High-redshift quasars in the cosmos survey: The space density of z > 3 X-ray selected QSOs

M. Brusa, A. Comastri, R. Gilli, G. Hasinger, K. Iwasawa, V. Mainieri, M. Mignoli, M. Salvato, G. Zamorani, A. Bongiorno, N. Cappelluti, F. Civano, F. Fiore, A. Merloni, J. Silverman, J. Trump, C. Vignali, P. Capak, M. Elvis, O. IlbertC. Impey, S. Lilly

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93 Scopus citations


We present a new measurement of the space density of high-redshift (z ≃ 3.0-4.5), X-ray-selected quasi-stellar objects (QSOs) obtained by exploiting the deep and uniform multiwavelength coverage of the COSMOS survey. We have assembled a large (40 objects), homogeneous sample of z > 3 QSOs with X-ray flux F0.5-2 keV > 10-15 erg cm-2 s-1, and available spectroscopic (22) or photometric (18) redshifts. We discuss their optical (color-color diagrams) and X-ray properties, their number counts and space densities and compare our findings with previous works and model predictions. We find that the optical properties of X-ray-selected quasars are not significantly different from those of optically selected samples. There is evidence for substantial X-ray absorption (logNH > 23 cm-2) in about 20% of the sources in the sample. We find that the comoving space density of luminous (LX ≳ 1044 erg s-1) QSOs declines exponentially (by an e-folding per unit redshift) in the z ∼ 3.0-4.5 range, with a behavior similar to that observed for optically bright unobscured QSOs selected in large area optical surveys. Prospects for future, large and deep X-ray surveys are also discussed.

Original languageEnglish (US)
Pages (from-to)8-22
Number of pages15
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2009


  • Galaxies: active
  • Surveys
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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  • High-redshift QSOs in the COSMOS survey

    Brusa, M. (Creator), Comastri, A. (Creator), Gilli, R. (Creator), Hasinger, G. (Creator), Iwasawa, K. (Creator), Mainieri, V. (Creator), Mignoli, M. (Creator), Salvato, M. (Creator), Zamorani, G. (Creator), Bongiorno, A. (Creator), Cappelluti, N. (Creator), Civano, F. (Creator), Fiore, F. (Creator), Merloni, A. (Creator), Silverman, J. (Creator), Trump, J. (Creator), Vignali, C. (Creator), Capak, P. (Creator), Elvis, M. (Creator), Ilbert, O. (Creator), Impey, C. D. (Creator), Lilly, S. (Creator), Gilli, R. (Creator), Silverman, J. (Creator), Vignali, C. (Creator) & Capak, P. (Creator), Centre de Donnees Strasbourg (CDS), 2011


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