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: firstname.lastname@example.org
Accepted: 24 April 2008
Context. The evidence of a decrease with increasing luminosity in the fraction fabs of absorbed and Compton-thin among X-ray-selected (2-10 keV) AGN is observationally well supported, while that of an increase in fabs 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 fabs and the black-hole mass MBH.
Aims. The most recent findings on the distribution of the Eddington ratio as a function of MBH and z are used to convert that relationship into one between fabs and both bolometric (Lb) and X-ray (LX) luminosities at various values of z.
Methods. The findings for are properly treated to ensure completeness in the prediction of fabs 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 fabs and LX, but they fail to reproduce an increase in fabs 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 fabs 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 fabs with z. However, fabs could hardly exceed about 0.3 at the highest luminosities.
Key words: galaxies: active / X-rays: galaxies / ISM: clouds
© ESO, 2008