CR Driven Multi-phase Gas Formed via Thermal Instability

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.

Dynamical Thermal Instability in Highly Supersonic Outflows

“…Third, based on recent conclusions about the effects of cosmic-ray (CR) physics on TI in the CGM/ICM (e.g., Butsky et al 2020;Kempski & Quataert 2020;Tsung et al 2021;Huang et al 2022), it will be important to determine the circumstances under which CR heating or pressure can alter the outcome of our simulations. According to Wagner et al (2005), the isobaric stability criterion for TI is unchanged, irrespective of the CR diffusivity or pressure.…”