The cosmic X-ray background: abundance and evolution of hidden black holes

TL;DR

This paper reviews the cosmic X-ray background to understand the abundance, evolution, and obscuration of hidden supermassive black holes, highlighting recent survey findings and future observational challenges.

Contribution

It synthesizes current models of AGN populations, discusses the role of galaxy processes in black hole growth, and explores limitations of current X-ray instrumentation.

Findings

Deep X-ray surveys reveal a large population of obscured, distant black holes.

Galaxy merging and secular processes both contribute to nuclear activity.

Current instruments have limitations in detecting the most obscured, high-redshift black holes.

Abstract

The growth of supermassive black holes across cosmic time leaves a radiative imprint recorded in the X-ray background (XRB). The XRB spectral shape suggests that a large population of distant, hidden nuclei must exist, which are now being revealed at higher and higher redshifts by the deepest surveys performed by Chandra and XMM. Our current understanding of the XRB emission in terms of AGN population synthesis models is here reviewed, and the evolutionary path of nuclear accretion and obscuration, as emerging from the major X-ray surveys, is investigated. The role of galaxy merging versus secular processes in triggering nuclear activity is also discussed in the framework of recent galaxy/black hole co-evolutionary scenarios. Finally, the limits of current instrumentation in the detection of the most obscured and distant black holes are discussed and some possible directions to overcome these limits are presented.