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S.A. Stanford, S.D. Croft, W.H. de Vries, W.J.M. van Breugel, R.H. Becker (IGPP-LLNL / UC Davis), N. Kassim, A. Cohen (NRL), A. Dey, B. Jannuzi (NOAO)
We have combined low-frequency (325 MHz) VLA radio imaging data with existing deep imaging data, across a large range of the electromagnetic spectrum, for the NOAO Deep Wide-Field Survey (NDWFS) Bootes and Spitzer First Look Survey (FLS) fields. In the radio, the availability of two flux density measurements at 325 and 1400 MHz allows for a direct handle on the radio source population properties. Radio sources can be broadly classified based on the steepness of their radio spectrum and the flux densities. Steep spectrum sources that are bright in the radio tend to be high-redshift objects, whereas low flux density - steep spectrum sources tend to be nearby star-forming galaxies.
This fact is corroborated in a direct way by matching our radio spectral index catalog (containing about 1200 radio sources) to the groundbased deep optical, and near-infrared Spitzer imaging data. A high fraction of radio sources, especially the flatter spectrum ones, are recovered in the optical / near-IR (up to 90%). However, this fraction drops dramatically toward the steepest radio sources (~ 40%), but interestingly enough, only for the subset of radio-bright steep-spectrum sources. The radio-faint ones are indeed local and easily identified. No selection is ever perfect: there are a few radio-faint and near-IR faint objects which turn out to be z~3, ~L* proto-galaxy AGN hosts.
We therefore have established the tools needed to effectively select radio sources at cosmologically early epochs; systems that by virtue of their mass and radio activity sign-post construction sites of galactic systems in the early universe. Up till now, it has been hard to construct a sample of high redshift systems that was not affected by obscuration in one way or the other. Our method, which relies mainly on the radio and near-IR detections is much less affected, and provides a new window on the early universe.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.