Surveying the Giant HII Regions of the Milky Way with SOFIA: I. W51A

Abstract: We discuss the first results from our mid-infrared imaging survey of Milky Way Giant HII regions with our detailed analysis of W51A, which is one of the largest GHII regions in our Galaxy. We used the FORCAST instrument on SOFIA to obtain 20 and 37$\mu$m images of the central $10' \times 20'$ area, which encompasses both of the G49.5-0.4 and G49.4-0.3 sub-regions. Based on these new data, and in conjunction with previous multi-wavelength observations, we conjecture on the physical nature of several individual sources and sub-components within W51A. We find that extinction seems to play an important role in the observed structures we see in the near- to mid-infrared, both globally and locally. We used the SOFIA photometry combined with Spitzer-IRAC and Herschel-PACS photometry data to construct spectral energy distributions (SEDs) of sub-components and point sources detected in the SOFIA images. We fit those SEDs with young stellar object models, and found 41 sources that are likely to be massive young stellar objects, many of which are identified as such in this work for the first time. Close to half of the massive young stellar objects do not have detectable radio continuum emission at cm wavelengths, implying a very young state of formation. We derived luminosity-to-mass ratio and virial parameters of the extended radio sub-regions of W51A to estimate their relative ages.