Physics & Astronomy Colloquium

3:30 PM, Friday, April 14, 2006

Room 155, Chem-Phys Building

Dr. Mark Lacy

Spitzer Science Center
Caltech

``How much supermassive black hole growth
is obscured by dust?''

Every large galaxy is thought to contain a black hole in its center, with a mass of millions to billions of times that of the sun, whose mass scales with the mass of the galaxy. These black holes are thought to grow by accretion of gas, and that accretion is accompanied by extremely luminous emission from an accretion disk around the black hole, known as a quasar. Until recently, it was thought that most luminous quasars could be seen directly in the optical, and that the fraction obscured by large columns of dust (in the optical) and gas (in the soft X-ray) was small. X-ray, optical and especially mid-infrared observations are now showing that this is not in fact the case, and that most quasars are hidden from our direct view. I shall discuss results from mid-infrared selection of quasars using the Spitzer Space Telescope, and their implications for the nature of black hole growth and its link to galaxy formation.

Refreshments will be served in CP 155 at 3:15 PM