X-ray absorption in Compton-thin AGN: the predictions of a model revisited

TL;DR

This paper revisits a model predicting X-ray absorption in Compton-thin AGN, examining how black hole mass, galaxy gas content, and redshift influence the fraction of absorbed AGN, with implications for understanding AGN evolution.

Contribution

It integrates recent Eddington ratio data into a model to predict absorption fractions, highlighting the role of galaxy gas content and morphology in AGN obscuration.

Findings

Model reproduces the anti-correlation between f_{abs} and L_X.

Fails to reproduce the increase of f_{abs} with redshift without considering gas content evolution.

At high luminosities, f_{abs} is limited to about 0.3.

Abstract

The evidence of a decrease with increasing luminosity of the fraction f_{abs} of absorbed and Compton-thin among X-ray (2-10 keV) selected AGN is observationally rather well supported, while that of an increase of f_{abs} with redshift is rather controversial. In Lamastra, Perola & Matt (2006) the gravitational effect of the SMBH on the molecular interstellar gas, in the central region of the host galaxy, was shown to predict an anti-correlation between f_{abs} and M_{BH}. The most recent findings on the distribution of the Eddington ratio \lambda=L_b/L_E as a function of M_{BH} and z are used to convert that relationship into one between f_{abs} and both bolometric (L_b) and X-ray (L_X) luminosities at various values of z. The findings on \lambda(M_{BH},z) are properly treated in order to ensure completeness in the prediction of f_{abs} above a certain luminosity, at values of z=0.1, 0.35, 0.7 and >1. To verify the consequence of these findings alone, we adopted in a first istance a distribution of gas surface density \Sigma, observed in a sample of local spiral galaxies, irrespective of the galaxy morphological type and z. Assuming in the \lambda(M_{BH},z) distribution the Eddington limit, \lambda=1, as a ``natural'' cut-off, the predictions are consistent with the existence of an anti-correlation between f_{abs} and L_X, but fail to reproduce an increase of f_{abs} with z. Because the early type galaxies on average are much poorer in molecular gas than late type ones, a quantitative agreement with the local value of f_{abs} requires the existence of a correlation between \Sigma and the central activity. An increase of typical values of \Sigma with z, correlated with the activity, might explain an increase of f_{abs} with z. However, at the highest luminosities f_{abs} could hardly exceed about 0.3.