Extended thermal radio emission from the Galactic plane

Abstract

A six square degree sample of the MOST southern Galactic plane radio survey at l= 312°, b = 0° is presented. In addition to discrete radio sources such as classical H II regions, supernova remnants and extragalactic background sources, the survey has revealed an abundance of low-surface-brightness extended structure, mostly too faint to have been recognized in earlier radio observations. Comparison between our 0.843-GHz data and IRAS observations suggests that these faint structures are likely to be thermal. The structures may be linked to the extended H II envelopes that have been postulated to explain both low-frequency spectral turnovers of Galactic supernova remnants and low-frequency radio recombination lines from apparently blank regions of the Galactic plane. They may also represent an intermediate state between classical H II regions and the warm ionized medium.