Past Colloquia
(in reverse chronological order)
Dec 14, 2007: Dr. Brant Robertson (Univ. of Chicago)
“Molecular Gas, Interstellar Radiation, and the Scale-Dependency of Star Formation”
We use hydrodynamical simulations of disk galaxies to study relations between star formation and properties of the molecular interstellar medium (ISM). We implement a model for the ISM that includes low-temperature (T < 10^4 K) cooling, directly ties the star formation rate to the molecular gas density, and accounts for the destruction of H2 by an interstellar radiation field from young stars. We demonstrate that the ISM and star formation model simultaneously produces a spatially resolved molecular gas surface density Schmidt-Kennicutt relation with a power law index of n_mol ~ 1.4 independent of galaxy mass, and a total gas surface density -- star formation rate relation with power-law index that steepens from n_tot ~ 2 for large galaxies to n_tot >~ 4 for small dwarf galaxies. We show that deviations from the disk-averaged, n_tot ~ 1.4 total gas correlation determined by Kennicutt (1998) owe primarily to spatial trends in the ISM molecular fraction and may explain observed deviations from the global Schmidt-Kennicutt relation. We discuss the potential implications of our results for dwarf galaxy formation and modeling, cosmological disk formation, and models of the luminosity and baryonic mass function of galaxies.
Dec 13, 2007: Dr. Brant Robertson (Univ. of Chicago)
“Merger, Supermassive Black Holes, and the Fundamental Scaling Relations of Simulated Elliptical Galaxies”
Elliptical galaxies display an intriguing combination of chaos and regularity. While these systems likely form in violent mergers of progenitor galaxies, they also obey a variety of striking scaling relations. Foremost among these relations are the Fundamental Plane, the relation between black hole mass and spheroid mass, and the relation between black hole mass and spheroid velocity dispersion. We explore the origin and evolution of these relationships that connect the properties of stellar spheroids and the supermassive black holes they host by simulating the formation of spheroidal galaxies in hydrodynamical simulations of disk galaxy mergers. These simulations demonstrate the crucial role that gas dissipation plays in both forming the spheroidal stellar components as well as feeding supermassive black hole growth. We also explore the properties of the highest-redshift quasars using hydrodynamical simulations of the hierarchical formation of a quasar host galaxy, and identify new observational methods to explore the formation scenario of billion solar mass supermassive black holes at redshift z~6.
Dec 7, 2007: Dr. Yosuke Mizuno (NASA/Marshall Space Flight Center)
“3D RMHD Simulations of Magnetized Spine-Sheath Relativistic Jets"
We have performed numerical simulations of weakly and strongly magnetized relativistic jets embedded in a weakly and strongly magnetized stationary or mildly relativistic (0.5c) sheath flow using the RAISHIN code. In the numerical simulations a jet with Lorentz factor gamma=2.5 is precessed to break the initial equilibrium configuration. Results of the numerical simulations are compared to theoretical predictions from a normal mode analysis of the linearized RMHD equations describing a uniform axially magnetized cylindrical relativistic jet embedded in a uniform axially magnetized sheath flow. The prediction of increased stability of a weakly-magnetized system with mildly relativistic sheath flow to Kelvin-Helmholtz instabilities and the stabilization of a strongly magnetized system with mildly relativistic sheath flow is confirmed by the numerical simulations.
Dec 3, 2007: Prof. Stephane Courteau (Queen’s University)
“Puzzles in Galaxy Scaling Relations"
Spiral galaxies exhibit global correlations between their total luminosity, maximum circular velocity, and disk scale length that pose a fundamental challenge to standard models of structure formation. On local scales, galaxy scaling relations that include velocity dispersion estimates (such as the M_BH-sigma and the Vcirc-sigma relations) are also challenging to both structure formation and galaxy dynamics models. I will discuss recent derivations of global and local galaxy scaling relations and our attempts to model them.
Nov 30, 2007: Dr. Robert Nobel (SLAC)
"Particle Acceleration in Relativistic Jet-Plasma Interactions"
Relativistic outflows are commonly associated with astrophysical sources such as galactic nuclei and gamma-ray bursts. Their interaction with ambient plasma is believed to give rise to particle acceleration producing the observed radiation spectrum ranging from radio to gamma-ray. Particle acceleration occurs when the energy carried by the outflow is transferred to the surrounding material. Understanding the nature of these acceleration mechanisms is one of the most outstanding issues today in cosmic ray physics. The fundamental issue we consider is how jet-plasma instabilities can lead to particle acceleration. In particular we report on the study of the physics occurring at the plasma wavelength scale for a charge-neutral, electron-positron jet propagating through an electron-ion plasma. A 3-D, relativistic particle-in-cell code is used to explore the physics within the jet interior and at the jet-plasma boundary. We observe the development of magnetic and electrostatic instabilities resulting in rapid particle acceleration and plasma heating, and we identify the underlying mechanisms. The particles we observe can be synchronously accelerated over large distances and are candidates for the relativistic particles that produce jet emission spectra. The process we observe may also serve as the first-stage injection mechanism for cosmic ray acceleration.
Oct 12, 2007: Dr. Tim Kallman (NASA Goddard)
“What Can We Learn from Active Galaxy Warm Absorbers?”
X-ray spectra of many Seyfert galaxies show evidence for absorption or scattering by gas which is flowing out from the nucleus and which is partially ionized. That is, light elements such as hydrogen and helium are fully ionized while some bound electrons remain on the abundant heavier elements such as oxygen and iron. This gas was unknown prior to the advent of X-ray spectroscopy. Our understanding of warm absorber flows is hindered by the fact that they are probably not spherical. However, spectra from several Seyfert galaxies can be combined in order to provide enough information about geometry to remove this uncertainty. In this talk I will summarize the available observational information about these flows, present models which allow the mass flux to be estimated, and discuss the implications for the mass budget and other properties of active galaxies.
Oct. 5, 2007: Dr. Dan Whalen (LANL)
“Local Radiative Feedback: The Photoevaporation of Clustered Minihalos by the First Stars”
Very massive primordial stars photoionize and evaporate other dark matter halos in their vicinity to varying degrees, raising the question of whether star formation in them is promoted or suppressed. I will present two-dimensional numerical simulations of the ionization of minihalos proximate to a Pop III star that incorporate photon conserving
multifrequency radiation hydrodynamics together with 9-species primordial chemistry. We find that halos with central densities above 2000 cm^-3 are impervious to both ionizing and Lyman-Werner (LW) UV while more diffuse halos with densities below 2 -3 cm^-3 are completely destroyed anywhere within the cluster. Star formation can be accelerated or delayed in halos of intermediate density depending on how the I-front and its shadow compress the core of the halo after the death of the star. Most of the
halos are photodissociated but this plays no long term role in the later collapse of their cores because H2 formation is rapidly catalyzed in the warm relic H II region after the death of the star.
Sep. 21, 2007: Dr. Paul O’Brien (University of Leicester)
“GRBs: a short story”
The origin of short bursts has been argued about for many years. Recent observations by Swift have revealed details about the location, duration and properties of short GRBs which suggest multiple progenitors. I will discuss the observations and present preliminary results linking the GRB duration with the location of the GRBs with respect to their host galaxies and how their light curves compare to feedback accretion models. I will also outline a few cases of possible very low redshift short bursts which are extragalactic SGR candidates.
Sep. 4, 2007: Dr.Werner Becker (MPI fuer extraterrestrische Physik)
“Future optical and X-ray observatories for neutron star and pulsar studies”
Optical and X-ray astronomy has made great progress in the past several years thanks to telescopes with larger effective areas and greatly improved spatial, temporal and spectral resolutions. The next generation instruments like XEUS, Constellation-X, Simbol-X, eROSITA, the James Webb Space Telescope and the ESOs Extremely Large optical telescope are supposed to bring again a major improvement in sensitivity. The purpose of my talk is to summarize the future plans for X-ray and optical telescopes with the emphasis of their application for neutron star astronomy.
Jun 25, 2007: Janusz Gil & Giorgi Melikidze (J. Kepler Astronomical Institite, Zielona Gora, Poland)
"Partially screened inner accelerating region in pulsars and its observational consequences"
We discuss the model of the pulsar polar gap in which the temperature of the polar cap surface is almost equal to the co called critical temperature, below which the surface elements are partially bound and thus strong acceleration potential drop can develop. This critical temperature is defined by the actual value of the magnetic field at the neutron star surface. Observations of thermal X-ray emission from hot polar cap imply an important correlation between the bolometric area and the temperature derived from the blackbody fit. In most cases the bolometric polar cap is much smaller than the canonical polar cap. Therefore, the actual surface magnetic field must differ essentially from that of the pure star centered dipole field. There are also cases with bolometric hot spot larger that the canonical polar cap, which can be also naturally explained within our model. We believe that, major characteristics of the pulsar radiation, including correlation between observed features of X-ray and radio emissions can be naturally explained in the frame of this model.
Jun 18, 2007: Prof. Shiko Kobayashi (Liverpool JMU, UK)
“Polarization in Early GRB Afterglow"
An open issue in the study of gamma-ray bursts is the energy content of GRB jets (e.g. baryonic or Poynting flux dominated). I will discuss how polarization measurements of early afterglow might help to understand the nature of GRB jets.
May 4, 2007: Prof. Rosalba Perna (Univ. of Colorado)
“Gamma-Ray Bursts in the Swift Era”
The launch of the Swift satellite has marked the beginning of a new era in Gamma-Ray Burst (GRB) science. I will start with a brief review of the status of GRB observations prior to Swift, and our understanding of their progenitors for the class of long bursts. I will then discuss how GRB data during the Swift era are improving our understanding of the properties of the progenitors of long GRBs, shedding light on the nature of short bursts, and impacting several areas of cosmology. In particular, I will discuss how GRBs can be used to trace the evolution of the mean density of the medium with redshift and the properties of dust in high-z galaxies. Detection of GRBs at very high redshifts can help set constraints on the small-scale power spectrum of density fluctuations. Statistical studies of short GRBs can improve our understanding of evolutionary binary scenarios. Finally, I will discuss how the new, unexpected observations of X-ray flares are beginning to probe the extreme conditions of the GRB engine.
Apr 19, 2007: Prof. Art Wolfe (UC San Diego)
“Star Formation in Neutral Gas Reservoirs at High Redshifts”
Apr 13, 2007: Dr. Rene Oudmaijer (Leeds University, UK)
“Pre-Main sequence stars probed at high resolution: disks, outflows and
binaries."
Apr 12, 2007: Dr. Dale Frail (NRAO)
“The Hot and Cold Universe"
In the next decade two major facilities will come on line at radio, millimeter and sub-millimeter wavelengths which are capable of studying the cold, obscured Universe and the hot, relativistic Universe. The Expanded Very Large Array (EVLA) and the Atacama Large Millimeter Array (ALMA) will have order of magnitude or more improvements in sensitivity, frequency range and resolution, and angular resolution. I will broadly describe the science capabilities of EVLA and ALMA and I will provide an update of the status of both of these projects. (Much of the work on these two projects is being done at NRAO in Socorro, a short 500 miles from Las Vegas.
Mar 9, 2007: Dr. Jason Prochaska (UC Santa Cruz)
“Probing the ISM Near Star Forming Regions with GRB Afterglow Spectroscopy”
I will describe studies of the interstellar medium in high redshift galaxies as revealed by spectroscopic studies of background quasars and gamma-ray burst afterglows. The emphasis of this talk will be on the velocity fields in young, star-forming galaxies but I will also review the surface density, metallicity, and molecular fraction of gas near gamma-ray bursts.
Mar 2, 2007: Dr. Fred Hamann (Univ. of Florida)
“Quasars, SMBHs and Host Galaxy Evolution"
Recent studies have shown that all massive galaxies (e.g., galactic spheroids) contain central super-massive black holes (SMBHs) of roughly commensurate mass. The formation of these SMBHs must be intimately connected to galaxy formation. But many aspects of this evolution are not understood. Quasars are bright beacons powered by rapid SMBH growth. They were common in the distant past, when massive galaxies where still being assembled and rapidly making stars. It has become a major goal of quasar research to understand how they fit into the larger context of galaxy formation. Our group has focussed on measures of the elemental abundances to constraint the extent to which stars have formed, evolved and enriched the gas prior to the quasar epoch. We find that quasars at all redshifts were preceded by substantial star formation - enough to enrich the gas to near or above solar levels. Other studies reach similar conclusions. I will review these recent results and discuss the implications for galaxy/SMBH evolution.
Feb 23, 2007:Dr. Eric Linder (University of California/Berkeley Lab)
“Concepts and Challenges of the Accelerating Universe”
The acceleration of the cosmic expansion is a fundamental challenge to standard models of particle physics and cosmology. The new physics of dark energy may lie in the nature of gravity (when is gravity no longer attractive?), the quantum vacuum (does nothing weigh something?), or extra dimensions (is nowhere somewhere?). I discuss some of the puzzles and possibilities of dark energy and the means for unraveling them through cosmological probes. These astronomical observations will explore what makes up the still unknown 95% of our universe, while producing astronomical images equal to a million Hubble Deep Fields.
Feb 2, 2007: Prof. Marta Volonteri (Univ. of Michigan)
“Hierarchical build-up of massive black holes”
Dec 8, 2006: Dr. Phil Armitage (JILA and Univ. of Colorado, Boulder)
“New insights into the Solar System from extrasolar planets”
The discovery of 200 extrasolar planets provides a statistical sample of planetary systems to compare with our own Solar System. Some properties of extrasolar planetary systems - such as the existence of some gas giants with day-long orbital periods, and others with orbits as eccentric as comets - are alien enough that we may question whether the Solar System is a typical or rare outcome of the planet formation process. I will discuss recent observations of massive extrasolar planets, their implications for theories of giant planet formation, and more speculative consequences for the abundance of habitable planets.
Dec 1, 2006: Prof. Chris Howk (Univ. of Notre Dame)
“Gas Phase Physics and the Evolution of Galaxies”
One of the major accomplishments of modern astrophysics is the development of a picture for the evolution of star formation in the Universe. The stars in the Universe formed from the gas within the galaxies containing stars. Indeed, the gas itself can be an important indicator of the physics of galactic evolution. I will describe some of our on-going work to use the gas phase of galaxies to study their evolution from the early Universe to today, including the physics of the interstellar gas in early galaxies and the creation and distribution of elements in the Universe.
Nov 3, 2006: Dr. Tom Jarrett (IPAC/JPL/CALTECH)
“Constructing the Norma Wall of Galaxies --the Newly Emerging Portrait of the Great Attractor”
Oct 25, 2006: Dr. Yu Gao (Purple Mountain Observatory, China)
“The Global Star Formation Law: from Dense Cores in the Milky Way to Extreme Starbursts at High Redshifts”
