Radio constraints on the identifications and redshifts of submillimeter galaxies

We present radio maps from the Very Large Array (VLA) for 16 sources detected in a submillimeter (submm) survey of the distant universe. Our deep VLA 1.4 GHz maps allow us to identify radio counterparts or place stringent limits (less than or similar to 20 mu Jy in the source plane) on the radio flux of the submm sources. We compare the spectral indices of our sources between 850 mu m and 1.4 GHz to empirical and theoretical models for distant starburst galaxies and active galactic nuclei (AGNs) as a function of redshift. In this way we can derive redshift limits for the submm sources, even in the absence of an optical or near-infrared counterpart. We conclude that the submm population brighter than similar to 1 mJy has a median redshift of at least (z) similar to 2, more probably (z) similar to 2.5-3, with almost all galaxies at z much greater than 1. This estimate is a strong lower limit as both misidentification of the radio counterparts and nonthermal emission from an AGN will bias our redshift estimates to lower values. The high median redshift means that the submm population, if predominately powered by starbursts, contributes a substantial fraction of the total star formation density at high redshifts. A comparison of the spectral index limits with spectroscopic redshifts for proposed optical counterparts to individual submm galaxies suggests that half of the submm sources remain unidentified and thus their counterparts must be fainter than 1 greater than or similar to 24.