CII fine-structure line observations of the Sagittarius C region in the Galaxy's Central Molecular Zone

Sagittarius C (Sgr C) is a massive but relatively quiescent complex at the western edge of the Galaxy's Central Molecular Zone (CMZ). While the Sgr B2 region on the opposite side of the CMZ has been extensively studied, Sgr C has received comparatively little attention. We aim to characterize the kinematics and physical state of the atomic gas in the Sgr C region using spatially and velocity-resolved emission from the CII line. This line traces the multi-phase gas that consists of the warm ionized medium, the warm and cold diffuse atomic medium, and the warm dense molecular gas and provides a complement to molecular line, dust, infrared, and radio observations. We present a fully sampled 74 times 47 pc map of the CII 158 μm fine-structure line toward Sgr C, observed with the upGREAT receiver onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) airborne observatory. The data feature a spatial resolution of 0.55 pc and a spectral resolution of 1 ̨ms. We analyzed these observations in conjunction with ancillary maps of the J=2-1 transition of CO and its isotopologues observed with the PI230 receiver at the Atacama Pathfinder Experiment (APEX) telescope. CII emission is widespread throughout the region and shows a continuous structure that extends from Sgr A to Sgr C with a complex morphology. The majority of the emission arises from gas at negative radial velocities, consistent with the direction of Galactic rotation. The most prominent feature is the giant Sgr C HII region, where CII reveals an expanding, ring-like shell structure interpreted as a photodissociation region (PDR). We modeled the shell's kinematics and derived an expansion velocity of ∼ 23 ̨ms and a dynamical age of ∼ 0.13 Myr. Our analysis suggests that stellar winds from the known massive stars are likely insufficient to power the observed expansion; this points toward alternative driving mechanisms such as a buried supernova. We find a striking spatial association between this shell and a nonthermal radio filament, which provides evidence that the shell's expansion into the surrounding medium has triggered high-mass star formation at its edge.