Talks & Events
Special Seminars: 2006
Radiation Pressure Supported Starburst Disks and the Fueling of Active Galactic Nuclei
The UV radiation produced by massive stars in the dense ISM of starburst galaxies is absorbed, scattered, and reprocessed into the infrared by dust grains. The radiation pressure associated with the diffusion of these photons out of the starburst disk can provide the dominant vertical support against gravity. This conclusion is particularly robust when the disk is optically thick to its own infrared radiation, as in the central regions of Ultra-luminous Infrared Galaxies (ULIRGs). I will discuss the implications of this important feedback process. In particular, because the disk radiates at its Eddington limit (for dust), the "Schmidt-law" for star formation changes qualitatively. I compare the model with observations of local and high-redshift ULRIGs. I extend the model from many-hundred parsec scales to sub-parsec scales and address the problem of AGN fueling. A strong bifurcation exists between models whose spectra are starburst dominated and those that are AGN dominated.
Models that fuel AGN contain a compact starburst on parsec scales. A natural consequence of this starburst component is the generation of a radiation pressure supported photosphere with vertical extent of order the disk radius. This structure may be the "obscuring torus" required in unified models of AGN. I further speculate on the origin of the disk(s) of young stars at the Galactic Center. Lastly, I show that radiation pressure may be important in driving galactic winds and in setting the "M-sigma" relation between the velocity dispersion of spheroids and the masses of the black holes they harbor.
The Search for Cosmic Supernova Explosions with SDSS-II
The Sloan Digital Sky Survey II has started a massive supernova search, where 300 square degrees of the sky are scheduled for multi-band imaging on a cadence of two days during the fall seasons of 2005 - 2008. A unique large-format imaging camera array with a total of 120 million pixels is capable of recording 200 GB of data in a single night of observing, allowing efficient discoveries of supernova explosions out to z = 0.4, or about 1/3 of the way back to the Big Bang. The first-year campaign, which ended in December 2005, was a remarkable success and a total of 126 SN Ia in the redshift interval of 0.01 < z < 0.42 were spectroscopically confirmed in the 3-month period. Although our primary focus this year has been SN Ia cosmology, the survey has produced an enormous amount of data of other types of transient sources as well. I will present some early results from the first year of the survey and discuss future prospects.
Harassing the Neighbors: The Impact of Relativistic Jets on the Universe at Large
Relativistic jets are powerful collimated flows emanating from the atmospheres of growing black holes. They are the prime mechanism by which black holes can interact with their large-scale environment, coupling black hole growth with the evolution of large-scale cosmic structure. X-ray evidence from galaxy clusters shows that this interaction is both violent and frequent. Computing resources and numerical methods have reached a level of fidelity where this interaction and its consequences for cosmology can be simulated directly. This provides us with a path towards a new, quantitative understanding of the detailed hydrodynamic processes around powerful jets and their impact on the universe.
Massive Black Holes and Galaxy Formation
Molecules in Wonderland: Nonequilibrium processes in the interstellar medium
The chemical and physical states of an interstellar cloud are far out of thermodynamical equilibrium. The distinctions between microscopic and macroscopic phenomena defy normal human experience: an interstellar cloud is a molecular wonderland in which the collisional mean-free-path can approach the size of our solar system. The abundances and excitations of interstellar molecules are controlled by processes that have a wide range of characteristic temperatures and that are sometimes intrinsically non-thermal. This seminar will concentrate on one example of molecular ions that may probe non-equilbrium processes. The formyl ion, HCO+, is widely observed and relatively abundant in diffuse molecular gas. In this environment the chemical time-scales are short enough that the abundances of HCO+ and its highly reactive isomer HOC+ might be expected to reflect a chemical steady state. Based on well studied chemical reactions and measured abundances of the potential parent molecules of these ions, we can make an empirical analysis of the budget of formation and destruction rates in order to test the assumption of steady state (A. Tappe & J. H. Black, in preparation).
Dust in Active Galactic Nuclei
The ultra-violet spectra of quasars have been widely assumed to be unreddened, yet in the optical and IR active galactic nuclei show signs of reddening. I show that the solution to this paradox is that the dust in active galactic nuclei is unlike any hitherto known. I discuss the variety of extinction curves seen in AGNs. The dust has been substantially modified by the harsh environment of the AGNs. Reddening of AGNs has wide-ranging implications both for understanding how black hole accretion works and the role of AGNs in galaxy formation. The continuum spectral energy distributions of different types of AGNs are not radically different. This supports unified pictures of accretion power from scales of stellar mass objects up to supermassive black holes. The presence of dust means that AGNs are more plentiful and more luminous than has hitherto been thought. This has implications for the formation of supermassive black holes in the nuclei of galaxies in the early universe.