Arecibo with the 305 m Telescope and lie within the SIRTF / GLIMPSE survey region

Fifty-four ultracompact (UC) H ii regions in the GLIMPSE survey region (jbj < 1 and 30 < l < 70 ) were observed in H2CO and H110 using the 305 m Arecibo telescope. By analyzing H2CO absorption against the UC H ii region continuum emission, we resolve the distance ambiguity toward 44 sources. This determination is critical to measure global physical properties of UCH ii regions (e.g., luminosity, size, mass) and properties of the Galaxy (e.g., spiral structure, abundance gradients). We find that the distribution of UC H ii regions in this survey is consistent with a ‘‘ local spur,’’ the Perseus, Sagittarius, and Scutum arms as delineated by Taylor & Cordes. However, departures from model velocities produce distance uncertainties only slightly smaller than the proposed arm separations. Subject headings:Galaxy: disk — radio lines: general — stars: formation On-line material:machine-readable tables