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Astron. Astrophys. 339, 34-40 (1998)

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1. Introduction

New radio source surveys covering large parts of the sky down to low flux densities are now rapidly becoming available. The NRAO VLA Sky Survey (NVSS) and Faint Images of the Radio Sky (FIRST) are both at 20 cm, but with different resolutions (Condon et al. 1998, Becker et al. 1995), and contain hundreds of thousands to several million radio sources. A parallel survey, at 92 cm, has been carried out with the Westerbork Radio Synthesis Telescope (WSRT) and is similar in size to the VLA surveys (Rengelink et al. 1997). Moreover, the Westerbork Northern Sky Survey (WENSS) has a resolution (of order one arcminute) similar to the NVSS.

It is clear that these new surveys will start a whole new chapter in the field of extragalactic research. The fact that the surveys are done at different frequencies will make it possible to study large numbers of sources with extreme spectra, like ultra-steep spectrum, peaked spectrum, and flat spectrum sources (the latter often being high-redshift quasars); it also allows us to do systematic searches for very rare events like gravitational lenses (see the CLASS project, Myers et al. 1995). Moreover, "classical" topics can now be studied in unprecedented detail. Optical/Infrared identification of radio sources can lead to very big complete samples of quasars and galaxies (both ellipticals and spirals), over a much wider range of intrinsic properties, such as radio power, than has hitherto been possible.

Although the observations of WENSS have now been completed, a subsample (the WENSS minisurvey) is in an advanced state of analysis (see Rengelink et al. 1997). Moreover, the minisurvey was crosscorrelated with optical data, resulting in a sample of "bright" ([FORMULA]) galaxy identifications. We report on this sample of galaxy identifications in the present paper.

A very brief description of the minisurvey of WENSS is given in Sect. 2; the construction of a sample of galaxies associated with minisurvey sources is discussed in Sect. 3. Although many galaxies have known redshifts (because they are bright) this is by no means true for all galaxies in the sample. We therefore started a program of optical spectroscopy, the first results of which are discussed in Sect. 4. Another readily available property is the radio spectral index between 325 MHz (the observing frequency of WENSS) and 1400 MHz (the observing frequency of the NVSS, which also has a very similar angular resolution); this is discussed in Sect. 5. Some first results, like a determination of the local radio luminosity functions of ellipticals and spirals, are given in Sect. 6, while an outline of future related work is described in Sect. 7.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998