@article{f193581689f849159141888d474dfbaa,
title = "High-redshift quasars in the cosmos survey: The space density of z > 3 X-ray selected QSOs",
abstract = "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.",
keywords = "Galaxies: active, Surveys, X-rays: galaxies",
author = "M. Brusa and A. Comastri and R. Gilli and G. Hasinger and K. Iwasawa and V. Mainieri and M. Mignoli and M. Salvato and G. Zamorani and A. Bongiorno and N. Cappelluti and F. Civano and F. Fiore and A. Merloni and J. Silverman and J. Trump and C. Vignali and P. Capak and M. Elvis and O. Ilbert and C. Impey and S. Lilly",
note = "Funding Information: This work is based on observations obtained with XMMNewton, 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 f{\"u}r Wirtschaft und Technologie/Deutsches Zentrum f{\"u}r 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 byASI-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 http://www.astro.caltech.edu/cosmos. 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 Information: 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 f{\"u}r Wirtschaft und Technologie/ Deutsches Zentrum f{\"u}r 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 http://www.astro.caltech.edu/ cosmos. 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. Publisher Copyright: {\textcopyright} 2009. The American Astronomical Society. All rights reserved. Printed in U.S.A.",
year = "2009",
month = mar,
day = "1",
doi = "10.1088/0004-637X/693/1/8",
language = "English (US)",
volume = "693",
pages = "8--22",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "American Astronomical Society",
number = "1",
}