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 galaxies, but identifying the optical/near-infrared (NIR) counterparts to these sources has proved difficult because of the poor submillimeter spatial resolution. However, the proportionality of both centimeter and submillimeter data to the star formation rate suggests that high-resolution radio continuum maps with subarcsecond positional accuracy can be exploited to locate submillimeter sources. In this paper we present results from a targeted SCUBA 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 observations (a 3 sigma detection limit of 6 mJy at 850 mu m), we were successful at making submillimeter detections of optical/NIR-faint (I greater than or similar 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 corollary is that a large fraction of the bright (>6 mJy) submillimeter sources in untargeted submillimeter surveys have extremely faint optical/NIR counterparts 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 distribution. This procedure, which we refer to as millimetric redshift estimation, places the bright submillimeter population at z = 1-3, where it forms the high-redshift tail of the faint radio population. The star formation rate 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 SFRD at high-redshift inferred from our >6 mJy submillimeter observations is comparable with that observed in the ultraviolet/optical.