Survey with large-scale radio continuum emission A.R. Duncan and R.F. Haynes, PASA, 15 (1), 50
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The Parkes survey
New, high-resolution radio surveys of the southern Milky Way, such as the Molonglo Observatory Synthesis Telescope (MOST) survey (Whiteoak et al. 1989), show the fine structure and detail of HII and star-forming regions well. However, as the recently completed Parkes 2.4-GHz survey has shown, a great deal of thermal structure exists on large angular sizes (of the order of 
and greater). Emission on such large angular scales cannot generally be detected by synthesis instruments.
The Parkes survey is a sensitive, polarimetric, radio continuum survey of the southern Milky Way, that has produced high-quality images with a minimum of instrumental artefacts. This work covers 
of Galactic longitude (
) and latitudes out to 
, although in some regions this has been extended to 
and 
.
At 10.4 arcmin, the angular resolution of the radio images is rather low - certainly much lower than the resolution of most synthesis telescopes. However, this large beamwidth allows impressive brightness sensitivity to be achieved (see also Cram 1997). The rms noise of the Parkes survey corresponds to a brightness sensitivity of approximately 1500 Jy sr
(17 mJy/beam area), which is approaching the confusion limit of the telescope at this wavelength. Indeed, for optically-thin thermal emission (at a temperature of 
K), this brightness sensitivity implies a limiting emission measure of approximately 10 cm
pc. Whilst the limiting emission measure of the H
survey is not yet known, the earlier work of Sivan (1974) quotes a value of 30 cm pc. The new AAO/UKST H survey could be expected to better this somewhat. As a result, the radio and H surveys should have comparable sensitivities to thermal emission.
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