The Evolution and Eddington Ratio Distribution of Compton Thick Active Galactic Nuclei

TL;DR

This study investigates the physical properties of Compton thick AGN, focusing on their Eddington ratios and black hole masses, to better understand their role in the cosmic X-ray background.

Contribution

It introduces a novel model linking Eddington ratios to the contribution of Compton thick AGN to the CXB, incorporating physical parameters previously unconsidered.

Findings

CT AGN are likely a mix of sources accreting at >90% or <1% Eddington ratios.

The model aligns with observed CT AGN densities and log N-log S relations.

Physical Eddington ratio modeling improves understanding of AGN contributions to the CXB.

Abstract

Previous studies of the active galactic nuclei (AGN) contribution to the cosmic X-ray background (CXB) consider only observable parameters such as luminosity and absorbing column. Here, for the first time, we extend the study of the CXB to physical parameters including the Eddington ratio of the sources and the black hole mass. In order to calculate the contribution to the CXB of AGN accreting at various Eddington ratios, an evolving Eddington ratio space density model is calculated. In particular, Compton thick (CT) AGN are modeled as accreting at specific, physically motivated Eddington ratios instead of as a simple extension of the Compton thin type 2 AGN population. Comparing against the observed CT AGN space densities and log N-log S relation indicates that CT AGN are likely a composite population of AGN made up of sources accreting either at >90% or <1% of their Eddington rate.