Galactic Structure As Probed By The GBT HRDS

Details Galactic Structure As Probed By The GBT HRDS Galactic Structure As Probed By The GBT HRDS

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21713202a&link_type=abstract

American Astronomical Society, AAS Meeting #217, #132.02; Bulletin of the American Astronomical Society, Vol. 43, 2011

Astronomy and Astrophysics

Astronomy

Scientific paper

Studies of external galaxies show that HII regions are excellent tracers of galactic structure. They are zero-age objects compared to the age of their host galaxy; they map the current locations of massive star formation, which trace spiral arms. Until recently the number of Milky Way HII regions with known distances was relatively small. The new GBT HII Region Discovery Survey (HRDS) has doubled the number of known first Galactic quadrant HII regions, giving us a much larger sample for Galactic structure studies. Here we report on kinematic distance determinations for HRDS sources located in the Galactic longitude range of 18° to 65°. These first quadrant sources suffer from the well known kinematic distance ambiguity (KDA): each measured recombination line velocity maps to two possible kinematic distances, a "near" and a "far" distance. We resolved the KDA for nearly 200 HRDS sources by making HI emission/absorption experiments using data from the VLA Galactic Plane Survey. The HRDS targets are nearly all at the far kinematic distance, which indicates that their small angular size is due to a large distance from the Sun rather than a small physical size. When combined with kinematic distances for the sample of previously known HII regions, the face-on map of first quadrant star forming regions shows strong evidence for Galactic structure. The map shows two spiral arc features that have mean Galactocentric radii of 4.5 and 6.0 kpc. Structures at these locations are generally associated with the Scutum and Sagittarius arms, respectively. The HRDS has also detected a significant number of very distant sources beyond the Solar orbit that show excellent agreement with what has been called the Outer Arm, and agree well with the warp and flaring seen in the CO and HI layers.

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