Looking for obscured QSOs in the X-ray emitting ERO population

¹ Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy e-mail: [paola;rdc;braito;saracco;marcella]@brera.mi.astro.it
² Universitäts-Sternwarte München, Scheiner Str. 1, 81679 München, Germany e-mail: [bender;feulner;hopp]@usm.uni-muenchen.de
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrae, 85748 Garching, Germany e-mail: [bender;hopp;maraston]@mpe.mpg.de
⁴ University of Texas at Austin, Austin, Texas 78712, USA e-mail: drory@astro.as.utexas.edu
⁵ IRA - CNR, Firenze, Italy e-mail: filippo@arcetri.astro.it

Received: 2 July 2004
Accepted: 7 October 2004

Abstract

We present XMM-Newton data centered on one of the MUNICS Near Infrared Cluster Survey fields (S2F1) and we discuss the X-ray properties of the 6 X-ray emitting EROs found. For one of them we have already obtained the redshift using near-infrared spectroscopic data, while for the remaining 5 EROs the analysis is based on photometric redshifts. We find evidence for the presence of an X-ray obscured QSO in at least 5 out of the 6 X-ray emitting EROs. For these 5 objects we derive intrinsic (2–10 keV) luminosities in excess of 10⁴⁴ erg s^-1 and intrinsic column densities higher than 10²² cm^-2. These values have been obtained through a basic X-ray spectral analysis for the three brightest sources and through the analysis of the hardness ratios for the remaining two. All of these 5 X-ray emitting EROs appear extended in the optical/near-infrared bands indicating that the host galaxy emission dominates at these wavelengths. This suggests that the hosted AGNs are likely to be absorbed also in the optical/near-infrared bands: i.e. they are likely X-ray obscured possible type 2 QSOs. For the remaining ERO the presence of an AGN is suggested both by its high 0.5–2 keV luminosity ( erg s^-1) and by its X-ray-to-optical flux ratio. In this case the quality of the present data prevents us from placing firm constraints on the AGN type hosted. Finally, the near-IR spectrum obtained for one of the 6 EROs classifies the host galaxy as an elliptical at with a stellar mass well in excess of 10¹¹ . This result corroborates the possible link between the QSO activity and the formation of massive spheroids.