RECENT DEVELOPMENTS IN THE SEARCH FOR COMPTON-THICK AGN

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 > 1.5 × 1024 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 μm excess sources relative to their optical emission b) Spitzer spectroscopy for the detection of high optical depth Si 9.7μm absorption features c) low X-ray to 6μm luminosity ratio.