A Compton-thick AGN at z~5 in the 4 Ms Chandra Deep Field South

TL;DR

This paper reports the discovery of the most distant confirmed Compton-thick AGN at z~5, revealing a heavily obscured supermassive black hole during early galaxy formation, with implications for understanding black hole and galaxy co-evolution.

Contribution

First confirmed Compton-thick AGN at z~5 based on X-ray spectral analysis, demonstrating the presence of heavily obscured black holes in the early universe.

Findings

Most distant heavily obscured AGN confirmed by X-ray spectrum

Column density N_H=1.4 x 10^{24} cm^{-2} indicating Compton-thick absorption

Associated with massive star formation and early black hole growth

Abstract

We report the discovery of a Compton-thick Active Galactic Nucleus (AGN) at z=4.76 in the 4 Ms Chandra Deep Field South. This object was selected as a V-band dropout in HST/ACS images and previously recognized as an AGN from optical spectroscopy. The 4 Ms Chandra observations show a significant (~4.2sigma) X-ray detection at the V-band dropout position. The X-ray source displays a hardness ratio of HR=0.23+-0.24, which, for a source at z~5, is highly suggestive of Compton-thick absorption. The source X-ray spectrum is seen above the background level in the energy range of ~0.9-4 keV, i.e., in the rest-frame energy range of ~5-23 keV. When fixing the photon index to Gamma=1.8, the measured column density is N_H=1.4^{+0.9}_{-0.5} x 10^{24} cm^{-2}, which is Compton-thick. To our knowledge, this is the most distant heavily obscured AGN, confirmed by X-ray spectral analysis, discovered so far. The intrinsic (de-absorbed), rest-frame luminosity in the 2-10 keV band is ~2.5 x 10^{44} erg s^{-1}, which places this object among type-2 quasars. The Spectral Energy Distribution shows that massive star formation is associated with obscured black hole accretion. This system may have then been caught during a major co-eval episode of black hole and stellar mass assembly at early times. The measure of the number density of heavily obscured AGN at high redshifts will be crucial to reconstruct the BH/galaxy evolution history from the beginning.