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U. Giveon, R. H. Becker (UC, Davis), D. J. Helfand (Columbia U.), R. L. White (STScI)
We utilize new VLA Galactic plane catalogs at 5 and 1.4 GHz covering the first Galactic quadrant (350< l< 42, |b|< 0.4) in conjunction with the MSX6C Galactic plane catalog to construct a large sample of ultra-compact HII regions. A radio catalog of this region was first published by Becker et al. (1994), but we have added new observations and re-reduced the data with significantly improved calibration and mosaicing procedures, resulting in a tripling of the number of 5 GHz sources detected. Comparison of the new 5 GHz catalog and the MSX6C Galactic plane catalog resulted in a sample of 687 matches, out of which we estimate only 15 to be chance coincidences. Most of the matches show red MSX colors and a thermal radio spectrum. The scale height of their Galactic latitude distribution is very small (FWHM of 16' or ~40 pc). These properties suggest that the sample is dominated by young ultra-compact HII regions, most of which are previously uncataloged.
Recently, new sub-arcsecond mid-infrared observations were made on a subset of this sample with Keck/LWS. These observations reveal considerable sub-structure in objects that were unresolved by MSX, and will lead to a more accurate determination of properties such as spectral types and dust temperatures.
This work was supported in part (R.H.B.) under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under contract W-7405-ENG-48, and the National Science Foundation under grant AST 0206309. D.J.H. acknowledges the support of the National Science Foundation under grant AST 02-6-55.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.