[5.09] The BU-FCRAO Galactic Ring Survey: Breaking the Near-Far Kinematic Distance Ambiguity

M.A. Kolpak, J.M. Jackson, T.M. Bania, D.P. Clemens, R. Simon (Boston University Institute for Astrophysical Research), M.H. Heyer (UMASS Amherst), J.M. Dickey (University of Minnesota)

Kinematic distance determinations in the Inner Galaxy are hampered by the near-far kinematic distance ambiguity. For axisymmetric rotation curves, each velocity corresponds to 2 distances equally spaced about the tangent point. To resolve the ambiguity, we observed the 21cm line of HI in absorption toward 39 radio continuum sources in the galactic plane between l=18 degrees and l=52 degrees with the VLA. Most of these sources are compact HII regions. The absorption features can be used to discriminate between near and far sources. If there is absorption up to the tangent point velocity, the continuum source must lie behind the tangent point at the far kinematic distance. We find that 6 of the sources are located at the near kinematic distance and 33 at the far distance. Using radio recombination line velocities, we assigned distances to each source and created a face-on map. We successfully use these near-far determinations as an independent confirmation of another technique that employs HI self-absorption features to place molecular clumps in the Inner Galaxy at the correct near or far kinematic distances.