THE SPACE DENSITY OF COMPTON-THICK ACTIVE GALACTIC NUCLEUS AND THE X-RAY BACKGROUND

Abstract

We constrain the number density and evolution of Compton-thick Active\nGalactic Nuclei (AGN). In the local Universe we use the wide area surveys from\nthe Swift and INTEGRAL satellites, while for high redshifts we explore\ncandidate selections based on a combination of X-ray and mid-IR parameters. We\nfind a significantly lower space density of Compton-thick AGN in the local\nUniverse than expected from published AGN population synthesis models to\nexplain the X-ray background. This can be explained by the numerous\ndegeneracies in the parameters of those models; we use the high-energy surveys\ndescribed here to remove those degeneracies. We show that only direct\nobservations of CT AGN can currently constrain the number of heavily-obscured\nsupermassive black holes. At high redshift, the inclusion of IR-selected\nCompton-thick AGN candidates leads to a much higher space density, implying (a)\na different (steeper) evolution for these sources compared to less-obscured\nAGN, (b) that the IR selection includes a large number of interlopers, and/or\n(c) that there is a large number of reflection-dominated AGN missed in the\nINTEGRAL and Swift observations. The contribution of CT AGN to the X-ray\nbackground is small, ~9%, with a comparable contribution to the total cosmic\naccretion, unless reflection-dominated CT AGN significantly outnumber\ntransmission-dominated CT AGN, in which case their contribution can be much\nhigher. Using estimates derived here for the accretion luminosity over cosmic\ntime we estimate the local mass density in supermassive black holes and find a\ngood agreement with available constraints for an accretion efficiency of ~10%.\nTransmission-dominated CT AGN contribute only ~8% to total black hole growth.\n