The Space Density of Compton‐thick AGN

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 $f24μm/fR>1000$ 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 $NH>5×1024 cm−2.$ 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 $LX>1044 erg/s$ sources from $z = 1.5$ to 2.5.