Cosmic Rays and Non-thermal Emission Induced by Accretion of Cool Gas onto the Galactic Disk

On both observational and theoretical grounds, the disk of our Galaxy should be accreting cool gas with temperature $\lesssim {10}^{5}$ K via the halo at a rate ∼$1\,{{M}_{\odot }\mathrm{yr}}^{-1}$. At least some of this accretion is mediated by high-velocity clouds (HVCs), observed to be traveling in the halo with velocities of a few 100 km s(−)(1) and occasionally impacting the disk at such velocities, especially in the outer regions of the Galaxy. We address the possibility of particle acceleration in shocks triggered by such HVC accretion events, and the detectability of consequent non-thermal emission in the radio to gamma-ray bands and high-energy neutrinos. For plausible shock velocities $\sim 300\,\mathrm{km}\,{{\rm{s}}}^{-1}$ and magnetic field strengths $\sim 0.3\mbox{--}10\,\mu {\rm{G}}$, electrons and protons may be accelerated up to ∼1–10 TeV and $\sim 30\mbox{--}{10}^{3}$ TeV, respectively, in sufficiently strong adiabatic shocks during their lifetime of $\sim {10}^{6}\,{\rm{yr}}$. The resultant pion decay and inverse Compton gamma-rays may be the origin of some unidentified Galactic GeV–TeV sources, particularly the “dark” source HESS J1503–582 that is spatially coincident with the anomalous H i structure known as “forbidden-velocity wings.” Correlation of their locations with star-forming regions may be weak, absent, or even opposite. Non-thermal radio and X-ray emission from primary and/or secondary electrons may be detectable with deeper observations. The contribution of HVC accretion to Galactic cosmic rays is subdominant, but could be non-negligible in the outer Galaxy. As the thermal emission induced by HVC accretion is likely difficult to detect, observations of such phenomena may offer a unique perspective on probing gas accretion onto the Milky Way and other galaxies.