Detection of 130 “Diffuse” Galactic H<scp>ii</scp>Regions

The paper reports the results of a search for di use 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 rst time ; most require at least one main-sequence O star to provide their ionization. The di use H II regions appear to have essentially the same distribution in the Galaxy as compact H II regions. There is some indication that the di use 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 D1 radio H II region for each 106 of molecular hydrogen. However, H II regions are a factor D2 more conned to the Galac-M _ tic plane than the molecular gas from which they form.