The Chandra deep field South/GOODS survey: Optically faint X-ray sources

V. Mainieri, P. Rosati, P. Tozzi, J. Bergeron, R. Gilli, G. Hasinger, M. Nonino, I. Lehmann, D. M. Alexander, R. Idzi, A. M. Koekemoer, C. Norman, G. Szokoly, W. Zheng

Research output: Contribution to journalReview article

52 Citations (Scopus)


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 use a large multi-wavelength data set, including the GOODS/ACS survey, the recently released Bubble 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 〈zphot〉 ∼ 1.9 and the remaining 29% with irregular or starburst galaxies mainly at Zphot > 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 ∼11% fraction of the [2-10] keV X-ray Background (XRB) whereas current XRB synthesis models predict a ∼38% contribution.

Original languageEnglish
Pages (from-to)805-821
Number of pages17
JournalAstronomy and Astrophysics
Issue number3
Publication statusPublished - Jul 1 2005



  • Cosmology: diffuse radiation
  • Galaxies: active
  • Galaxies: quasars: general
  • Surveys
  • X-ray: galaxies
  • X-rays: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Mainieri, V., Rosati, P., Tozzi, P., Bergeron, J., Gilli, R., Hasinger, G., Nonino, M., Lehmann, I., Alexander, D. M., Idzi, R., Koekemoer, A. M., Norman, C., Szokoly, G., & Zheng, W. (2005). The Chandra deep field South/GOODS survey: Optically faint X-ray sources. Astronomy and Astrophysics, 437(3), 805-821.