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

Dipartimento di Fisica “E. Amaldi”, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy e-mail: lamastra@fis.uniroma3.it

Received: 28 November 2007
Accepted: 24 April 2008

Abstract

Context. The evidence of a decrease with increasing luminosity in the fraction f_abs of absorbed and Compton-thin among X-ray-selected (2-10 keV) AGN is observationally well supported, while that of an increase in f_abs with redshift is fairly controversial. In a previous paper, 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 the black-hole mass M_BH.

Aims. The most recent findings on the distribution of the Eddington ratio 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.

Methods. The findings for are properly treated to ensure completeness in the prediction of f_abs above a certain luminosity, at values of , 0.35, 0.7, and ≥1. To verify the consequence of these findings alone, we first adopted a distribution of gas surface density Σ, observed in a sample of local spiral galaxies, irrespective of the galaxy morphological type and z.

Results. Assuming the Eddington limit, , in the distribution as a “natural” cut-off, the predictions are consistent with the existence of an anti-correlation between f_abs and L_X, but they fail to reproduce an increase in f_abs with z. Because the early type galaxies are on average 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 Σ and the central activity. An increase in typical values of Σ with z, correlated with the activity, might explain an increase in f_abs with z. However, f_abs could hardly exceed about 0.3 at the highest luminosities.