We have constructed three spatially separated samples of USS sources containing a total of 669 objects. High-resolution radio observations of more than half of these show that the median size is 6 , independent of 1.4 GHz flux density, which is consistent with results of similar resolution surveys of samples without spectral index selection. The absence of a downturn in angular size at the lowest fluxes indicates that we do not include significant numbers of spiral galaxies in our sample. A USS sample fainter than ours would therefore include more of these foreground sources, and be less efficient to find HzRGs.
The identification fraction on the POSS is 15%, with no clear dependence on spectral index, indicating that the HzRGs in the sample are all too distant to be detected, and the POSS detections consist of different classes of objects. A correlation of our USS samples with X-ray catalogs showed that at least 85% of the X-ray identifications seem to be galaxy clusters known from the literature or by inspection of the POSS. We conclude that (1) the majority of the "non HzRG'' USS sources in our sample are clusters, and (2) the combined selection of USS and X-ray sources is an extremely efficient technique to select galaxy clusters.
The above results indicate that up to 85% of our USS sources might be HzRGs. To identify these objects, we have started an intensive program of R- and K-band imaging on 3-10 m class telescopes. Initial results from optical spectroscopy indicate that 2/3 are indeed z>2 radio galaxies (De Breuck et al. 1998a), and K-band imaging of optically undetected (R > 25) sources (see e.g., van Breugel et al. 1999a) has already lead to the discovery of the first radio galaxy at z>5 (van Breugel et al. 1999b).
We are grateful for the excellent help provided by the staff of the VLA and ATCA observatories, with special thanks to Chris Carilli and Greg Taylor (NRAO), and Ray Norris and Kate Brooks (ATNF) for help in with observation planning and data reduction. We thank Hien "Napkin'' Tran for his comments on the manuscript. The VLA is a facility of the National Radio Astronomy Observatory, which is operated by Associated Universities Inc. under cooperative agreement with the National Science Foundation. The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. The authors made use of the database CATS (Verkhodanov et al. 1997) of the Special Astrophysical Observatory, the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. The work by C.D.B. and W.V.B. was performed under auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
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