Black Holes in Galaxy Mergers: Evolution of Quasars

Based on numerical simulations of gas-rich galaxy mergers, we discuss a modelin which quasar activity is tied to the self-regulated growth of supermassiveblack holes in galaxies. Nuclear inflow of gas attending a galaxy collisiontriggers a starburst and feeds black hole growth, but for most of the durationof the starburst, the black hole is heavily obscured by surrounding gas anddust which limits the visibility of the quasar, especially at optical and UVwavelengths. Eventually, feedback energy from accretion heats the gas andexpels it in a powerful wind, leaving a 'dead quasar'. Between buried and deadphases there is a window during which the galaxy would be seen as a luminousquasar. Because the black hole mass, radiative output, and distribution ofobscuring gas and dust all evolve strongly with time, the duration of thisphase of observable quasar activity depends on both the waveband and imposedluminosity threshold. We determine the observed and intrinsic lifetimes as afunction of luminosity and frequency, and calculate observable lifetimes ~10Myr for bright quasars in the optical B-band, in good agreement with empiricalestimates and much smaller than the black hole growth timescales ~100 Myr,naturally producing a substantial population of 'buried' quasars. However,observed and intrinsic energy outputs converge in the IR and hard X-ray bandsas attenuation becomes weaker and chances of observation greatly increase. Weobtain the distribution of column densities along sightlines in which thequasar is seen above a given luminosity, and find that our result agreesremarkably well with observed estimates of the column density distribution fromthe SDSS for appropriate luminosity thresholds. (Abridged)Comment: 12 pages, 7 figures. Accepted for publication in ApJ (September 2005). Replacement with minor revisions from referee repor