Mapping the evolution of high-redshift dusty galaxies with submillimeter observations of a radio-selected sample

Direct submillimeter imaging has recently revealed the 850 mu m background to be mostly composed of a population of distant ultraluminous infrared gal axies, but identifying the optical/near-infrared (NIR) counterparts to thes e sources has proved difficult because of the poor submillimeter spatial re solution. However, the proportionality of both centimeter and submillimeter data to the star formation rate suggests that high-resolution radio contin uum maps with subarcsecond positional accuracy can be exploited to locate s ubmillimeter sources. In this paper we present results from a targeted SCUB A survey of microjansky radio sources in the flanking fields of the Hubble Deep Field. The sources were selected from the uniform (8 mu Jy at 1 sigma) 1.4 GHz VLA image of Richards. Even with relatively shallow SCUBA observat ions (a 3 sigma detection limit of 6 mJy at 850 mu m), we were successful a t making submillimeter detections of optical/NIR-faint (I greater than or s imilar to 24 and K similar or equal to 21-22) radio sources, and our counts closely match the bright counts from submillimeter surveys. An important c orollary is that a large fraction of the bright (>6 mJy) submillimeter sour ces in untargeted submillimeter surveys have extremely faint optical/NIR co unterparts and hence are inaccessible to optical imaging and spectroscopy. However, redshift estimates can be made from the ratio of the submillimeter flux to the radio flux across the 100 GHz break in the spectral energy dis tribution. This procedure, which we refer to as millimetric redshift estima tion, places the bright submillimeter population at z = 1-3, where it forms the high-redshift tail of the faint radio population. The star formation r ate density (SFRD) due to ultraluminous infrared galaxies increases by more than 2 orders of magnitude from z similar to 0 to z similar to 1-3. The SF RD at high-redshift inferred from our >6 mJy submillimeter observations is comparable with that observed in the ultraviolet/optical.