Elaine M. Sadler , V.J. McIntyre , C.A. Jackson , R.D. Cannon, PASA, 16 (3), 247.
Next Section: Two kinds of radio
Title/Abstract Page: Radio Sources in the
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The radio data
The NRAO VLA Sky Survey (NVSS; Condon et al. 1998) is a 1.4GHz (20cm) radio imaging survey of the entire sky north of declination
. The survey catalogue contains sources as weak as 2.5mJy, and is essentially complete above 3.5mJy. We used the NVSS source catalogue to identify candidate radio-emitting galaxies in the 2dFGRS. Subsets of the NVSS catalogue were extracted to match the RA and Dec range covered by each of the 2dF fields. The NVSS source density is roughly 60 per square degree, so each of these sub-catalogues contained about 200 radio sources. At this stage, we did not attempt to identify `double' NVSS sources. However, we estimate (using an algorithm similar to that adopted by Magliocchetti et al. (1998) for identifying double radio sources in the FIRST survey) that these represent only a small fraction of NVSS sources (of order 1%), so the presence of double sources does not significantly affect our results in this small sample.
We compared the NVSS and 2dFGRS catalogues for each field and identified the galaxies for which there is a candidate radio `match', i.e. an NVSS radio source lying within 15 arcsec of the position of a 2dF galaxy. The 15 arcsec limit was chosen because our earlier Monte Carlo tests using the COSMOS database suggested that most candidate matches of bright galaxies (i.e. 
19.4 mag) with radio-optical separation up to 10 arcsec are real associations, together with a substantial fraction of those with offsets of 10-15 arcsec.
The uncertainty in the NVSS radio source positions increases from a 1
error of 1-2 arcsec at 10mJy to 4-5 arcsec (and occasionally up to 10 arcsec) for the faintest (2-3mJy) sources (Condon et al. 1998). In addition, centroiding of bright radio sources with extended radio structure can be somewhat uncertain and may make optical identification difficult. Determining the optical centroid also becomes imprecise for large nearby galaxies. In these cases, overlaying the radio contours on an optical image usually makes it clear whether or not the candidate ID is correct.
We found a total of 127 candidate matches in the 30 fields studied, i.e. 1.5% of the 8362 2dF galaxies in the survey area. Of these, 99 had radio-optical offsets of less than 10 arcsec.
We also ran the matching program twice more, offsetting all the radio positions by 3 and 5 arcmin. Any matches produced from this `off-source' catalogue should be chance coincidences, allowing us to estimate the number of matches expected purely by chance. Table 1 lists the results of this test, and shows the distribution of the offset D (difference between radio and optical positions) for matches found in the `on-source' and `off-source' tests.
A further check comes from predicting the expected number of chance coincidences, based on the average surface densities of objects. Since 15 arcsec is 1/240 of the 1degree radius of each 2dF field, and there are about 60 NVSS sources per square degree, the chance that a given GRS target will fall within 15 arcsec of an unrelated radio source is about 60
/2402 (the resolution of the NVSS is such that at most one source can be identified with each optical galaxy). Thus about 27 chance coincidences are expected in a total of 8362 galaxies (see column 5 of Table 1). This calculation ignores the known clustering of galaxies on the sky, although this will only invalidate the result if there is significant clustering on scales comparable to the identification range of 15 arcsec, or if there are substantial differences between the spatial distribution of radio-loud and quiet galaxies. We believe these effects are unlikely to give an error of more than 10%.
The results suggest that candidate matches with an offset of up to 10arcsec are highly likely to be real associations. We therefore use a simple 10arcsec cutoff in radio-optical position difference for the analysis which follows. This gives us 99 radio-detected galaxy matches in the 30 2dF fields. It also means that we have probably omitted about a dozen real identifications with larger offsets, but this is not a problem here since our aim is to make a first qualitative exploration of the faint radio galaxy population.
Next Section: Two kinds of radio
Title/Abstract Page: Radio Sources in the
Previous Section: The optical data
© Copyright Astronomical Society of Australia 1997