Detection of 130 “Diffuse” Galactic H ii Regions

Abstract

The paper reports the results of a search for diffuse Galactic H II regions using radio recombination lines. Unlike most previous surveys, which have been directed toward fairly bright, compact radio continuum sources, we observed unprepossessing objects that might be optically obscured analogs to the Rosette Nebula. The sources observed here are typically a factor of 2 fainter in the continuum than those that have been detected before. One hundred thirty nebulae have been measured in radio recombination lines for the first time; most require at least one main-sequence 0 star to provide their ionization. The diffuse H II regions appear to have essentially the same distribution in the Galaxy as compact H II regions. There is some indication that the diffuse nebulae have a low abundance of ionized helium, possibly due to their low excitation. Several newly discovered nebulae are associated with peculiar kinematic features in the Galactic nucleus. The surface density of radio H II regions in the inner Galaxy follows the surface density of CO fairly closely from 4 ≤ R ≤ 8 kpc. In this area, there is ˜1 radio H II region for each 106 Msun of molecular hydrogen. However, H II regions are a factor ˜2 more confined to the Galactic plane than the molecular gas from which they form.