2007ApJ...654..764B

We use highly spectroscopically complete observations of the radio sources from the VLA 1.4 GHz survey of the Hubble Deep Field-North region to study the faint radio galaxy population and its evolution. The fraction that can be optically spectroscopically identified is fairly independent of radio flux, with about 60%-80% identified at all fluxes. We spectrally classify the sources as absorbers, star formers, Seyfert galaxies, and broad-line AGNs, and we analyze their properties by type. We supplement the spectroscopic redshifts with photometric redshifts measured from the rest-frame ultraviolet to mid-infrared spectral energy distributions. Using deep X-ray observations of the field, we do not confirm the existence of an X-ray-radio correlation for star-forming galaxies. We also do not observe any correlations between 1.4 GHz flux and R magnitude or redshift. We find that the radio powers of the host galaxies rise dramatically with increasing redshift, while the optical properties show at most small changes. Assuming that the locally determined far-infrared-radio correlation holds at high redshifts, we estimate total FIR luminosities for the radio sources. We note that the FIR luminosity estimates for any radio-loud AGNs, which we conservatively do not try to remove from the sample, will be overestimates. Considering only the radio sources with quasar-like bolometric luminosities, we find a maximum ratio of candidate highly obscured AGNs to X-ray-luminous (L_0.5–2keVor L_2–8keV≥10⁴² ergs/s) sources of about 1.9. Finally, we use source-stacking analyses to measure the X-ray surface brightnesses of various populations. We find the contributions to the 4-8 keV light from our candidate highly obscured AGNs to be very small, and hence these sources are unable to account for the light that has been suggested may be missing at these energies.