The Canada-France Redshift Survey - VII. Optical counterparts of microjansky radio sources

Deep imaging and spectroscopy have been carried out for optical counterparts of a complete sample of <e1>S<~</e1>16 muJy radio sources during the course of the Canada-France Redshift Survey (CFRS). All 36 sources but two have been optically identified, and spectra have been obtained for 23 of them. The objects brighter than I_AB<22.5 for which we have spectra reveal three populations dominating the muJy radio counts: <e1>z<~</e1>0.7 early-type galaxies with radio emission powered by an AGN, intermediate-redshift post-starburst galaxies, and lower redshift blue emission-line objects. From their radio and optical properties, it is argued that the 11 objects fainter than I_AB<~22.5 are mostly at <e1>z<~</e1>1, and about half of them are probably early-type galaxies. We conclude that ~40 per cent of the muJy sources are likely to be at <e1>z<~</e1>1. Between one-third and one-half of the luminous ellipticals in this field beyond z=0.7 show moderately powerful radio emission (P~5x10^23 W Hz^-1) which is at least 10 times more powerful than seen in local samples, and probably reflects evolution of the activity in their nuclei. Only one classical starburst galaxy is identified in the sample; the rest of the blue emission-line objects show optical and radio activity more typical of low-power AGNs than starbursts. The number of post-starburst galaxies at muJy levels is considerably higher than the surface density of mJy starburst galaxies, suggesting that the latter are the parent population of the former. While starburst galaxies are considered to be major contributors to the mJy radio source counts, the majority of the muJy radio sources appear to be related to AGN activity rather than to normal star formation.