Galaxy Evolution in the Reddest Possible Filter

Abstract

We describe an observational program aimed at understanding the radio emission from distant, rapidly evolving galaxy populations. These observations were carried out at 1.4 and 8.5 GHz with the VLA centered on the Hubble Deep Field, obtaining limiting flux densities of 40 and 8 $μ$Jy respectively. Using high resolution 1.4 GHz observations obtained with MERLIN, we resolve all radio sources detected in the VLA complete sample and measure a median angular size for the microjansky radio population of 1-2\arcs . This clue coupled with the steep spectral index of the 1.4 GHz selected sample suggests diffuse synchrotron radiation in $z \sim 1$ galactic disks. The wide-field HST and ground-based optical exposures show that the radio sources are identified primarily with disk systems composed of irregulars, peculiars, interacting/merging galaxies, and a few isolated field spirals. About 20% of the radio sources remain unidentified to I = 26-28 in the HDF and flanking fields. Several of these objects have extremely red counterparts. We suggest that these are high redshift dusty protogalaxies.