Arecibo Observations of Formaldehyde and Radio Recombination Lines toward Ultracompact H ii Regions

We report observations of the H110α radio recombination line and H2CO (110-111) toward 21 ultracompact H II regions with the Arecibo 305 m radio telescope. We detect the H110α line in 20 sources, and for each of these we also detect a H2CO 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 H II 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 H II regions. In general, the single-dish fluxes exceed the interferometric values, consistent with an extended component of radio continuum emission.