The Changing Active Galactic Nucleus Population

We investigate how the fraction of broad-line sources in the active galactic nucleus (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 43 ergs s -1 and have a mean luminosity of 1.3 × 10 44 ergs s -1 . Type II AGNs can become an important component of the X-ray population only 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 L X > 10 44 ergs s -1 sources (quasars) steadily declines with decreasing redshift, while the number density of L X = 10 43 -10 44 ergs s -1 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.