SOFIA/FORCAST Imaging of the Circumnuclear Ring at the Galactic Center

We present 19.7, 31.5, and 37.1 μm images of the inner 6 pc of the Galactic center of the Milky Way with a spatial resolution of 3.''2-4.''6 taken by the Faint Object Infrared Camera on the Stratospheric Observatory for Infrared Astronomy. The images reveal in detail the "clumpy" structure of the circumnuclear ring (CNR)—the inner edge of the molecular torus orbiting the supermassive black hole at the Galactic center—and the prominent streamers of hot, ionized gas and dust within the CNR that compose the H II region Sgr A West. The CNR exhibits features of a classic H II region: the dust emission at 19.7 μm closely traces the ionized gas emission observed in the radio while the 31.5 and 37.1 μm emission traces the photo-dissociation region beyond the ionized gas. The 19.7/37.1 color temperature map reveals a radial temperature gradient across the CNR with temperatures ranging from 65 to 85 K, consistent with the prevailing paradigm in which the dust is centrally heated by the inner cluster of hot, young stars. We model the 37.1 μm intensity of the CNR as an inclined (θ _i = 67°) ring with a thickness and radius of 0.34 pc and 1.4 pc, respectively, and find that it is consistent with the observed 37.1 μm map of the CNR. The 37.1 μm optical depth map also reveals the clumpy dust distribution of the CNR and implies a total gas mass of ~610 M _⊙. Dense (5-9 × 10⁴ cm^-3) clumps with an FWHM of ~0.15 pc exist along the inner edge of the CNR and shadow the material deeper into the ring. We find that the clumps are unlikely to be long-lived structures since they are not dense enough to be stable against tidal shear from the supermassive black hole and will be sheared out on a timescale of an orbital period (~10⁵ yr).