Galactic winds driven by cosmic ray streaming

Abstract

Galactic winds are observed in many spiral galaxies with sizes from dwarfs up\nto the Milky Way, and they sometimes carry a mass in excess of that of newly\nformed stars by up to a factor of ten. Multiple driving processes of such winds\nhave been proposed, including thermal pressure due to supernova-heating, UV\nradiation pressure on dust grains, or cosmic ray (CR) pressure. We here study\nwind formation due to CR physics using a numerical model that accounts for CR\nacceleration by supernovae, CR thermalization, and advective CR transport. In\naddition, we introduce a novel implementation of CR streaming relative to the\nrest frame of the gas. We find that CR streaming drives powerful and sustained\nwinds in galaxies with virial masses M_200 < 10^{11} Msun. In dwarf galaxies\n(M_200 ~ 10^9 Msun) the winds reach a mass loading factor of ~5, expel ~60 per\ncent of the initial baryonic mass contained inside the halo's virial radius and\nsuppress the star formation rate by a factor of ~5. In dwarfs, the winds are\nspherically symmetric while in larger galaxies the outflows transition to\nbi-conical morphologies that are aligned with the disc's angular momentum axis.\nWe show that damping of Alfven waves excited by streaming CRs provides a means\nof heating the outflows to temperatures that scale with the square of the\nescape speed. In larger haloes (M_200 > 10^{11} Msun), CR streaming is able to\ndrive fountain flows that excite turbulence. For halo masses M_200 > 10^{10}\nMsun, we predict an observable level of H-alpha and X-ray emission from the\nheated halo gas. We conclude that CR-driven winds should be crucial in\nsuppressing and regulating the first epoch of galaxy formation, expelling a\nlarge fraction of baryons, and - by extension - aid in shaping the faint end of\nthe galaxy luminosity function. They should then also be responsible for much\nof the metal enrichment of the intergalactic medium.\n