The Changing Agn Population

We investigate how the fraction of broad-line sources in the AGN population changes with X-ray luminosity and redshift. We first construct the rest-frame hard-energy (2 − 8 keV) X-ray luminosity function (HXLF) at z = 0.1 − 1 using Chandra Lockman Hole-Northwest wide-area data, Chandra Deep Field-North 2 Ms data, other Chandra deep field data, and the ASCA Large Sky Survey data. We find that broad-line AGNs dominate above ∼ 3 × 10 ergs s and have a mean luminosity of 1.3 × 10 ergs s. Type II AGNs can only become an important component of the X-ray population at Seyfert-like X-ray luminosities. We then construct z = 0.1− 0.5 and z = 0.5− 1 HXLFs and compare them with both the local HXLF measured from HEAO-1 A2 survey data and the z = 1.5 − 3 HXLF measured from soft-energy (0.5−2 keV) Chandra and ROSAT data. We find that the number density of Lx > 10 44 ergs s sources (quasars) steadily declines with decreasing redshift, while the number density of Lx = 10 43 − 10 ergs s sources peaks at z = 0.5 − 1. Strikingly, however, the number density of broad-line AGNs remains roughly constant with redshift while their average luminosities decline at the lower redshifts, showing another example of cosmic downsizing. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: active