MUSTANG-2 Galactic Plane survey at 3mm (MGPS90)

We identify 126 sources that have mm-wavelength counterparts but do not have cm-wavelength counterparts and are therefore candidate HCHII regions; of these, 10 are morphologically compact and are strong candidates for new HCHII regions. Given the limited number of candidates in the extended area in this survey compared to the relatively large numbers seen in protoclusters W51 and W49, it appears that most HCHII regions exist within dense protoclusters.

Comparing the counts of HCHII to ultracompact HII (UCHII) regions, we infer the HCHII region lifetime is 16%-46% that of the UCHII region lifetime. We additionally separated the 3mm emission into dust and free-free emission by comparing with archival 870{mu}m and 20cm data. In the selected pilot fields, most (>~80%) of the 3mm emission comes from plasma, either through free-free or synchrotron emission.