2004ApJS..154..553S

We report simultaneous H110α and H₂CO line observations with the NRAO Green Bank Telescope toward 72 H II regions in the Spitzer Space Telescope GLIMPSE survey area (|l|=10°-65° and|b|≤1°). We used the H110α line to establish the velocity of the H II regions and H₂CO absorption lines to distinguish between near and far distances. Accurate distances are crucial for the determination of physical properties of massive star formation regions. We resolved the distance ambiguity of 44 H II regions. We detected multiple H II regions along 18 lines of sight located in the longitude interval 12°-31°, primarily a result of the relatively large telescope beam width. We could not resolve distance ambiguities for lines of sight with multiple H II regions, since we could not determine which H₂CO lines were being absorbed against which H II region.

We examined the projected location of H II regions whose distance ambiguities have been resolved (in this work and other similar studies) in the Galactic plane and in a longitude-velocity diagram for a recognizable spiral arm pattern. Although the highest density of points in the position-position plot approximately follows the spiral arms proposed by Taylor & Cordes, the dispersion is still about as large as the separation between their proposed arms. The longitude-velocity plot shows an increase in the density of sources at the points where the spiral arm loci proposed by Taylor & Cordes are approaching the locus of tangent point velocities and a lower density between the arm loci. However, it is not possible to trace spiral arms over significant segments of Galactic longitude in the longitude-velocity plot. We conclude that a very large number of H II regions in combination with more sophisticated Galactic rotation models will be required to obtain a more continuous spiral pattern from kinematic studies of H II regions than from fully sampled surveys of H I or CO.