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S.G. Djorgovski (CIT), D.J. Thompson (CIT/LBTO), D. Stern (JPL), A.A. Mahabal, M. Bogosavljevic (CIT), A.S. Cohen, W.M. Lane, N.E. Kassim, T.J.W. Lazio (NRL)
We obtained IR IDs and spectroscopy for a sample of steep-spectrum (\alpha < -1.3), compact (\theta < 10 arcsec) sources selected from the VLSS 74 MHz survey. The radio sky at such low frequencies is relatively poorly understood, and may become an important cosmological arena in the LOFAR, LWA, and SKA era. We find that the sample contains a variety of object types. In general, the IDs are as faint or fainter than in any of the previous surveys for high-z radio galaxies. Some sources are associated with early-type or possible post-merger galaxies at moderate redshifts, z ~0.1 – 1. Some are situated in what looks like distant clusters of galaxies, but do not coincide with any of the apparent cluster members; their nature is as yet unknown, but they may represent aging radio lobes, cluster halos or relics, or perhaps something new and more interesting. Some are seemingly ``classical'' powerful radio galaxies at z ~2 – 3.5, but with one intriguing difference: they show a significantly lower ionization in their emission line spectra. The cause of this is not understood, but it may reflect a systematic difference in the UVX continuum properties of their central engines. This is the first time that such a correlation between the radio spectral index (for the lobes) and the UV/optical spectra of the hosts is seen for radio galaxies. Finally, some of the faint IDs may be at very high redshifts, z > 5 – 7. Therefore, we find that the low-frequency radio sky has a diverse, rich, and interesting phenomenology, with a great scientific promise for the future instruments and surveys in this wavelength regime. This work was supported in part by the NSF grant AST-0407448 at Caltech, and by the ONR at NRL.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.