4.3. Faint radio galaxies

As discussed above, the faintest radio galaxies should be detectable in the submm waveband if the far-IR-radio correlation remains valid at high redshifts. The narrow dispersion of this correlation suggests that submm galaxies and faint radio galaxies are perhaps the most likely populations of high-redshift galaxies to overlap substantially. Surveys made using SCUBA to search for optically-faint, and thus presumably high-redshift, galaxies with radio flux densities close to the detection threshold of the deepest radio surveys (Barger et al., 2000; Chapman et al., 2001b) have been used to detect many tens of high-redshift dusty galaxies much more rapidly than blank-field surveys. The selection effects at work when making a radio-detected, optically-faint cut from a radio survey are not yet sufficiently well quantified to be sure that these catalogs are representative of all submm galaxies. The typical optical magnitudes of the radio-selected objects with submm detections are clustered around I 24 and greater. Hence, bright optical counterparts to mJy-level submm galaxies are rare (Chapman et al., 2002b). Because relatively accurate positions are available from the radio observations, it should be possible to determine spectroscopic redshifts for a significant number of these galaxies, providing a valuable contribution to our knowledge of the distances to at least a subset of submm galaxies.