Keck Observations of Microjansky Radio Sources

Abstract

Millions of times fainter than classical double-lobed radio monsters, µJy sources cover the sky at densities approaching faint optical galaxy counts. To investigate their redshifts, star formation characteristics, optical morphologies, and evolution, we have used the 10-meter Keck telescope to obtain BRI broad-band images and optical spectra of a complete sample of µJy sources detected in three fields at the VLA. In one field, only 1 of 16 sources remains unidentified down to I ~ 26.5, and in another field only 3 of 14 sources lack redshifts. The µJy sources appear to be dominated (~ 50% of the sample) by blue star-forming galaxies with median redshift z ~ 0.5, with a large fraction of interacting and close-pair systems. About 25% are high-redshift QSOs, including one with strong Mg II absorption at z _abs = z _em = 1.8, and the remaining 25% show signs of both starformation-induced emission and absorption lines characteristic of evolved stellar populations (“S+A” galaxies). The colors of the sources are consistent with those of field galaxies, which are dominated by Sbc galaxies, though the µJy radio galaxies are at least 1 mag brighter than field galaxies. It appears that the typical µJy source is similar in most respects to M82, the canonical starburst galaxy, moved out to z ~ 0.5. Evolution of the sources, which could have been avoided only if z _med (i < 0.1, now appears inevitable.