The HELLAS2XMM survey^*,**

IV. Optical identifications and the evolution of the accretion luminosity in the Universe

¹ INAF-Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio-Catone (RM), 00040, Italy
² Dip. di Astronomia Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Dip. di Fisica Università di Roma Tor Vergata, via della ricerca scientifica, 1, 00133 Roma, Italy
⁵ IASF/CNR, via Bassini 15, Milano, Italy
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Dip. di Fisica, Università Roma Tre, via della vasca navale 84, 00146 Roma, Italy
⁸ INAF-Osservatorio Astrofisico di Arcetri, L. go E. Fermi 5, 50125 Firenze, Italy
⁹ INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
¹⁰ Dept. of Astronomy and Astrophysics Pennsylvania State University, 525 Davey, University Park, PA 16802, USA

Corresponding author: F. Fiore, fiore@mporzio.astro.it

Received: 22 April 2003
Accepted: 1 July 2003

Abstract

We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2–10 keV band (the HELLAS2XMM 1dF sample). Their flux cover the range   and the total area surveyed is 0.9 square degrees. One of the most interesting results (which is found also in deeper sourveys) is that about 20% of the hard X-ray selected sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes more than one order of magnitude lower than the HELLAS2XMM survey sources, many of the extreme X/O sources in our sample have and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with (to be compared with 9 sources known from the deeper, pencil-beam surveys). Eight of them are narrow line QSO (seemingly the extension to very high luminosity of the type 2 Seyfert galaxies), four are broad line QSO. The results from our survey are also used to make reliable predictions about the luminosity of the sources not yet spectroscopically identified, both in our sample and in deeper Chandra and XMM-Newton samples. We then use a combined sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, Chandra Deep Field North 1Msec, Chandra SSA13 and XMM-Newton Lockman Hole flux limited samples), 221 with measured redshifts, to evaluate the cosmological evolution of the hard X-ray source's number and luminosity densities. Looking backward in time, the low luminosity sources ( erg s^-1) increase in number at a much slower rate than the very high luminosity sources ( erg s^-1), reaching a maximum around and then levelling off beyond . This translates into an accretion driven luminosity density which is dominated by sources with  erg s^-1 up to at least , while the contribution of the same sources and of those with  erg s^-1 appear, with yet rather large uncertainties, to be comparable between and 4.