The Chandra deep field South/GOODS survey

Optically faint X-ray sources

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse PF 1312, 85748 Garching bei Muenchen, Germany e-mail: vmainieri@mpe.mpg.de
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
³ Dip. di Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁴ INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
⁵ Institut d'Astrophysique de Paris, 98bis boulevard, 75014 Paris, France
⁶ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy
⁷ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Center for Astrophysical Sciences, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA

Received: 2 May 2004
Accepted: 25 February 2005

Abstract

We provide important new constraints on the nature and redshift distribution of optically faint () X-ray sources in the Chandra Deep Field South Survey. We use a large multi-wavelength data set, including the GOODS/ACS survey, the recently released Hubble Ultra Deep Field (UDF) data, and the new public VLT/ISAAC imaging. 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 () 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 and the remaining 29% with irregular or starburst galaxies mainly at . We estimate that of the optically faint sources are X-ray absorbed QSOs. The overall population of X-ray absorbed QSOs contributes a ∼ fraction of the [2–10] keV X-ray Background (XRB) whereas current XRB synthesis models predict a ∼ contribution.