EDP Sciences
Free access
Volume 437, Number 3, July III 2005
Page(s) 805 - 821
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20041212

A&A 437, 805-821 (2005)
DOI: 10.1051/0004-6361:20041212

The Chandra deep field South/GOODS survey

Optically faint X-ray sources
V. Mainieri1, 2, 3, P. Rosati2, P. Tozzi4, J. Bergeron5, R. Gilli6, G. Hasinger1, M. Nonino4, I. Lehmann1, D. M. Alexander7, R. Idzi8, A. M. Koekemoer8, C. Norman9, G. Szokoly1 and W. Zheng9

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)

We provide important new constraints on the nature and redshift distribution of optically faint ($R\ge 25$) 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 (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 $\langle z_{\rm phot}\rangle \sim 1.9$ and the remaining 29% with irregular or starburst galaxies mainly at $z_{\rm phot}>3$. We estimate that $15 \%$ of the optically faint sources are X-ray absorbed QSOs. The overall population of X-ray absorbed QSOs contributes a $\sim $$11 \%$ fraction of the [2-10] keV X-ray Background (XRB) whereas current XRB synthesis models predict a $\sim $$38 \%$ contribution.

Key words: surveys -- galaxies: active -- galaxies: quasars: general -- cosmology: diffuse radiation -- X-ray: galaxies -- X-rays: general

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© ESO 2005

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