A new catalog of radio compact HII regions in the Milky Way

We use new VLA Galactic plane catalogs at 5 and 1.4 GHz covering the first Galactic quadrant (350degrees less than or equal to l less than or equal to 42degrees, \b\ less than or equal to 0degrees.4) in conjunction with the MSX6C Galactic plane catalog to construct a large sample of ultracompact H II regions. A radio catalog of this region was first published by Becker et al., but we have added new observations and rereduced the data with significantly improved calibration and mosaicking 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 Midcourse Space Experiment colors and a thermal radio spectrum. The scale height of their Galactic latitude distribution is very small (FWHM of 16' or similar to40 pc). These properties suggest that the sample is dominated by young ultracompact H II regions, most of which are previously uncataloged.