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

Free access article

Issue		A&A Volume 431, Number 1, February III 2005


Page(s)		87 - 95
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20041587

A&A 431, 87-95 (2005)
DOI: 10.1051/0004-6361:20041587

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

P. Severgnini¹, R. Della Ceca¹, V. Braito¹, P. Saracco¹, M. Longhetti¹, R. Bender^{2, 3}, N. Drory⁴, G. Feulner², U. Hopp^{2, 3}, F. Mannucci⁵ and C. Maraston³

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, Giessenbachstra $\beta$ e, 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 10 ⁴⁴ erg s ^-1 and intrinsic column densities higher than 10 ²² 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 ( $L_{0.5-2\rm keV}\sim10^{43}$ 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 $z\simeq1.7$ with a stellar mass well in excess of 10 ¹¹ $M_\odot$ . This result corroborates the possible link between the QSO activity and the formation of massive spheroids.

Key words: galaxies: active -- X-rays: galaxies

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