The Chandra Deep Field South/GOODS survey. Optically faint X-ray sources

TL;DR

This study analyzes optically faint X-ray sources in the Chandra Deep Field South, revealing their nature, redshift distribution, and contribution to the X-ray background, with many being absorbed AGN and some being high-redshift QSOs.

Contribution

It provides new photometric redshifts for optically faint X-ray sources, improving understanding of their distribution and properties, and compares findings with X-ray background models.

Findings

Optically faint X-ray sources are mainly absorbed AGN.

Many sources have high X-ray-to-optical flux ratios.

Approximately 23% are X-ray absorbed QSOs.

Abstract

We provide important new constraints on the nature and redshift distribution of optically faint (R>25) X-ray sources in the Chandra Deep Field South Survey. We show that we can derive accurate photometric redshifts for the spectroscopically unidentified sources thus maximizing the redshift completeness for the whole X-ray sample. Our new redshift distribution for the X-ray source population is in better agreement with that predicted by X-ray background synthesis models; however, we still find an overdensity of low redshift (z<1) sources. The optically faint sources are mainly X-ray absorbed AGN, as determined from direct X-ray spectral analysis and other diagnostics. Many of these optically faint sources have high (>10) X-ray-to-optical flux ratios. We also find that ~71% of them are well fitted with the SED of an early-type galaxy with <z_phot>~1.9 and the remaining 29% with irregular or starburst galaxies mainly at z_phot>3. We estimate that 23% of the optically faint sources are X-ray absorbed QSOs. The overall population of X-ray absorbed QSOs contributes a ~15% fraction of the [2-10] keV X-ray Background (XRB) whereas current XRB synthesis models predict a ~38% contribution.