by Matthew Hankins
While the Milky Way galaxy is “average” in several regards, the environment in the central ~200 pc is unlike any other part of our galaxy. This region, known as the central molecular zone (CMZ), has high molecular gas densities, high gas and dust temperatures, significant turbulence, and a strong gravitational potential well, which combine to create a very unusual and complex star formation environment that has more commonalities with actively star forming galaxies compared to the Milky Way as a whole (Mills 2017 and references therein). Our relative proximity to the CMZ provides a unique opportunity to study star formation on size scales that are simply inaccessible in other galaxies.
However, in stating these similarities, there is an important difference that should be considered, because the CMZ is relatively inefficient at forming stars. In fact, the observed global star formation rate in this region is deficient by an order of magnitude or more compared to theoretical expectations based on its molecular gas reservoir (Longmore+ 2013). This is a significant conundrum in our understanding of this Galactic Center (GC), and potentially has broad implications for observational star formation tracers that are used to study other galaxies.
In order to study this star formation quandary in further detail, one of SOFIA’s first Legacy Programs targeted infrared bright regions within the GC to conduct a census of recent high-mass star formation.
The survey has produced high quality mosaics of the GC at 25 and 37 μm using the Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST) instrument. The SOFIA/FORCAST observations have an angular resolution of 2.3” and 3.4” for the 25 and 37 μm observations, respectively. The spatial resolution provided by SOFIA/FORCAST — 2.3” or 0.09 pc, assuming a distance of 8 kpc — is key for tracing warm dust associated with young stellar objects which might be confused with nearby molecular cloud material in lower resolution maps. Additionally, the improved spatial resolution allows for morphology studies for a variety of objects in the region, ranging from compact bubbles to arc-like structures which could be indicative of a bow shock.
These SOFIA/FORCAST observations cover many important regions within Sagittarius A, B, and C which were badly saturated in archival Spitzer/MIPS 24 μm observations. Previous studies of these regions at about 20 μm relied on earlier observations from the Midcourse Space Experiment, which are much lower spatial resolution — about 20” at 21.3 μm — compared to the SOFIA/FORCAST GC Legacy mosaics. The recent survey overview paper (Hankins+2020) provides researchers with a useful guide to the observations and survey mosaics in these and other regions of interest within the GC, including the Arched Filaments HII region and the Sickle HII region. Additionally, there are numerous lesser-known sources that show spatially resolved structures that are visible for the first time in the SOFIA/FORCAST mosaics.
The Legacy Program team is in the process of completing the survey catalog and expect it to be released in early 2023. The team is also creating updated survey mosaics which combine later SOFIA/FORCAST observations taken in SOFIA Cycles 8 and 9 with the original survey that was observed in Cycle 7. These additional observations increase the original survey area for sources of interest near Sgr B and C and provide significant added legacy value to the data for future research. Interested researchers can access the SOFIA/FORCAST Galactic Center survey data on the IRSA SOFIA portal .