probe their signature with WMAP and JWST. The first supernovae are responsible for the initial metal enrichment of the IGM, and I address the impact of this initial enrichment event on the subsequent history of structure formation. Finally, I describe the properties and statistics of high redshift GRBs and SNe that result from the first generation of stars.


Date:   Monday 08-Mar-2004
Speaker:   Dr. A. Slyz (Oxford)
Title:  "Star formation in the multiphase ISM"

As a first step to a more complete understanding of star formation in the interstellar medium (ISM), we have performed hydrodynamical simulations of a kilo-parsec scale, periodic, highly supersonic and "turbulent" three-dimensional flow. Using simple but physically motivated recipes for identifying star forming regions, we convert gas into stars which we follow self-consistently as they impact their surroundings through supernovae and stellar winds. We investigate how various processes (turbulence, radiative cooling, self-gravity, and supernovae feedback) structure the ISM, determine its energetics, and consequently affect its star formation rate (SFR). We find that we can parameterize the SFR by three quantities: the porosity, gas velocity dispersion, and gas density. In addition we find that algorithms that use a gas density criterion to determine sites of supernovae explosions give dramatically different results from those which self-consistently determine supernovae events from the true locations of star particles.


Date:   Wednesday 17-Mar-2004
Speaker:   Dr. C. Gammie (U. Illinois)
Title:  "Numerical Models of Black Hole Accretion Flows"

Quasars, AGN, galactic black hole binaries, and gamma ray bursts are all probably powered by a rotating black hole surrounded by a magnetized plasma. I will describe recent fully relativistic, time-dependent numerical models of these systems. I will compare the numerical results with analytic models, such as the Blandford-Znajek force-free magnetosphere, and describe how the black hole exchanges energy and angular momentum with the surrounding plasma.


Date:   Wednesday 31-Mar-2004
Speaker:   Dr. A. Evans (SUNY, Stony Brook)
Title:  "Molecular Gas and the Host Galaxies of Infrared Excess Quasi-Stellar Objects"

Quasi-stellar Objects (QSOs) and Ultraluminous Infrared Galaxies (ULIGs) make up the bright end of the local luminosity function. A significant amount of work has been done over the last 15 years characterizing the stellar populations, the dynamical state of the host galaxies, and the dominant sources of atomic gas ionization in both ULIGs and QSOs; these studies have attempted to establish whether or not ULIG and QSOs are two phases of an evolutionary sequence of massive galaxy mergers.

In order to establish the presence of fuel for both star formation and active galactic nuclear activity, a molecular gas survey of z < 0.2 optically selected QSOs has been initiated. This sample of QSOs have infrared-excesses (L_IR [8-1000um] / L [0.1-1um]) > 0.36, making them likely transition sources between the ULIG and UV-excess QSO phenomena. I will compare the millimeter data and recently obtained optical and near-infrared imaging observations of this IR-excess sample with that of local ULIGs and discuss these in the context of the dust enshrouded-quasar model of ULIGs.


Date:   Wednesday 07-Apr-2004
Speaker:   Dr. D. Saumon (Los Alamos National Laboratory)
Title:  "Partly cloudy weather on brown dwarfs"

Brown dwarfs are the new kids on the block of stellar astrophysics and are remarkable hybrids of stars and planets. Their most striking planetary property is the presence of condensates and clouds in their atmospheres. The spectroscopic and photometric evidence for clouds for effective temperatures between 1800K and 1400K is very strong. This corresponds more or less to the full L spectral class. Even more interesting is rising evidence for patchy cloud cover around the transition between the L and T spectral classes.

Models of brown dwarf atmospheres have reached the level where their observed characteristics in terms of effective temperature are fairly well understood. They also explain the non-LTE abundances of CO observed in some brown dwarfs and predict spectral features in the mid-IR that can be studied very well with the Spitzer Space Telescope.


Date:   Monday 12-Apr-2004 [*** Note special date ***]
Speaker:   Dr. M. Strauss (Princeton U.)
Title:  "Quasars near and far with the Sloan Digital Sky Survey"

The Sloan Digital Sky Survey, with comprehensive five-band imaging of thousands of square degrees of sky, and spectroscopy of over 600,000 objects, is revolutionizing our understanding of the nature of active galaxies. I will discuss several on-going projects which use these data. Among the results which will be discussed: -The search for the very highest redshift quasars, and the epoch of reionization. -The nature of Type II quasars, and populations of reddened quasars. -The luminosity distribution of quasars.


Date:   Wednesday 21-Apr-2004
Speaker:   Dr. R. Greenberg (University of Arizona)
Title:  "Tides and the permeable ice crust of Europa"


Date:   Wednesday 28-Apr-2004
Speaker:   Dr. F. Schweizer (Observatories of the Carnegie Institution of Washington)
Title:  "Formation of Globular Clusters in Merging Galaxies"

During mergers of gas-rich galaxies globular clusters form by the hundreds. Their subsequent evolution can be traced through observations of merger galaxies and their remnants of different ages all the way to old ellipticals. In this talk I will review what we have learned about the globular-cluster formation process itself and about the subsequent evolution of the clusters' properties. In summary, the observations suggest that "second-generation" metal-enriched globulars form from giant molecular clouds shocked by the rapid pressure increase in the merger-induced starbursts. This pressure-induced formation lends credence to arguments that the general pressure increase during cosmological reionization at z = ~15 triggered the near-simultaneous formation of the first-generation metal-poor globulars observed in galaxies of all types.


Date:   Wednesday 05-May-2004
Speaker:   Dr. J. Mather (NASA Goddard Space Flight Center)
Title:  "James Webb Space Telescope, SAFIR, SPECS, and the Future of Space Astronomy"

The James Webb Space Telescope (JWST) is the first in a possible series of deployable infrared to millimeter wave space telescopes. The design process for JWST has already produced ultralight mirrors, deployment methods, and cooling approaches that could lead to much larger and more capable equipment in the future. We will describe the progress on JWST in the context of its history and illustrate some new concepts for future missions that spring from it. These include the SAFIR (Single Aperture Far Infrared) telescope and the SPECS (Submillimeter Probe of the Evolution of Cosmic Structure). The JWST will operate at the Sun-Earth Lagrange point L2, where radiative cooling lowers the telescope and instrument temperatures to about 35 K. It will have an 18-segment beryllium primary mirror with a 25 m^2 area fitting inside a 6.6 m circumscribed circle, and will provide spectroscopy and imaging over the wavelength range from 0.6 to 28 um. It is planned for launch in 2011 on an Ariane 5 rocket. The project is a partnership of NASA, ESA, and CSA, and the prime contractor is Northrop Grumman. See http://www.jwst.nasa.gov for more details on JWST.

Missions to follow JWST will be able to draw on a greatly expanded technological base. Other uses ranging from Earth sciences to surveillance demand large space telescopes and interferometric systems, and the infrastructure for remote assembly and astronaut servicing will continue to improve as the Space Station is completed and experience is gained.

The SAFIR (http://safir.jpl.nasa.gov, http://safir.gsfc.nasa.gov) and SPECS (http://space.gsfc.nasa.gov/astro/specs) missions have been approved by NASA for Vision Mission studies. SAFIR was mentioned prominently in the 2000 NRC Decadal Report on Astronomy as the recommended next step in exploring the cosmos at far-infrared wavelengths. The report furthermore states that SAFIR could form the basis for developing a far-infrared interferometer in the succeeding decade. I will summarize both of these concepts and the unique science capabilities enabled by these missions as well as outline how they might further develop as other projects come on line, scientific priorities evolve, and technological capabilities expand.


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