Powerful, obscured active galactic nuclei among X-ray hard, optically dim serendipitousChandrasources

Abstract

We present a small sample of Chandra X-ray sources selected from the fields of ACIS observations which probe fluxes around the break in the hard band source counts. The targets of these fields include nine nearby galaxy clusters, one distant cluster and two powerful radio galaxy fields. The follow-up of this serendipitous sample was biased towards X-ray hard and optically dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray fluxes, optical and near-infrared photometry leading to 51 photometric redshifts in all and 18 independently measured spectroscopic redshifts. Few sources are associated with the target fields themselves. 56 of 58 sources imaged in the K-band are detected at K 20 with K median = 18, and of these, 38 have hard X-ray count ratios and 24 of these are significantly hard with most of the counts emerging about 2 keV. We find that almost all are active galactic nuclei (AGN) hosted in massive early-type host galaxies with a photometric redshift distribution peaking at z ∼ 1. Two type 2 quasars with intrinsic X-ray luminosity L 10 45 erg s −1 ,F e Kα emission lines and absorbing column density N H > 10 23 cm −2 ‐ and N H 10 24 cm −2 in one case ‐ are discussed in detail; the sample contains at least 12 potential type 2 quasars in all. We discuss various detection strategies for type 2 quasars and calculate their inferred space density. This combines and extends a number of results from subsamples already published by us.