Luminosity-dependent evolution of soft X-ray selected AGN

New Chandra and XMM-Newton surveys

¹ Max-Planck-Institut für extraterrestrische Physik, Postf. 1312, 85741 Garching, Germany e-mail: ghasinger@mpe.mpg.de
² Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburg, PA 15213, USA e-mail: miyaji@cmu.edu
³ California Institute of Technology, Pasadena, CA 91125, USA e-mail: mxs@astro.caltech.edu

Received: 7 October 2004
Accepted: 10 June 2005

Abstract

We present new results on the cosmological evolution of unabsorbed (type-1) active galactic nuclei (AGN) selected in the soft (0.5-2 keV) X-ray band. From a variety of ROSAT, XMM-Newton and Chandra surveys we selected a total of ~1000 AGN with an unprecedented spectroscopic and photometric optical/NIR identification completeness. For the first time we are able to derive reliable space densities for low-luminosity (Seyfert-type) X-ray sources at cosmological redshifts. The evolutionary behaviour of AGN shows a strong dependence on X-ray luminosity: while the space density of high-luminosity AGN reaches a peak around , similar to that of optically selected QSO, the space density of low-luminosity AGNs peaks at redshifts below . This confirms previous ROSAT findings of a luminosity-dependent density evolution. Using a rigorous treatment of the optical identification completeness we are able to show that the space density of AGN with X-ray luminosities  erg s^-1 declines significantly towards high redshifts.