Elaine M. Sadler , V.J. McIntyre , C.A. Jackson , R.D. Cannon, PASA, 16 (3), 247.
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Title/Abstract Page: Radio Sources in the
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Introduction
A new generation of sensitive, large-area radio-source surveys at milliJanksy levels (1Jy = 10-26 Wm-2Hz-1) is now becoming available. They include the FIRST survey (Becker et al. 1995), WENSS (Rengelink et al. 1997), NVSS (Condon et al. 1998) and SUMSS (Bock et al. 1999). These surveys offer some important advantages for cosmological studies - they reach sufficiently high source densities that detection of large-scale structure is possible (Cress et al. 1996, Magliocchetti et al. 1998), and also probe a second cosmologically-significant radio source population, that of star-forming galaxies, which are rarely seen in strong-source surveys. Deep radio surveys of a few small areas of sky at 1.4GHz (Condon 1984, Windhorst et al. 1985; see also Condon 1992) have shown that there are two distinct populations of extragalactic radio sources. Over 95% of radio sources above about 50mJy are classical radio galaxies and quasars (median redshift z
1) powered by active galactic nuclei (AGN), while the remainder are identified with star-forming galaxies (median z0.1). The fraction of star-forming galaxies increases rapidly below 10mJy, and below 1mJy they begin to be the dominant population.
The scientific return from radio continuum surveys is enormously increased if the optical counterparts of the radio sources can be identified and their redshift distribution measured. In the past, this has been a slow and tedious process which could only be carried out for relatively small samples. However, the Anglo-Australian Observatory's Two-degree Field (2dF) spectrograph now makes it possible to carry out spectroscopy of several hundred galaxies simultaneously. Here, we describe the first step in this process - the identification of faint radio-source counterparts among galaxies whose spectra have been obtained in the 2dF Galaxy Redshift Survey (2dFGRS).
The 2dFGRS (Colless 1999, Maddox 1998) is a large-scale survey of 250,000 galaxies covering 2000 square degrees in the southern hemisphere. The survey is designed to be almost complete down to a limiting apparent magnitude of
. The median redshift of the galaxies is about 0.1 and the great majority have z < 0.3. Spectra are being obtained using the 2dF multi-object fibre optic spectroscopic system on the 3.9m Anglo-Australian Telescope (AAT) (Lewis et al. 1998; Smith & Lankshear 1998). 2dF enables the AAT to obtain spectra for 400 objects simultaneously, spread over a field which is two degrees in diameter. The survey will cover two large strips of sky at high Galactic latitude, one each in the southern and northern Galactic hemispheres, plus outlying random fields in the south. The first test data were obtained in 1997 and the survey is expected to be substantially complete by the end of 2000.
This paper presents the first results from what will eventually be a much larger study. When the 2dFGRS is complete, it will yield around 4000 good-quality spectra of galaxies associated with faint radio sources -- by far the largest sample of radio-galaxy spectra ever compiled. Our aim in this paper is to present a first, qualitative exploration of the faint radio source population as observed by 2dF and the NVSS.
Throughout this paper, we use Ho=75 km/s/Mpc and qo=0.5.
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