High-energy radiation from the impact of high-velocity clouds on the galactic disk

High-velocity clouds (HVCs) are HI clouds with velocities of more than 100 km s⁻¹. These clouds do not partake of the differential Galactic rotation; a significant fraction of them are falling down towards the Galactic disk. The typical mass of these clouds is ∼ 10⁴ M_⊙, so in a collision with the disk energies of the order of ∼ 10⁵¹ erg can be released into the interstellar medium. Such collisions should produce strong shocks propagating through both the cloud and the disk. Under adequate conditions, these shocks can accelerate particles up to relativistic energies by Fermi mechanism. In this work, we study the hydrodynamical inter-actions and the relevant radiative processes (thermal and non-thermal) associated with HVC-disk collisions. We find that a shock propagating through a typical cloud should give rise to significant non-thermal radio emission, whereas the protons accelerated there diffuse and might emit elsewhere. A shock propagating through the disk, on the other hand, produces extended gamma-ray emission and injects protons with energies from 10 GeV to ∼1 TeV. Taking into account the injected mass rate of HI in our Galaxy by cloud bombardement, we found that ∼ 10 % of the Galactic cosmic ray power could be generated by these cloud-disk collisional events.