Cosmological shock waves

TL;DR

Cosmological shock waves from large-scale structure formation and AGN activity are collisionless, capable of heating gas, amplifying magnetic fields, and accelerating particles, significantly impacting galaxy cluster thermodynamics.

Contribution

This paper reviews the properties and microscopic processes of collisionless cosmological shocks, highlighting their role in gas heating, magnetic field amplification, and particle acceleration.

Findings

Collisionless shocks cause excessive gas compression.

They accelerate energetic particles and produce non-thermal emission.

Shocks modify thermodynamic scaling relations in galaxy clusters.

Abstract

Large-scale structure formation, accretion and merging processes, AGN activity produce cosmological gas shocks. The shocks convert a fraction of the energy of gravitationally accelerated flows to internal energy of the gas. Being the main gas-heating agent, cosmological shocks could amplify magnetic fields and accelerate energetic particles via the multi-fluid plasma relaxation processes. We first discuss the basic properties of standard single-fluid shocks. Cosmological plasma shocks are expected to be collisionless. We then review the plasma processes responsible for the microscopic structure of collisionless shocks. A tiny fraction of the particles crossing the shock is injected into the non-thermal energetic component that could get a substantial part of the ram pressure power dissipated at the shock. The energetic particles penetrate deep into the shock upstream producing an extended shock precursor. Scaling relations for postshock ion temperature and entropy as functions of shock velocity in strong collisionless multi-fluid shocks are discussed. We show that the multi-fluid nature of collisionless shocks results in excessive gas compression, energetic particle acceleration, precursor gas heating, magnetic field amplification and non-thermal emission. Multi-fluid shocks provide a reduced gas entropy production and could also modify the observable thermodynamic scaling relations for clusters of galaxies.