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

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 nonthermal 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.

Gamma rays from reaccelerated cosmic rays in high velocity clouds colliding with the Galactic disk

“…Also in the case of subsolar metallicities cooling times are expected to be longer and hence shocks might radiate less efficiently. The case of particles accelerated in adiabatic shocks in HVCs induced by cloud-disk collisions has been extensively studied in del Valle et al (2018).…”

“…The collision with the disk produces a system of two shocks, propagating through the disk and the cloud (Tenorio-Tagle 1981). Under some conditions, these shocks could be the site of particle acceleration and the accelerated particles might produce nonthermal emission (e.g., Inoue et al 2017;del Valle et al 2018). Also the clouds can act as passive sources for the cosmic rays, producing gamma rays from neutral pion decay as observed in molecular clouds.…”

“…Recently, we modeled the nonthermal emission produced in the collision of high-velocity clouds with the Galactic disk (Müller et al 2017;del Valle et al 2018). We considered the case of an adiabatic shock accelerating protons and electrons.…”

“…The shocks formed within the clouds after the collisions with the Galactic disk have velocities of hundreds of kilometers per second and will be radiative in many cases (e.g., del Valle et al 2018). Cosmic rays from the so called Cosmicray Sea permeate the cloud, and they can be reaccelerated at the shocks independent of their energy.…”

Radiation from the impact of broad-line region clouds onto AGN accretion disks

“…The power injected per impact can be calculated as (del Valle et al 2018). The kinetic energy obtained using the set of parameters of our model is L s = 3.9 × 10 40 erg s −1 .…”