The evolution of obscured accretion

TL;DR

This paper reviews the evolution of obscured supermassive black hole accretion, discussing detection methods, current knowledge gaps, and future prospects for understanding heavily obscured AGN populations.

Contribution

It provides a comprehensive review of methods to detect heavily obscured AGN and compares observational results with theoretical models, highlighting current knowledge gaps.

Findings

Measured space density of heavily obscured AGN varies across techniques.

Preliminary results from 3 Ms XMM observations of CDFS are presented.

Future X-ray missions could significantly improve population studies.

Abstract

Our current understanding of the evolution of obscured accretion onto supermassive black holes is reviewed. We consider the literature results on the relation between the fraction of moderately obscured, Compton-thin AGN and redshift, and discuss the biases which possibly affect the various measurements. Then, we discuss a number of methods - from ultradeep X-ray observations to the detection of high-ionization optical emission lines - to select the population of the most heavily obscured, Compton-thick AGN, whose cosmological evolution is basically unknown. The space density of heavily obscured AGN measured through different techniques is discussed and compared with the predictions by current synthesis models of the X-ray background. Preliminary results from the first half of the 3 Ms XMM observation of the Chandra Deep Field South (CDFS) are also presented. The prospects for population studies of heavily obscured AGN with future planned or proposed X-ray missions are finally discussed.