Cosmological evolution of X-ray emitting active galactic nuclei

The Chandra Multi-wavelength Project (ChaMP) is carrying out a wide (>10 deg2) field X-ray survey of the extragalactic universe using Chandra archival images. The ChaMP probes flux levels intermediate between past surveys (ROSAT, ASCA) and the current deep surveys with Chandra. With Chandra's sensitivity out to 8 keV and small point spread function (∼1″ resolution on-axis), the ChaMP is finding populations that have been missed in past optical, UV and soft X-ray surveys.;In this thesis, I present results from the optical followup campaign of 1431 X-ray sources in 23 ChaMP fields. I have primarily led the optical spectroscopic program to identify 452 counterparts with magnitude r' < 22.5. This depth allows us to detect Quasi-Stellar Objects up to z ∼ 5 and heavily obscured Active Galactic Nuclei (AGN) at z < 0.8.;Using a hard band selected sample, we have determined the demographics of the sources responsible for the bulk of the 2--8 keV Cosmic X-ray Background: broad emission line AGN (69%), narrow emission line galaxies (24%), absorption line galaxies (7%). We find that most X-ray unabsorbed AGN have optical properties characterized by broad emission lines and blue colors, similar to optically-selected quasars. We also find a significant population of redder (g'--i ' > 1.0) AGN with broad optical emission lines. Most of the X-ray absorbed AGN are associated with narrow emission line galaxies, with red optical colors characteristically dominated by luminous, early type galaxy host. Overall, we find that 81% of X-ray selected AGN can be explained by current AGN unification models.;We have measured the evolution of the AGN population by extending the known X-ray luminosity function out to higher redshifts (z ∼ 5) than any previous survey. We confirm that the lower luminosity (L 0.3-8kev < 1044 erg s-1) AGN are more prevalent at low redshifts (z < 1), similar to ongoing deep Chandra and XMM- Newton surveys. We find that high luminosity L 0.3-8kev < 1044 erg s-1 AGN evolve in a manner similar to optical surveys. The comoving space density rises from the present epoch to a peak at z ∼ 2.5 then drops off at high redshift. This is the first X-ray survey to detect a decline in the comoving space density at high redshift.