INFRARED OBSERVATIONS OF 1-JANSKY RADIO-SOURCE IDENTIFICATIONS AND EMPTY FIELDS

Infrared observations of 53 '1-jansky' radio sources are presented and discussed. Particular attention is paid to the observations of 12 'empty field' sources that have not been optically identified on CCD images to a limiting magnitude of r∼23.5. Seven of these empty field sources were detected in the K passband at significance levels greater than 4σ and all of the remainder produced net positive signals at lower significance levels. A comparison of the data on the identified sources with existing data on the more radio-luminous 3C radio galaxies has shown first that the 1 Jy radio galaxies have the same absolute magnitudes as the 3C radio galaxies, and secondly that at z∼1 they display the same range of active and passive evolution of their stellar populations. These results indicate that the changes seen in the optical/infrared properties of the high-redshift radio galaxies are primarily changes with redshift rather than radio luminosity. In the case of the empty-field observations, additional infrared colour measurements of two of these strongly suggest that they are high-redshift radio galaxies. If this is the case for all of the empty-field sources, then the lower limits to the (r-K) colours for the brighter infrared detections indicate that these are passively evolving giant elliptical galaxies at z∼2 that have no continuing star-formation activity and hence that the active evolution seen in radio galaxies at 0.5 z ≤ 1.3 is not the tail end of universal violent star formation at z∼2. The fainter infrared detections are probably galaxies with z 2. The high fraction of infrared detections suggests that this sample does not contain a significant number of sources with z 5. © 1985 Royal Astronomical Society.