The Space Density of Compton-thick AGN

TL;DR

This paper investigates the abundance and evolution of Compton-thick AGN across cosmic time, using multi-wavelength surveys and candidate selection techniques to estimate their contribution to the X-ray background.

Contribution

It provides new estimates of the space density and evolution of Compton-thick AGN, especially at high redshift, using a combination of X-ray and mid-IR data.

Findings

The space density of Compton-thick AGN at z~2 is about 10^-5 Mpc^-3.

Strong evolution in the number of luminous CT AGN from z=1.5 to 2.5.

Candidate selection methods effectively identify heavily obscured AGN.

Abstract

We constrain the number density and evolution of Compton-thick Active Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray background. 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 mid-IR parameters. We present the properties of a sample of 211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South (ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ~2 orders of magnitude smaller than for the general AGN population, suggesting column densities of NH>5x10^24 cm^-2. The space density of CT AGN at z~2 derived from these observations is ~10^-5 Mpc^{-3}, finding a strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.