CR Driven Multi-phase Gas Formed via Thermal Instability

TL;DR

This study uses two-dimensional CR-magnetohydrodynamic simulations to explore how cosmic rays influence the formation, morphology, and dynamics of multi-phase gas in the circum-galactic medium, highlighting their role in cloud acceleration and suppression of cold gas growth.

Contribution

It provides new insights into the effects of cosmic rays on multi-phase gas dynamics, including their influence on cloud morphology, cold gas growth suppression, and velocity achievement in the CGM.

Findings

CRs produce lower density contrast and filamentary morphology in gas clouds.

CRs suppress cold gas growth depending on magnetic field strength.

CRs can accelerate cold clouds to observed velocities without rapid destruction.

Abstract

Cosmic rays (CRs) are an important energy source in the circum-galactic medium (CGM) that impact the multi-phase gas structure and dynamics. We perform two-dimensional CR-magnetohydrodynamic simulations to investigate the role of CRs in accelerating multi-phase gas formed via thermal instability. We compare outflows driven by CRs to those driven by a hot wind with equivalent momentum. We find that CRs driven outflow produces lower density contrast between cold and hot gas due to non-thermal pressure support, and yields a more filamentary cloud morphology. While entrainment in a hot wind can lead to cold gas increasing due to efficient cooling, CRs tend to suppress cold gas growth. The mechanism of this suppression depends on magnetic field strength, with CRs either reducing cooling or shredding the clouds by differential acceleration. Despite the suppression of cold gas growth, CRs are able to launch the cold clouds to observed velocities without rapid destruction. The dynamical interaction between CRs ad multi-phase gas is also sensitive to the magnetic field strength. In relatively strong fields, the CRs are more important for direct momentum input to cold gas. In relatively weak fields, the CRs impact gas primarily by heating, which modifies gas pressure.