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

Abstract

Context. The evidence of a decrease with increasing luminosity in the fraction f abs of absorbed and Compton-thin among X-rayselected (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 λ = 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. Methods. The findings for λ(M BH , z) are properly treated 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 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, λ = 1, in the λ(M BH , z) 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.

Cosmological Evolution of Supermassive Black Holes. Ii. Evidence for Downsizing of Spin Evolution

“…Ballantyne et al (2006), modeling the X-ray background, also report a similar evolution trend of (1 + z) 0.3 . However, other studies cast doubt on the redshift evolution of the obscured AGN fraction (Ueda et al 2003;Dwelly & Page 2006;Gilli et al 2007;Lamastra et al 2008).…”

“…Ballantyne et al (2006), modeling the X-ray background, also report a similar evolution trend of (1 + z) 0.3 . However, other studies cast doubt on the redshift evolution of the obscured AGN fraction (Ueda et al 2003;Dwelly & Page 2006;Gilli et al 2007;Lamastra et al 2008). Based on their AGN population model, Gilli et al (2010) argue that by including a proper K−correction an intrinsically constant obscured AGN fraction can also represent the data.…”

The X-ray variability history of Markarian 3

“…We again constrained Z F e =0.43±0.09Z ⊙ . In astrophysical terms, M or represents a scenario whereby the line-ofsight to the inner far side of the torus grazes the rim of its near side or crosses a dust lane in the host galaxy (Lamastra et al 2008). Although gas from the torus rim is likely to be evaporated by the AGN radiation pressure, and therefore significantly ionized, we used in M or a photoelectric model for cold matter, as the data do not require the ionization parameter as a further degree of freedom.…”

Cosmological Evolution of Supermassive Black Holes: Mass Functions and Spins

“…Ballantyne et al (2006), modeling the X-ray background, also report a similar evolution trend of (1 + z) 0.3 . However, other studies cast doubt on the redshift evolution of the obscured AGN fraction (Ueda et al 2003;Dwelly & Page 2006;Gilli et al 2007;Lamastra et al 2008). Based on their AGN population model, Gilli et al (2010) argue that by including a proper K−correction an intrinsically constant obscured AGN fraction can also represent the data.…”