The Properties of Microjansky Radio Sources in the Hdf-n, Ssa13, and Ssa22 Fields

We present multiwavelength observations for a large sample of microjansky radio sources detected in ultradeep 1.4GHz maps centered on the Hubble Deep Field-North (HDF-N) and the Hawaii Survey Fields SSA13 and SSA22. Our spectroscopic redshifts for 169 radio sources reveal a flat median redshift distribution, and these sources are hosted by similarly luminous optical L galaxies, regardless of redshift. We suggest that the absence of low optical luminosity galaxies at low redshifts, where there are no selection effects, is due to small galaxies not being efficient at retaining the cosmic rays necessary to host microjansky radio sources. This is a serious concern for radio estimates of the local star formation rate density, as a substantial fraction of the ultraviolet luminosity density is generated by sub-L galaxies at low redshifts. From our submillimeter measurements for 278 radio sources, we find error-weighted mean 850μm fluxes of 1.72±0.09mJy for the total sample, 2.37±0.13mJy for the optically-faint (I > 23.5) subsample, and 1.04±0.13mJy for the optically-bright (I < 23.5) subsample. We significantly (> 3σ) detect in the submillimeter 50 of the radio sources, 38 with I > 23.5. Spectroscopic redshifts for three of the I < 23.5 submillimeter-detected radio sources are in the range z = 1.0–3.4, and all show AGN signatures. Using only the submillimeter mapped regions we find that 69 ± 9% of the submillimeterdetected radio population are at I > 23.5. We also find that 66±7% of the S850μm> 5mJy (> 4σ) sources are radio-identified. We use our spectroscopic sample to determine the evolution with redshift of the radio power, and hence the far-infrared (FIR) luminosity (through the local FIR-radio correlation). We find that millimetric redshift estimates at low redshifts are best made with a FIR template intermediate between a Milky Way type galaxy and a starburst galaxy, and at high redshifts with an Arp 220 template. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: starburst