A Caltech Library Service

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

Brusa, M. and Comastri, A. and Gilli, R. and Hasinger, G. and Iwasawa, K. and Mainieri, V. and Mignoli, M. and Salvato, M. and Zamorani, G. and Bongiorno, A. and Cappelluti, N. and Civano, F. and Fiore, F. and Merloni, A. and Silverman, J. and Trump, J. and Vignali, C. and Capak, P. and Elvis, M. and Ilbert, O. and Impey, C. and Lilly, S. (2009) High-redshift quasars in the cosmos survey: the space density of z > 3 x-ray selected QSOs. Astrophysical Journal, 693 (1). pp. 8-22. ISSN 0004-637X.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


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 F_0.5-2 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 (logN_H > 23 cm^(–2)) in about 20% of the sources in the sample. We find that the comoving space density of luminous (L_X ≳ 10^(44) 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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Additional Information:© 2009 American Astronomical Society. Print publication: Issue 1 (2009 March 1); received 2008 May 16; accepted for publication 2008 October 28; published 2009 February 27. This work is based on observations obtained with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). In Germany, the XMM-Newton project is supported by the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft- und Raumfahrt (BMWI/DLR, FKZ 50 OX 0001), and the Max-Planck Society. M.B. acknowledges support from the XMM-Newton DLR grant 50-G-0502, G.H. acknowledges the contribution from the Leibniz Prize of the Deutsche Forschungsgemeinschaft under the grant HA 1850/28-1. In Italy, the XMM-COSMOS project is supported by ASI-INAF and PRIN/MIUR under grants I/023/05/00 and 2006-02-5203. The zCOSMOS ESO Large Program Number 175.A-0839 is acknowledged. These results are also based in part on observations obtained with the Walter Baade telescope of the Magellan Consortium at Las Campanas Observatory, and with the MMT which is operated by the MMT Observatory (MMTO), a joint venture of the Smithsonian Institution and the University of Arizona. We gratefully acknowledge the contributions of the entire COSMOS collaboration; more information on the COSMOS survey is available at This research has made use of the NASA/IPAC Extragalactic Database (NED) and the SDSS spectral archive. We thank the anonymous referee for his/her useful comments and suggestions.
Funding AgencyGrant Number
XMM-Newton DLR50-G-0502
Deutsche ForschungsgemeinschaftHA 1850/28-1
Subject Keywords:galaxies: active; surveys; X-rays: galaxies
Record Number:CaltechAUTHORS:20090428-095914449
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14099
Deposited By: Jason Perez
Deposited On:07 Aug 2009 20:58
Last Modified:06 Jul 2015 20:50

Repository Staff Only: item control page