H_2_CO and H{alpha} observations of UC HII

We detect the H110{alpha} line in 20 sources, and for each of these we also detect a H_2_CO absorption feature at nearly the same velocity, demonstrating the association between molecular and ionized gas. We determine kinematic distances and resolve the distance ambiguity for all observed HII regions, as well as for 19 intervening molecular clouds.

A plot of the Galactic distribution of these objects traces part of the spiral structure in the first Galactic quadrant. We compare flux densities and velocities as measured with the Arecibo Telescope with interferometric measurements of our sample of ultracompact HII regions. In general, the single-dish fluxes exceed the interferometric values, consistent with an extended component of radio continuum emission.