Resolution of the distance ambiguity for galactic H II regions

Abstract

We resolve the kinematic distance ambiguity for 266 inner Galaxy H II regions out of a sample of 291 using existing H I and 13CO sky surveys. Our sample contains all H II regions with measured radio recombination line emission over the extent of the 13CO Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (18° < l < 55° and |b| < 1) and contains ultra compact (UC), 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 versus near distances. The ratio of far-to-near distances for UC 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 UC and compact H II regions is largely due to distance, and that the large angular size of diffuse H II regions is not solely due to proximity to the Sun. © 2009. The American Astronomical Society. All rights reserved.