Space density evolution of X-ray selected AGN as a function of redshift. Each panel corresponds to AGN with different levels of line-of-sight obscuration, parametrised by the hydrogen column density N_H as indicated by the legend at the top. The curves show observational constraints presented by Buchner et al. (2015). The red curves correspond to unobscured AGN logN_H<22 (cm^-2), blue is for mildly obscured sources logN_H=22-24 (cm^-2) and green is for the most heavily obscured systems, the Compton thick AGN, logN_H>24 (cm^-2).

The diffuse X-ray background (XRB) is the superposition of the X-ray emission of individual Active Galactic Nuclei (AGN) across cosmic time. AGN population synthesis models (e.g. Akylas et al. 2012) can reconstruct the shape and overall normalisation of the XRB spectrum by invoking a population of heavily obscured AGN. There are still uncertainties on the evolution of AGN, particularly heavily obscured ones, with direct impact on models for the composition of the XRB.

Work is currently in progress to provide tighter observational constraints on the space density of AGN as a function of accretion luminosity, redshift and level of obscuration, which can then feedback to XRB population synthesis models. Of particular interest are estimates of the space density of the most deeply shrouded (Compton thick) AGN, i.e. those that are embedded within cocoons of dust and gas clouds with equivalent hydrogen column density logN_H>24 (cm^-2).

X-ray surveys have been extensively used over the years to improve our knowledge on the most heavily obscured AGN. They are advantageous over selections at other wavelengths because they provide clean AGN samples, minimal contamination by non-AGN and also allow direct measure of the level of obscuration of the central source via X-ray spectroscopy. Work is currently underway to improve analysis methodology and expand the survey data used in luminosity function calculations.