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

Abstract

On both observational and theoretical grounds, the disk of our Galaxy is believed to be accreting cool gas with temperature ~< 10^5 K via the halo at a rate of order 1 M_sun/yr. High velocity clouds (HVCs), observed to be traveling in the halo at velocities of a few 100 km/s relative to the disk, are likely manifestations of this process, some of which can directly impact 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 from the radio to gamma-ray bands as well as high-energy neutrinos. For plausible values of shock velocity ~ 300 km/s and magnetic field strength ~ 0.3 - 10 mu G, electrons and protons may be accelerated up to ~ 1-10 TeV and ~ 30 - 10^3 TeV, respectively, in sufficiently strong adiabatic shocks during their lifetime of ~ 10^6 yr. The resultant pion decay and inverse Compton gamma-rays may be the origin of some unidentified GeV-TeV sources in the Galactic Plane, particularly the "dark" source HESS J1503-582 that is spatially coincident with anomalous HI structure known as a "forbidden-velocity wing". Correlation of their locations with star-forming regions may be weak, absent, or even opposite. Non-thermal radio and X-ray emission due to primary and/or secondary electrons may be detectable in some cases with deeper observations. The contribution of HVC accretion to Galactic cosmic rays is expected to be subdominant, ~< 5 % in total, but could be non-negligible in the outer Galaxy. As the thermal emission induced by HVC accretion could be difficult to detect, observations of such phenomena may offer a unique perspective on probing gas accretion processes onto the Milky Way and other galaxies.