The Physics of Galactic Winds Driven by Cosmic Rays II: Isothermal Streaming Solutions

Abstract

We use analytic calculations and time-dependent spherically-symmetric simulations to study the properties of isothermal galactic winds driven by cosmic-rays (CRs) streaming at the Alfvén velocity. The simulations produce time-dependent flows permeated by strong shocks; we identify a new linear instability of sound waves that sources these shocks. The shocks substantially modify the wind dynamics, invalidating previous steady state models: the CR pressure $p_c$ has a staircase-like structure with $d{p}_c/dr\simeq 0$ in most of the volume, and the time-averaged CR energetics are in many cases better approximated by $p_c\propto {\rho }^{1/2}$, rather than the canonical $p_c\propto {\rho }^{2/3}$. Accounting for this change in CR energetics, we analytically derive new expressions for the mass-loss rate, momentum flux, wind speed, and wind kinetic power in galactic winds driven by CR streaming. We show that streaming CRs are ineffective at directly driving cold gas out of galaxies, though CR-driven winds in hotter ISM phases may entrain cool gas. For the same physical conditions, diffusive CR transport (Paper I) yields mass-loss rates that are a few-100 times larger than streaming transport, and asymptotic wind powers that are a factor of $\simeq 4$ larger. We discuss the implications of our results for galactic wind theory and observations; strong shocks driven by CR-streaming-induced instabilities produce gas with a wide range of densities and temperatures, consistent with the multiphase nature of observed winds. We also quantify the applicability of the isothermal gas approximation for modeling streaming CRs and highlight the need for calculations with more realistic thermodynamics.

26 pages (incl. Appendices), MNRAS submitted

References in corpus (12)

• Cosmological Simulations of Intergalactic Medium Enrichment from Galactic Outflows
• Ubiquitous outflows in DEEP2 spectra of star-forming galaxies at z=1.4
• The Athena++ Adaptive Mesh Refinement Framework: Design and Magnetohydrodynamic Solvers
• Feedback Heating by Cosmic Rays in Clusters of Galaxies
• Fermi Large Area Telescope observations of Local Group galaxies: Detection of M31 and search for M33
• Cosmic ray-driven galactic winds: streaming or diffusion?
• Magnetohydrodynamic-Particle-in-Cell Simulations of the Cosmic-Ray Streaming Instability: Linear Growth and Quasi-linear Evolution
• The Physics of Galactic Winds Driven by Cosmic Rays I: Diffusion
• The Launching of Cold Clouds by Galaxy Outflows. IV. Cosmic-Ray-Driven Acceleration
• The impact of astrophysical dust grains on the confinement of cosmic rays
• The Characteristic Momentum of Radiatively Cooling Energy-Driven Galactic Winds
• The Launching of Cosmic Ray Driven Outflows

Cited by in corpus (25)

• The Structure of Multiphase Galactic Winds
• Hot-mode accretion and the physics of thin-disk galaxy formation
• Cosmic ray transport in large-amplitude turbulence with small-scale field reversals
• Standard Self-Confinement and Extrinsic Turbulence Models for Cosmic Ray Transport are Fundamentally Incompatible with Observations
• Cosmic-Ray Transport in Varying Galactic Environments
• CHANG-ES XXVI: Insights into cosmic-ray transport from radio halos in edge-on galaxies
• Plasmoid Instability in the Multiphase Interstellar Medium
• CR Driven Multi-phase Gas Formed via Thermal Instability
• Cosmic ray streaming in the turbulent interstellar medium
• Towards First-principle Characterization of Cosmic-ray Transport Coefficients from Multi-scale Kinetic Simulations
• The Launching of Cosmic Ray Driven Outflows
• The Cosmic Ray Staircase: the Outcome of the Cosmic Ray Acoustic Instability
• Turbulent Reacceleration of Streaming Cosmic Rays
• Deciphering the physical basis of the intermediate-scale instability
• Cosmic-Ray Driven Galactic Winds from the Warm Interstellar Medium
• The high-velocity clouds above the disk of the outer Milky Way: misty precipitating gas in a region roiled by stellar streams
• Observational Constraints on Cool Gas Clouds in M82's Starburst-Driven Outflow
• Cosmic-ray transport in inhomogeneous media
• Lévy Flights and Leaky Boxes: Anomalous Diffusion of Cosmic Rays
• Revisiting the Galactic Winds in M82 II: Development of Multiphase Outflows in Simulations
• AGN cool feedback and analogy with X-ray binaries: from radiation pressure to cosmic ray driven outflows
• Linear Analysis and Simulations of the Cosmic-Ray Streaming Instability: the Importance of Oblique Waves
• A new buoyancy instability in galaxy clusters due to streaming cosmic rays
• Velocity decorrelation functions of high-energy cosmic rays propagating in magnetic fields
• Time-Dependent Cosmic Ray Halos from Bursty Star Formation and Active Galactic Nuclei: Semi-Analytic Formalism and Galaxy Formation Implications