  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. High reliability and low
reliability source lists are available for download with the paper.
