The Space Density of Compton Thick AGN and the X-ray Background

TL;DR

This study constrains the density and evolution of Compton-thick AGN using local and high-redshift surveys, revealing lower local densities than models predict and highlighting the importance of direct observations for understanding heavily-obscured black holes.

Contribution

It provides new constraints on the space density and evolution of Compton-thick AGN, emphasizing the role of direct observations and IR selection in understanding their contribution to the X-ray background.

Findings

Lower local density of CT AGN than models predict

High-redshift IR-selected CT AGN suggest different evolution

CT AGN contribute about 9% to the X-ray background

Abstract

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