2018MNRAS.475.3543G

A significant number of active galactic nuclei (AGNs) are observed to be hidden behind dust and gas. The distribution of material around AGNs plays an important role in modelling the cosmic X-ray background (CXB), especially the fraction of type 2 AGNs (f₂). One of the possible explanations for obscuration in Seyfert galaxies at intermediate redshifts is dusty starburst discs. We compute the two-dimensional (2D) hydrostatic structure of 768 nuclear starburst discs (NSDs) under various physical conditions and also the distribution of column density along the line of sight (N_H) associated with these discs. Then the N_H distribution is evolved with redshift by using the redshift-dependent distribution function of input parameters. Parameter f₂ shows a strong positive evolution up to z = 2, but only a weak level of enhancement at higher z. The Compton-thin and Compton-thick AGN fractions associated with these starburst regions increase ∝ (1 + z)^δ, where δ is estimated to be 1.12 and 1.45, respectively. The reflection parameter R_f associated with column density N_H >= 10^23.5 cm^–2 extends from 0.13 at z = 0 to 0.58 at z = 4. A CXB model employing this evolving N_H distribution indicates that more compact (R_out < 120 pc) NSDs provide a better fit to the CXB. In addition to 'Seyfert-like' AGNs obscured by nuclear starbursts, we predict that 40-60 per cent of quasars must be Compton-thick to produce a peak of the CXB spectrum within the observational uncertainty. The predicted total number counts of AGNs in 8-24 keV bands are in fair agreement with observations from the Nuclear Spectroscopic Telescope Array (NuSTAR).