# Cosmic Ray Acceleration of Cool Clouds in the Circumgalactic Medium

## TL;DR

This study explores how cosmic rays can accelerate cool clouds in the circumgalactic medium through magnetohydrodynamic simulations, revealing that CR pressure buildup can propel clouds to high velocities, with implications for understanding high velocity clouds.

## Contribution

First 2D MHD simulations of cosmic ray bottlenecks in the CGM, demonstrating cloud acceleration and the effects of radiative cooling on cloud stability.

## Key findings

- CR pressure buildup can accelerate cool clouds to hundreds of km/s.
- Radiative cooling helps keep clouds intact against CR wave heating.
- Cloud acceleration scales with CR flux, roughly linear at low fluxes.

## Abstract

We investigate a mechanism for accelerating cool (10$^4$ K) clouds in the circumgalactic medium (CGM) with cosmic rays (CRs), possibly explaining some characteristics of observed high velocity clouds (HVCs). Enforcing CRs to stream down their pressure gradient into a region of slow streaming speed results in significant buildup of CR pressure which can accelerate the CGM. We present the results of the first two-dimensional magnetohydrodynamic (MHD) simulations of such `CR bottlenecks,' expanding on simpler simulations in 1D from \cite{wiener17a}. Although much more investigation is required, we find two main results. First, radiative cooling in the interfaces of these clouds is sufficient to keep the cloud intact to CR wave heating. Second, cloud acceleration depends almost linearly with the injected CR flux at low values (comparable to that expected from a Milky Way-like star formation rate), but scales sublinearly at higher CR fluxes in 1D simulations. 2D simulations show hints of sublinear dependence at high CR fluxes but are consistent with pure linear dependence up to the CR fluxes tested. It is therefore plausible to accelerate cool clouds in the CGM to speeds of hundreds of km s$^{-1}$.