The Blast Wave Model for AGN Feedback: Effects on AGN Obscuration

Abstract

We compute the effect of galactic absorption on active galactic nucleus (AGN) emission in a cosmological context by including a physical model for AGN feeding and feedback in a semianalytic model of galaxy formation. This is based on galaxy interactions as triggers for AGN accretion and on expanding blast waves as a mechanism to propagate outwards the AGN energy injected into the interstellar medium at the center of galaxies. We first test our model against the observed number density of AGNs with different intrinsic luminosities as a function of redshift. The model yields a downsizing behavior in close agreement with the observed one for z 2. At higher redshifts, the model predicts an overall abundance of AGNs (including Compton-thick sources) larger than the observed Compton-thin sources by a factor of ≈2 for z 2 and LX ≤ 1044 erg s−1. Thus, we expect that at such luminosities and redshifts, Compton-thick sources contribute to about 1/2 of the total AGN population. We then investigate the dependence of the absorbing column density NH associated with cold galactic gas (and responsible for the Compton-thin component of the overall obscuration) on AGN luminosity and redshift. We find that the absorbed fraction of AGNs with NH ≥ 1022 cm−2 decreases with luminosity for z ≤ 1. In addition, the total (integrated over luminosity) absorbed fraction increases with redshift up to z ≈ 2, and saturates to the value ≈0.8 at higher redshifts. Finally, we predict that the luminosity dependence of the absorbed fraction of AGNs with LX ≤ 3 × 1044 erg s−1 will weaken with increasing redshift. We compare our results with recent observations and discuss their implications in the context of cosmological models of galaxy formation.