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E. A. Richards (NRAO/UVa)
I 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 \muJy respectively. The differential count of the radio sources is marginally sub-Euclidean to the completeness limits (\gamma = -2.3 ± 0.1) and fluctuation analysis suggests nearly 100 sources per arcmin2 at the 1 \muJy level. Using the high resolution 1.4 GHz observations, I measure a median angular size for the microjansky radio population of about 2\arcsec . This clue coupled with the steep spectral index of the 1.4 GHz selected sample may be indicative of diffuse synchrotron radiation in z~1 galactic disks.
The wide-field HST and ground-based optical exposures show that the radio sources are identified primarily with disk systems. Only 20% of the radio sources can be attributed to AGN activity -- the majority are likely associated with starburst activity. The available redshifts range from 0.1-3, with a mean of about 0.8. We are likely witnessing a major episode of starburst activity in these luminous (L > L*) systems, occassionaly accompanied by an embedded AGN. Star-formation rates calculated from the radio luminosity of these sources indicate rates an order of magnitude higher than those from UV studies. The implication is that the majority of starbursts out to z~1 are missed in optical studies, and hence the global star-formation rate severely underestimated.