DETECTION OF 130 DIFFUSE GALACTIC H-II REGIONS

The paper reports the results of a search for diffuse Galactic H II re gions using radio recombination lines. Unlike most previous surveys, w hich have been directed toward fairly bright, compact radio continuum sources, we observed unprepossessing objects that might be optically o bscured analogs to the Rosette Nebula. The sources observed here are t ypically a factor of 2 fainter in the continuum than those that have b een detected before. One hundred thirty nebulae have been measured in radio recombination lines for the first time; most require at least on e main-sequence O star to provide their ionization. The diffuse H II r egions appear to have essentially the same distribution in the Galaxy as compact H II regions. There is some indication that the diffuse neb ulae have a low abundance of ionized helium, possibly due to their low excitation. Several newly discovered nebulae are associated with pecu liar kinematic features in the Galactic nucleus. The surface density o f radio H II regions in the inner Galaxy follows the surface density o f CO fairly closely from 4 less than or equal to R less than or equal to kpc. In this area, there is similar to 1 radio H II region for each 10(6) M. of molecular hydrogen. However, H II regions are a factor si milar to 2 more confined to the Galactic plane than the molecular gas from which they form.