Resolution of the Distance Ambiguity for Galactic H II Regions

We resolve the kinematic distance ambiguity for 266 inner Galaxy H ii regions out of a sample of 291 using existing H i and CO sky surveys. Our sample contains all H ii regions with measured radio recombination line (RRL) emission over the extent of the CO Boston University—Five College Radio Astronomy Observatory Galactic Ring Survey (18 < l < 55 and |b| < 1) and contains ultra compact, compact, and diffuse H ii regions. We use two methods for resolving the distance ambiguity for each H ii region: H i emission/absorption (H i E/A) and H i self-absorption (H i SA). We find that the H i E/A and H i SA methods can resolve the distance ambiguity for 72% and 87% of our sample, respectively. When projected onto the Galactic plane, this large sample appears to reveal aspects of Galactic structure, with spiral arm-like features at Galactocentric radii of 4.5 and 6 kpc, and a lack of H ii regions within 3.5 kpc of the Galactic center. Our H ii regions are approximately in the ratio of 2 to 1 for far verses near distances. The ratio of far to near distances for ultra-compact H ii regions is 2.2 to 1. Compact H ii regions are preferentially at the near distance; their ratio of far to near distances is 1.6 to 1. Diffuse H ii regions are preferentially at the far distance; their ratio of far to near distances is 3.8 to 1. This implies that the distinction between ultra compact and compact H ii regions is due largely to distance, and that the large angular size of diffuse H ii regions is not due solely to proximity to the Sun. Subject headings: H ii regions — ISM: molecules — radio lines: ISM — stars: formation Institute for Astrophysical Research, 725 Commonwealth Ave., Boston University, Boston MA 02215, USA.