EDP Sciences
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Volume 418, Number 3, May II 2004
Page(s) 827 - 840
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20035622

A&A 418, 827-840 (2004)
DOI: 10.1051/0004-6361:20035622

The HELLAS2XMM survey

V. Near-Infrared observations of X-ray sources with extreme X/O ratios
M. Mignoli1, L. Pozzetti1, A. Comastri1, M. Brusa1, 2, P. Ciliegi1, F. Cocchia3, F. Fiore3, F. La Franca4, R. Maiolino5, G. Matt4, S. Molendi6, G.C. Perola4, S. Puccetti3, P. Severgnini7 and C. Vignali1

1  INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
    e-mail: lucia,comastri,ciliegi,l_vignali@bo.astro.it
2  Dipartimento di Astronomia Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
    e-mail: brusa@bo.astro.it
3  INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
    e-mail: cocchia,fiore,puccetti@mporzio.astro.it
4  Dipartimento di Fisica Università di Roma Tre, via Vasca Navale 84, 00146 Roma, Italy
    e-mail: lafranca,matt,perola@fis.uniroma3.it
5  INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy
    e-mail: maiolino@arcetri.astro.it
6  IASF-CNR, Istituto di Fisica Cosmica, via Bassini 15, 20133, Milano, Italy
    e-mail: silvano@mi.iasf.cnr.it
7  INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
    e-mail: paola@brera.mi.astro.it

(Received 3 November 2003 / Accepted 19 January 2004 )

We present the results of deep near-infrared observations (with ISAAC at VLT) of eleven hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude ( $R~\ga$ 24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the $K_{\rm s}$ band, with bright counterparts ( $K_{\rm s}$  < 19) and very red colors ( R-K>5), and therefore belong to the ERO population. The quality of the near-infrared data is such that we can take advantage of the sub-arcsec seeing to obtain accurate morphological information. A detailed analysis of the surface brightness profiles allows us to classify all of the near-infrared counterparts. There are two point-like objects, seven elliptical (bulge) galaxies and one source with an exponential profile. None of the extended sources shows any evidence of the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R-K colors and the morphological information, we have estimated for all the sources a "minimum photometric redshift", ranging between 0.8 and 2.4; the elliptical hosts have $z_{\rm min}=0.9{-}1.4$. We computed the X-ray properties using these redshifts: most of the sources have $N_{\rm H}>10^{22}$ cm -2, with unabsorbed X-ray luminosities up to 10 45 erg s -1 in the intrinsic 2-10 keV band. These objects therefore belong to the long-sought population of obscured (type II) quasars and, from a statistical point of view, they are a non-negligible fraction (about 10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the near-infrared is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN, whose obscured nuclei do not affect the host galaxy morphologies. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at $z\sim1$, and that these near-IR bright objects would be among the most massive spheroids at these epochs.

Key words: cosmology: observations -- galaxies: active -- infrared: galaxies -- X-rays: galaxies

Offprint request: M. Mignoli, mignoli@bo.astro.it

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