A New Distance Technique for Galactic Plane Objects

Abstract

We present a new method based on H I column densities for determination of distances within the disk of the Galaxy. The technique is useful for all Galactic plane objects, including H II regions and supernova remnants (SNRs), provided a line-of-sight velocity can be assigned to the object. Our method uses 21 cm spectral-line data to find the atomic hydrogen column density to an object, and beyond it to the Galactic edge. A model of the smooth large-scale Galactic distribution of H I material seen in emission (which principally traces the smooth structure of the Galaxy) is constructed. Our model accounts for scale-height flaring with increasing Galactocentric radius and includes the Galactic warp, which is prominent in the first and second quadrants of the Galaxy. The model's ability to trace the observed distribution of H I is demonstrated on lines of sight toward SNR DA 530 (l = 933, b = 7°) and H II region Sh 121 (l = 902, b = 17). We then apply the new technique to 29 Sharpless H II regions with known photometric distances across the second quadrant. We measure line-of-sight velocities for the H II regions from associated 12CO emission, using 1' resolution 12CO (J = 1-0) data from the Canadian Galactic Plane Survey. Our distance method yields distances to these objects that are consistent with their photometric distances and which are markedly smaller than the kinematic distances found from a flat Galactic rotation curve.