EDP Sciences
Free access
Volume 487, Number 1, August III 2008
Page(s) 109 - 118
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20079162
Published online 09 June 2008

A&A 487, 109-118 (2008)
DOI: 10.1051/0004-6361:20079162

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

A. Lamastra, G. C. Perola, and G. Matt

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

Context. The evidence of a decrease with increasing luminosity in the fraction $f_{\rm abs}$ of absorbed and Compton-thin among X-ray-selected (2-10 keV) AGN is observationally well supported, while that of an increase in $f_{\rm abs}$ 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 $f_{\rm abs}$ and the black-hole mass $M_{\rm BH}$.
Aims. The most recent findings on the distribution of the Eddington ratio $\lambda=L_{\rm b}/L_{\rm E}$ as a function of $M_{\rm BH}$ and z are used to convert that relationship into one between $f_{\rm abs}$ and both bolometric ($L_{\rm b}$) and X-ray ($L_{\rm X}$) luminosities at various values of z.
Methods. The findings for $\lambda(M_{\rm BH},z)$ are properly treated to ensure completeness in the prediction of $f_{\rm abs}$ above a certain luminosity, at values of z=0.1, 0.35, 0.7, and $\geq$1. To verify the consequence of these findings alone, we first adopted a distribution of gas surface density $\Sigma$, observed in a sample of local spiral galaxies, irrespective of the galaxy morphological type and z.
Results. Assuming the Eddington limit, $\lambda=1$, in the $\lambda(M_{\rm BH},z)$ distribution as a "natural" cut-off, the predictions are consistent with the existence of an anti-correlation between $f_{\rm abs}$ and $L_{\rm X}$, but they fail to reproduce an increase in $f_{\rm abs}$ 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 $f_{\rm abs}$ requires the existence of a correlation between $\Sigma$ and the central activity. An increase in typical values of $\Sigma$ with z, correlated with the activity, might explain an increase in $f_{\rm abs}$ with z. However, $f_{\rm abs}$ could hardly exceed about 0.3 at the highest luminosities.

Key words: galaxies: active -- X-rays: galaxies -- ISM: clouds

© ESO 2008