Absorption Properties and Evolution of Active Galactic Nuclei

Context. Aims. Intrinsic absorption is a fundamental physical property to understand the evolution of active galactic nuclei (AGN). Here a sample of 1290 AGN, selected in the 2–10 keV band from different flux-limited surveys with very high optical identification completeness is studied. Methods. The AGN are grouped into two classes, unabsorbed (type-1) and absorbed (type-2), depending on their optical spectroscopic classification and X–ray absorption properties, using hardness ratios. Utilizing the optical to X–ray flux ratios, a rough correction for the ∼ 8% redshift incompleteness still present in the sample is applied. Then the fraction of absorbed sources is determined as a function of X-ray luminosity and redshift. Results. A strong decrease of the absorbed fraction with X-ray luminosity is found. This can be represented by an almost linear decrease from ∼ 80% to ∼ 20% in the luminosity range logLX=42–46 and is consistent with similar derivations in the optical and MIR bands. Several methods are used to study a possible evolution of the absorption fraction. A significant increase of the absorbed fraction with redshift is found, which can be described by a power law ∼ (1 + z)0.76±0.12, saturating at a redshift of z∼2. Conclusions. The variation of the AGN absorption with luminosity and redshift is described with higher statistical accuracy and smaller systematic errors than previous results. The findings have important consequences for the broader context of AGN and galaxy co-evolution. Here it is proposed that the cosmic downsizing in the AGN population is due to two different feeding mechanisms: a fast process of merger driven accretion at high luminosities and high redshifts versus a slow process of gas accretion from gravitational instabilities in galactic disks rebuilding around pre-formed bulges and black holes.