XMM-Newton observations of Extremely Red Objects and the link with luminous, X-ray obscured quasars^*

¹ Dipartimento di Astronomia Universita' di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: marcella@mpe.mpg.de
² INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany
⁴ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
⁶ Sterrewacht Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands

Received: 14 June 2004
Accepted: 28 October 2004

Abstract

We present the results of a deep (about 80 ks) XMM-Newton survey of the largest sample of near-infrared selected Extremely Red Objects () available to date to . At the relatively bright X-ray fluxes ( erg cm^-2 s^-1) and near-infrared magnitude probed by the present observations, the fraction of AGN (i.e. X-ray detected) among the ERO population is small (~3.5%); conversely, the fraction of EROs among hard X-ray selected sources is much higher (~20%). The X-ray properties of EROs detected in our XMM-Newton observation indicate absorption in excess of 10²² cm^-2 in a large fraction of them. We have also considered additional samples of X-ray detected EROs available in the literature. X-ray spectral analysis of the highest S/N sources unambiguously indicates that large columns of cold gas (even >10²³ cm^-2) are the rule rather than the exception. The X-ray, optical and near-infrared properties of those X-ray selected EROs with a spectroscopic or photometric redshift nicely match those expected for type 2 quasars, the high-luminosity, high-redshift obscured AGNs predicted in baseline XRB synthesis models. A close correlation is detected between X- and K-band fluxes. For the AGN EROs this is consistent, under reasonable assumptions, with the relation established locally between the host galaxies and their central black holes. This suggests that the majority of EROs are powered by massive black holes accreting, on average, at about 0.03-0.1 of the Eddington limit.