High-energy radiation from collisions of high velocity clouds and the Galactic disk

TL;DR

This study investigates how collisions between high-velocity clouds and the Galactic disk produce shocks that accelerate particles, resulting in detectable non-thermal radiation and contributing to galactic cosmic rays.

Contribution

It provides a detailed analysis of particle acceleration mechanisms and non-thermal emissions resulting from HVC-disk collisions, highlighting their potential significance in galactic cosmic-ray populations.

Findings

Shocks from cloud-disk collisions can accelerate particles via diffusive shock acceleration.

Significant synchrotron radio and soft gamma-ray emissions are produced in shocked clouds.

Accelerated protons may locally exceed cosmic-ray background levels.

Abstract

High-velocity clouds (HVCs) are interstellar clouds of atomic hydrogen that do not partake of the Galactic rotation and have velocities of a several hundred kilometers per second. A considerable number of these clouds are falling down towards the Galactic disk. HVCs form large and massive complexes, so their collisions with the disk must release a great amount of energy into the interstellar medium. The cloud-disk interaction produces two shocks, one propagates through the cloud and the other through the disk; the properties of these shocks depend mainly on the cloud velocity and the disk-cloud density ratio. In this work we study the conditions necessary for these shocks to accelerate particles by diffusive shock acceleration and the produced non-thermal radiation. We analyze particle acceleration in both the cloud and disk shocks. Solving a time-dependent 2-D transport equation for both relativistic electrons and protons we obtain particle distributions and non-thermal spectral energy distributions. In a shocked cloud significant synchrotron radio emission is produced along with soft gamma rays. In the case of acceleration in the shocked disk, the non-thermal radiation is stronger; the gamma rays, of leptonic origin, might be detectable with current instruments. A large number of protons are injected into the Galactic interstellar medium, and locally exceed the cosmic-ray background. We conclude that under adequate conditions the contribution from HVC-disk collisions to the galactic population of relativistic particles and the associated extended non-thermal radiation might be important.