Astron. Astrophys. 355, L47-L51 (2000)

1. Introduction

X-ray background (XRB) synthesis models ascribe most of the high energy flux to radio quiet, absorbed Active Galactic Nuclei (AGNs) at intermediate and high redshifts (e.g. Comastri et al. 1995, Gilli et al. 1999, Pompilio et al. 2000). Observationally the available information on AGNs at these redshifts refers mostly to unabsorbed nuclei, since the current samples of radio quiet AGNs have been selected mostly with color techniques, or in the soft X rays. Only recently have selection criteria less sensitive to absorption been used. Examples are the radio quiet red QSOs (Kim & Elvis 1999), analogous to the ones already found in radio loud samples (eg. Webster et al. 1995), and the spectroscopic identifications in the ELAIS field (Rowan-Robinson et al. 1999). Yet, most of our knowledge about absorbed, radio quiet AGNs is limited to low redshifts and low luminosities, where spectroscopic surveys of bright galaxies have been performed. The High Energy LLarge Area Survey [HELLAS, Comastri et al. 2000, Fiore et al. 2000 (paper II)] aims at providing a useful sample of hard X-ray selected (5-10 keV), optically identified AGNs while waiting for the Chandra and XMM results. The survey instrument is the BeppoSAX MECS. The sky coverage is 1-50 square degrees at  5-30 , respectively, and is 84 deg² at fluxes higher than . The cataloged sources amount to 147, and at the fainter limit the source density is  deg^-2, implying that about 20-30% of the XRB at these energies has been resolved. A program of optical identification is underway, which includes optical/near-IR broad band photometry and near-IR imaging, beside optical spectroscopy of all candidates down to R = 20. So far, 63 optical counterparts have been identified, in about two thirds of the examined errorboxes. About half of the spectra are typical of QSOs, with a blue continuum and broad lines, about half are of intermediate type (1.8-1.9), generally with red continua, and a few of them contain only narrow lines [Fiore et al. 1999 (paper I), La Franca et al. 2000 (paper III)].

In this paper we present and discuss preliminary results of the near-IR photometry and imaging observations of the spectroscopically identified counterparts. Combined with the optical information presented in papers I and III these data give a broad-band view of the properties of the HELLAS sources and allow a preliminary census of the hard XRB contributors.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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