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

P. Severgnini, R. Della Ceca, V. Braito, P. Saracco, M. Longhetti, R. Bender, N. Drory, G. Feulner, U. Hopp, F. Mannucci, C. Maraston

Research output: Contribution to journalArticlepeer-review


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 (L0.5−2keV ∼ 1043 erg s−1) and by its X-rayto- 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 1.7 with a stellar mass well in excess of 1011 M. This result corroborates the possible link between the QSO activity and the formation of massive spheroids.
Original languageEnglish
Pages (from-to)87-95
JournalAstronomy and Astrophysics
Issue number1
Publication statusPublished - 3 Feb 2005


  • galaxies : active
  • X-rays : galaxies


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