The Chandra COSMOS Legacy Survey: Compton Thick AGN at high redshift

TL;DR

This study identifies and analyzes a large sample of high-redshift Compton thick AGN using Chandra X-ray data, revealing an increasing fraction with redshift and luminosity, and linking CT AGN to galaxy mergers.

Contribution

It presents a novel method to select high-redshift CT AGN candidates using X-ray spectroscopy and Monte Carlo sampling, providing new insights into their evolution and host galaxy properties.

Findings

The intrinsic CT AGN fraction increases with redshift and luminosity.

Approximately 42 effective CT AGN are identified in the sample.

CT AGN hosts show a higher incidence of mergers and interactions.

Abstract

The existence of a large population of Compton thick (CT, $N_{H}>10^{24} cm^{-2}$) AGN is a key ingredient of most Cosmic X-ray background synthesis models. However, direct identification of these sources, especially at high redshift, is difficult due to the flux suppression and complex spectral shape produced by CT obscuration. We explored the Chandra COSMOS Legacy point source catalog, comprising 1855 sources, to select via X-ray spectroscopy, a large sample of CT candidates at high redshift. Adopting a physical model to reproduce the toroidal absorber, and a Monte-Carlo sampling method, we selected 67 individual sources with >5% probability of being CT, in the redshift range $0.04<z<3.5$. The sum of the probabilities above $N_{H}>10^{24} cm^{-2}$, gives a total of 41.9 effective CT, corrected for classification bias. We derive number counts in the 2-10 keV band in three redshift bins. The observed logN-logS is consistent with an increase of the intrinsic CT fraction ($f_{CT}$) from $\sim0.30$ to $\sim0.55$ from low to high redshift. When rescaled to a common luminosity (log(L$_{\rm X}$/erg/s)$=44.5$) we find an increase from $f_{CT}=0.19_{-0.06}^{+0.07}$ to $f_{CT}=0.30_{-0.08}^{+0.10}$ and $f_{CT}=0.49_{-0.11}^{+0.12}$ from low to high z. This evolution can be parametrized as $f_{CT}=0.11_{-0.04}^{+0.05}(1+z)^{1.11\pm0.13}$. Thanks to HST-ACS deep imaging, we find that the fraction of CT AGN in mergers/interacting systems increases with luminosity and redshift and is significantly higher than for non-CT AGN hosts.