Spectroscopy of Ultra–steep-Spectrum Radio Sources

We present optical spectroscopy of 62 objects selected from several samplesof ultra steep spectrum (USS) radio sources. 46 of these are from our primarycatalog, consisting of 669 sources with radio spectral indices alpha < -1.30(S_nu ~ nu^alpha); this first spectroscopic sub-sample was selected on thebasis of their faint optical and near-IR identifications. Most are identifiedas narrow-lined radio galaxies with redshifts ranging from z=0.25 to z=5.19.Ten objects are at z>3, nearly doubling the number of such sources known todate. Four of the USS radio sources are identified with quasars, of which atleast three have very red spectral energy distributions. The source TNJ0936-2242 is identified with an extremely red object (ERO, R-K>5); both it anda close companion are at z=1.479. The spectrum of the ERO closely resemblesthat of previously discovered radio galaxies at z~1.5. Five sources showcontinuum emission, but fail to show any clear emission or absorption features,despite integrations of ~1 h with the Keck telescope. We suggest that theseobjects could be (i) radio galaxies with faint emission lines in the ``redshiftdesert'' at 1.5 <~ z <~ 2.3, (ii) radio galaxies with an obscured AGN, whichare dominated by a stellar continuum observed with insufficient S/N, or (iii)pulsars. Three radio sources identified with faint objects in the K-band imagesremain undetected in 50-90 min spectroscopic integrations with the Kecktelescope, and are possible z>7 candidates.Comment: 59 Pages, including 59 PostScript figures. Accepted for publication in the Astronomical Journal (March 2001 issue). Tables 5 and 6 will be available in electronic format on the AJ website or upon reques