Evidence for Cosmic-Ray Escape in the Small Magellanic Cloud Using Fermi Gamma Rays.

Galaxy formation simulations demonstrate that cosmic-ray (CR) feedback may be important in the launching of galactic-scale winds. CR protons dominate the bulk of the CR population, yet most observational constraints of CR feedback come from synchrotron emission of CR electrons. In this paper, we analyze 105 months of Fermi Gamma-ray Space Telescope observations of the Small Magellanic Cloud (SMC), with the aim of exploring CR feedback and transport in an external galaxy. We produce maps of the 2–300 GeV emission and detect statistically significant extended emission along the “Bar” and the “Wing,” where active star formation is occurring. Gamma-ray emission is not detected above 13 GeV, and we set stringent upper limits on the flux above this energy. We find the best fit to the gamma-ray spectrum is a single-component model with a power law of index and an exponential cutoff energy of GeV. We assess the relative contribution of pulsars and CRs to the emission, and we find that pulsars may produce up to 10% of the flux above 100 MeV. Thus, we attribute most of the gamma-ray emission (based on its spectrum and morphology) to CR interactions with the interstellar medium. We show that the gamma-ray emissivity of the SMC is at least five times smaller than that of the Milky Way and that the SMC is far below the “calorimetric limit,” where all CR protons experience pion losses. We interpret these findings as evidence that CRs are escaping the SMC via advection and diffusion. [ABSTRACT FROM AUTHOR]