List of Past CTC Theory Lunches : 01-Sep-2009 to 31-Dec-2009

Date:   Monday 14-Sep-2009
Speaker:   Cole Miller and Stratos Boutloukos
Title:  Summary of "Matter and Electromagnetic Fields in Strong Gravity" workshop


Date:   Monday 21-Sep-2009
Speaker:   Soko Matsumura
Title:  Tidal evolution of planetary systems


Date:   Monday 28-Sep-2009
Speaker:   Sean O'Neill and Chris Reynolds
Title:  Summary of "The Monster's Fiery Breath: Feedback in galaxies, groups, and clusters" conference


Date:   Tuesday 06-Oct-2009
Speaker:   Dr. Pablo Laguna (Center for Relativistic Astrophysics, Georgia Tech)
Title:  Numerical Relativity Preludes: Golden Black Holes, The Kerr Limit and Wet Binaries


Date:   Monday 12-Oct-2009
Speaker:   Doug Hamilton
Title:  Saturn's largest ring


Date:   Monday 19-Oct-2009
Speaker:   Bruno Giacomazzo (joint seminar with the physics gravity group)
Title:  General Relativistic Simulations of Binary Neutron Stars: gravitational waves and matter dynamics

Binary neutron stars are among the most important sources of gravitational waves which are expected to be detected by the current or next generation of gravitational wave detectors, such as LIGO and Virgo, and they are also thought to be at the origin of very important astrophysical phenomena, such as short gamma-ray bursts. I will present results obtained with the use of the fully general relativistic magnetohydrodynamic code Whisky in simulating binary neutron stars that inspiral, merge and eventually form a black hole surrounded by an hot accretion disk. I will describe in particular the gravitational waves that are emitted by both equal and unequal mass systems and the effects that magnetic field can have on them. I will also discuss the properties of the tori that can be formed by these systems.

Date:   Wednesday 21-Oct-2009
Speaker:   Asaf Pe'er (STScI)
Title:  A Model for Emission from Microquasar Jets: Consequences of a Single Acceleration Episode

There are strong evidence for powerful jets in the low/hard state of black-hole candidates X-ray binaries (BHXRBs). I present a model in which electrons are accelerated once at the base of the jet, and are cooled by synchrotron emission and possible adiabatic energy losses. The accelerated electrons assume a Maxwellian distribution at low energies and possible energetic power law tail. These characteristic energies, combined with the decay of the magnetic field along the jet, introduce a wealth of spectra. I will identify critical values of the magnetic field, and show that: (I) the decay of the magnetic field along the jet enables, for wide jets, production of flat radio spectra without the need for electrons re-acceleration along the jet. (II) An increase of the magnetic field above a critical value of ~10^5 G leads to a sharp decrease in the flux at the radio band, while the flux at higher frequencies saturates to a constant value. (III) For strong magnetic field, the flux decays in the optical/UV band as F_\nu ~ \nu^{-1/2}, irrespective of the electrons initial distribution. (IV) For B_0 ~ 10^4 G, the flux at the X-ray band gradually steepens. (V) With adiabatic energy losses, flat spectrum can be obtained only at a limited frequency range, and under certain conditions (VI) For narrow jets, r(x) ~ x^{\alpha} with \alpha < 1/2, flat radio spectrum cannot be obtained. Reference: Pe'er & Casella, 2009, ApJ, 699, 1919 Casella & Pe'er, 2009, ApJ, 703, L63

Date:   Monday 26-Oct-2009
Speaker:   Susan Lamb and Stacy McGaugh
Title:  Mass distribution in disk galaxies


Date:   Monday 02-Nov-2009
Speaker:   Ed Shaya
Title:  Hard to see, the Dark Matter is.


Date:   Monday 09-Nov-2009
Speaker:   Dusan Keres (CfA)
Title:  Cold mode of gas accretion

Most galaxies are actively star forming at all epochs. However, observations of cold gas reservoirs indicate that, at any epoch, there is not enough gas in dense galactic component to support evolution of star formation activity over time. This suggests that galactic gas is being replenished from the intergalactic medium. I use fully cosmological simulations of galaxy formation to study the gas supply into galactic component from high redshift to present. At high redshift "smooth" infall of cold filamentary gas dominates the gas supply of all galaxies. This "cold mode accretion" is a major driver of very active star formation of high-z galaxies enabling such activity to proceed for a significant fraction of the Hubble time. Gas accretion rates at a given halo and galaxy mass decrease with time, causing the drop in star formation rates. Properties and geometry of infalling gas change with halo mass and redshift. At low redshift some of the halos are able to cool hot virialized gas but filaments are still indirectly supplying galaxies with gas via cold gaseous clouds that form from infaling cold/warm filamentary gas. In this talk I will describe properties, physics and consequences of such gas accretion for the formation and evolution of galaxies. Finally, I will point out promising directions for future research in this area and discuss several observational probes of cold halo gas that can provide strong constraints on the physics of gas accretion in galaxies.

Date:   Monday 16-Nov-2009
Speaker:   Fred Lamb
Title:  The puzzle of millisecond pulsars


Date:   Monday 23-Nov-2009
Speaker:   Manuel Tiglio (gravity group seminar)
Title:  Modeling non-linearities in the ringdown of colliding black holes

I will present a new covariant and gauge invariant formalism for arbitrary second order perturbations of (non-rotating) black holes. Next, I will discuss a numerical implementation of this formalism and studies of the gravitational radiation generated by the self-coupling of linear perturbations. Finally, I will review some ongoing comparisons with full non-linear simulations of colliding black holes. One of the goals of this ongoing effort is to gain further insights into the late stages of colliding black holes and its analytical modeling. A second motivation comes from the possibility of LISA being able to detect a second ringdown mode.

Date:   Monday 30-Nov-2009
Speaker:   Peter van Ham
Title:  Spinning supermassive black holes


Date:   Tuesday 08-Dec-2009
Speaker:   Joe Wolf (UC Irvine)
Title:  Testing galaxy formation scenarios with a new mass estimator

We present the recently derived Wolf et al. (2009) mass estimator, which is applicable for spherical pressure-supported stellar systems spanning over ten orders of magnitude in luminosity, as a tool to test galaxy formation theories. We show that all of the Milky Way dwarf spheroidal galaxies (MW dSphs) are consistent with having formed within a halo of mass approximately 3 x 10^9 Msun in LCDM cosmology. The faintest MW dSphs seem to have formed in dark matter halos that are at least as massive as those of the brightest MW dSphs, despite the almost five orders of magnitude spread in luminosity. We expand our analysis to the full range of observed pressure-supported stellar systems and examine their half-light I-band mass-to-light ratios. The M/L vs. half-light mass M_1/2 relation for pressure-supported galaxies follows a U-shape, with a broad minimum near M/L ~ 3 that spans dwarf elliptical galaxies to normal ellipticals, a steep rise to M/L ~ 3,200 for ultra-faint dSphs, and a more shallow rise to M/L ~ 800 for galaxy cluster spheroids.

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