The HELLAS2XMM survey V. Near-infrared observations of X-ray sources with extreme X/O ratios

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 24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks 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 zmin = 0.9-1.4. We computed the X-ray properties using these redshifts: most of the sources have NH > 1022 cm-2, with unabsorbed X-ray luminosities up to 1045 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 ∼ 1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.