CENSORS: A Combined EIS–NVSS Survey of Radio Sources – II. Infrared imaging and the K – z relation

The Combined EIS–NVSS Survey of Radio Sources (CENSORS) is a 1.4‐GHz radio survey selected from the National Radio Astronomy Observatories (NRAO) Very Large Array (VLA) Sky Survey (NVSS) and complete to a flux density of 7.2 mJy. It targets the European Southern Observatory (ESO) Imaging Survey (EIS) Patch D, which is a 3 × 2‐deg 2 field centred on right ascension 09 h 51 m 36 s · 0 and declination −21°00′ 00″ (J2000). This paper presents K ‐band imaging of 142 of the 150 CENSORS sources. The primary motivation for beginning infrared imaging of the sample was to identify the host galaxies of ∼30 per cent of sources for which the EIS I ‐band imaging failed to produce a likely candidate. In addition, K ‐band magnitudes allow photometric redshift estimation and I − K colours aid the identification of host galaxies (which are typically old, red ellipticals). Of the sources observed in the I and K bands, four remain undetected, possibly indicating high redshifts for the host galaxies, and eight involve complicated radio structures, or several candidate host galaxies, which have yet to be resolved. Thus, the host galaxy identifications are brought to 92 per cent completeness. In conjunction with spectroscopic observations, the K ‐band magnitudes have been aperture corrected and used to establish a K – z relation for the CENSORS radio galaxies. This relation is of interest because of its variation, at z > 1, between radio surveys of different flux‐density limit. Establishing this relation for CENSORS may shed light on the origin of this variation and will allow an appropriate K – z redshift estimator for any CENSORS source which remains without a spectroscopic redshift. It is shown that whilst the K – z relation for CENSORS is fainter than that of 3CRR at all redshifts, it agrees well with that of 7C over all redshifts studied.