Extended Galactic emission at l=312°: a comparison of mid-infrared and radio continuum (843 MHz) images

We report on the comparison of images of a region of the Galactic plane (centred on l =312°) as seen by the Midcourse Space Experiment (MSX) at 8.3 μm and by the Molonglo Observatory Synthesis Telescope (MOST) at 843 MHz in the radio continuum. We note that the survey from each telescope is without peer and occupies a niche in panoramic coverage with high spatial resolution. Using independent classification of sources in the selected region, a detailed comparison of the two surveys was made. The aim of the project was to seek global characteristics for different types of source, with a view to establishing predictive criteria for identification and hence emission mechanisms. Several strong trends were found. There is a complete absence in this field of any detected MSX counterparts to non-thermal radio sources. Almost every H  ii region in the radio image has its MSX counterpart, in the form of a polycyclic aromatic hydrocarbon halo in the neutral zone surrounding the ionized gas. Both surveys show large-scale 'braided' filamentary structures, extending over 1°, which appear to be produced by thermal processes. These filaments may be structures in the warm ionized phase of the interstellar medium or extended haloes around H  ii regions. The comparisons in this paper were made using both preliminary MSX 8.3-μm results with 46-arcsec resolution and final MSX images with the intrinsic 20-arcsec resolution of the instruments.