2011ApJ...736..149G

We present a pilot survey of 21 lines of sight toward ultracompact H II (UCH II) regions and three toward continuum-free lines of sight in the formaldehyde (H₂CO) 1₁₀-1₁₁(6 cm) and 2₁₁-2₁₂(2 cm) transitions, using the H₂ CO centimeter lines as a molecular gas densitometer. Using Arecibo and Green Bank beam-matched observations, we measure the density of 51 detected H₂ CO line pairs and present upper limits on density for an additional 24 detected 1₁₀-1₁₁lines. We analyze the systematic uncertainties in the H₂ CO densitometer, achieving H₂density measurements with accuracies ∼0.1-0.3 dex. The densities measured are not correlated with distance, implying that it is possible to make accurate density measurements throughout the galaxy without a distance bias. We confirm that UCH II regions are associated with, and possibly embedded in, gas at densities n(H₂) ≳ 10⁵/cm3. The densities measured in line-of-sight molecular clouds suggest that they consist of low volume filling factor (f ∼ 10^–2) gas at high (n(H₂) > 10⁴/cm3) density, which is inconsistent with purely supersonic turbulence and requires high-density clumping greater than typically observed in gravoturbulent simulations. We observe complex line morphologies that indicate density variations with velocity around UCH II regions, and we classify a subset of the UCH II molecular envelopes as collapsing or expanding. We compare these measurements to Bolocam Galactic Plane Survey 1.1 mm observations, and note that most UCH II regions have 1.1 mm emission consisting of significant (5%-70%) free-free emission and are therefore not necessarily dominated by optically thin dust emission as is often assumed when computing clump masses. A comparison of our data with the Mangum et al. starburst sample shows that the area filling factor of dense (n(H₂) ∼ 10⁵/cm3) molecular gas in typical starburst galaxies is ≲ 0.01, but in extreme starburst galaxies like Arp 220, is ∼0.1, suggesting that Arp 220 is physically similar to an oversized UCH II region.