Surveying the Giant H ii Regions of the Milky Way with SOFIA. II. M17

We present our second set of results from our mid-infrared imaging survey of Milky Way giant H ii regions. We used the FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy (SOFIA) to obtain 20 and 37 μm images of the central ∼10′ × 10′ area of M17. We investigate the small- and large-scale properties of M17 using our data in conjunction with previous multiwavelength observations. The spectral energy distributions of individual compact sources were constructed with Spitzer-IRAC, SOFIA-FORCAST, and Herschel-PACS photometry data and fitted with massive young stellar object (MYSO) models. Seven sources were found to match the criteria for being MYSO candidates, four of which are identified here for the first time, and the stellar mass of the most massive object, UC 1, is determined to be 64 M☉. We resolve the extended mid-infrared emission from the KW object and suggest that the angle of this extended emission is influenced by outflow. It is shown that IRS 5 decreases in brightness as a function of wavelength from the mid- to far-infrared and has several other indicators that point to it being an intermediate-mass Class II object and not an MYSO. We find that the large-scale appearance of emission in M17 at 20 μm is significantly affected by contamination from the [S iii] emission line from the ionized gas of the giant H ii region. Finally, a number of potential evolutionary tracers yield a consistent picture suggesting that the southern bar of M17 is likely younger than the northern bar.