A radio continuum and infrared study of Galactic HII regions

We present observations of the 4.8 and 8.6 GHz continuum emission towards 11 southern H II regions made with the Australian Telescope Compact Array. The observed objects were selected from the Infrared Space Observatory (ISO) spectral catalogue of compact H II regions (Peeters et al. 2002b). The morphologies observed for practically all the sources are consistent with them being ionized by a cluster of stars, rather than by a single star. The linear diameters of the regions range from 0.03 pc to 3 pc, the electron densities from 300 to 2.5x10(4) cm(-3), and the Lyman continuum photon flux from 10(47) to 10(50) s(-1). We confirm the existence of a relation between the density and size of H II regions which can be fit by a power law shallower than that predicted by the classic Stromgren theory. The radio observations provide, in addition, information about the distribution of the ionized gas within the ISO apertures. As a result of the combined radio and infrared study, estimates of the extinction in the infrared and the metal content of the nebular gas were calculated. In this analysis, we also included several (ultra) compact H II regions previously observed by the Very Large Array. Values for extinction in the K-band between similar to0 and 6 mag are found. The elemental abundances of nitrogen, neon, argon and sulphur were found to decrease with Galactocentric distance. Finally, the degree of ionization of the nebulae is confirmed to be correlated with the metal content.