Optically faint microjansky radio sources

We report on the identifications of radio sources from our survey of the Hubble Deep Field (HDF) and the Small Selected Area 13 fields, both of which comprise the deepest radio surveys to date at 1.4 and 8.5 GHz, respectively. About 80% of the microjansky radio sources are associated with moderate-redshift starburst galaxies or active galactic nuclei within the I-magnitude range of 17-24 with a median of I = 22 mag. Thirty-one (20%) of the radio sources are (1) fainter than I > 25 mag, with two objects in the HDF I-AB > 28.5, (2) often identified with very red objects I-K > 4, and (3) not significantly different in radio properties than the brighter objects. We suggest that most of these objects are associated with heavily obscured starburst galaxies with redshifts between 1 and 3. However, other mechanisms are discussed and cannot be ruled out with the present observations.