Galactic winds driven by cosmic ray streaming: Galactic winds driven by cosmic ray streaming

Abstract

Galactic winds are observed in many spiral galaxies with sizes from dwarfs up to the Milky Way, and they sometimes carry a mass in excess of that of newly formed stars by up to a factor of 10. Multiple driving processes of such winds have been proposed, including thermal pressure due to supernova heating, ultraviolet radiation pressure on dust grains or cosmic ray (CR) pressure. We here study wind formation due to CR physics using a numerical model that accounts for CR acceleration by supernovae, CR thermalization by Coulomb and hadronic interactions, and advective CR transport. In addition, we introduce a novel implementation of CR streaming relative to the rest frame of the gas. Streaming CRs excite Alfvén waves on which they scatter, thereby limiting the CRs’ effective bulk velocity. We find that CR streaming drives powerful and sustained winds in galaxies with virial masses . In dwarf galaxies ( ) the winds reach a mass loading factor of ∼5, expel ∼60 per cent of the initial baryonic mass contained inside the halo’s virial radius and suppress the star formation rate by a factor of ∼5. In dwarfs, the winds are spherically symmetric while in larger galaxies the outflows transition to biconical morphologies that are aligned with the disc’s angular momentum axis. We show that damping of Alfvén waves excited by streaming CRs provides a means of heating the outflows to temperatures that scale with the square of the escape speed, . In larger haloes ( ), CR streaming is able to drive fountain flows that excite turbulence, providing another means of heating the halo gas. For halo masses , we predict an observable level of Hα and X-ray emission from the heated halo gas. We conclude that CR-driven winds should be crucial in suppressing and regulating the first epoch of galaxy formation, expelling a large fraction of baryons, and – by extension – aid in shaping the faint end of the galaxy luminosity function. They should then also be responsible for much of the metal enrichment of the intergalactic medium.