A population of high-redshift type-2 quasars-I. Selection Criteria and Optical Spectra

Abstract

We discuss the relative merits of mid-infrared and X-ray selection of type-2 quasars. We describe the mid-infrared, near-infrared and radio selection criteria used to find a population of redshift z~2 type-2 quasars which we previously argued suggests that most supermassive black hole growth in the Universe is obscured (Martinez-Sansigre et al., 2005). We present the optical spectra obtained from the William Herschel Telescope, and we compare the narrow emission line luminosity, radio luminosity and maximum size of jets to those of objects from radio-selected samples. This analysis suggests that these are genuine radio-quiet type-2 quasars, albeit the radio-bright end of this population. We also discuss the possibility of two different types of quasar obscuration, which could explain how the ~2-3:1 ratio of type-2 to type-1 quasars preferred by modelling our population can be reconciled with the ~1:1 ratio predicted by unified schemes.