15.4.6. Optical Constraints

The magnitude distributions of galaxies identified with faint radio sources are often used to estimate redshift distributions. These estimates can be refined if broadband colors and optical morphologies are available, since the absolute magnitudes of the giant elliptical galaxies associated with the most luminous radio sources are somewhat brighter than those of elliptical galaxies identified with less luminous sources (Auriemma et al. 1977) and the "blue" population of galaxies investigated by Kron et al. (1985). Windhorst et al. (1984a) have shown that the fraction of radio sources identified with galaxies brighter than J 23.7 is about 50%, nearly independent of 1.4-GHz flux density in the range of 1 to 100 mJy. Thus, the median redshift in this flux-density range is roughly equal to the redshift of a giant elliptical galaxy at their plate limit, z 0.8. Unless the amount of evolution changes discontinuously at redshifts just beyond z 0.8, most of the remaining objects fainter than J 23.7 must have redshifts only slightly larger. In fact, the identification rate approaches 100% at V 26 (Windhorst 1986), and few galaxies in this flux-density range should have redshifts greater than 1.9.

The identification content appears to change below S 1 mJy from primarily red elliptical galaxies to bluer, morphologically distorted galaxies with fainter absolute magnitudes (Windhorst 1984, Kron et al. 1985). The nature of these galaxies and their redshift range is still uncertain (Wall et al. 1985, Windhorst 1986, Weistrop et al. 1987).