Recent developments in the search for Compton-thick AGN

TL;DR

This review summarizes recent advancements in identifying Compton-thick AGN using X-ray and mid-infrared surveys, highlighting key techniques and findings across various observational campaigns and their implications for understanding obscured supermassive black holes.

Contribution

It provides a comprehensive overview of recent survey methods and results in detecting Compton-thick AGN, emphasizing new techniques and high-redshift discoveries.

Findings

Identification of candidate Compton-thick AGN in deep X-ray surveys.

Development of mid-IR techniques for selecting obscured AGN.

Detection of high optical depth features in mid-IR spectra.

Abstract

I present a review of X-ray and mid-IR surveys for Compton-thick Active Galactic Nuclei (AGN). These are the most highly obscured sources having hydrogen column densities >1.5x10^24 cm-2. Key surveys in the local Universe are presented including the high energy SWIFT/BAT and INTEGRAL surveys, mid-IR and also optical surveys. Recently, deep X-ray surveys with Chandra and XMM-Newton have produced a number of candidate Compton-thick AGN at higher redshift primarily in the Chandra Deep Field South region. In addition, mid-IR surveys with Spitzer have helped to develop novel complementary techniques for the selection of Compton-thick AGN. The mid-IR techniques used to identify Compton-thick AGN include: a) 24 micron excess sources relative to their optical emission b) Spitzer spectroscopy for the detection of high optical depth Si 9.7 micron absorption features c) low X-ray to 6 micron luminosity ratio.