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 Alfv´en 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 massesM200 1011M . In dwarf galaxies (M200 ∼ 109M ) 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 Alfv´en waves excited by streaming CRs provides a means of heating the outflows to temperatures that scale with the square of the escape speed, kT ∝ υ2 esc. In larger haloes (M200 1011M ), CR streaming is able to drive fountain flows that excite turbulence, providing another means of heating the halo gas. For halo masses M200 1010M , 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

Variable interstellar radiation fields in simulated dwarf galaxies: supernovae versus photoelectric heating

“…However, this is much more challenging owing to the complicated behavior of CR propagation which depends on the configuration of the magnetic fields and some uncertainties in its propagation mechanisms (Uhlig et al 2012;Hanasz et al 2013;Girichidis et al 2016a;Simpson et al 2016). A fully self-consistent treatment of the CR ionization heating therefore requires substantial further investigation.…”

Dark-ages reionization and galaxy formation simulation – III. Modelling galaxy formation and the epoch of reionization

“…We have also imposed the additional constraint that the mass loading factor, ε mass , cannot exceed an upper limit which we nominally set to be ε max mass =10 based on reasonable expectations for typical high-z galaxies (e.g. Martin 1999; Uhlig et al 2012). For a standard energy-driven wind, β mass =2 (Murray et al 2005).…”

Supernovae driven galactic outflows

“…The damping of Alfvén waves excited by streaming cosmic rays can lead to the heating of the ISM and subsequently to outflows. Recently, a detailed numerical simulation that includes the physics of cosmic ray streaming, it was found that this process can drive galactic winds from galaxies with halo masses 10 11 M (Uhlig et al 2012). This process also suppresses the star formation rate, particularly in dwarf galaxies (10 9 M ), in which ∼ 60% of the baryonic matter is expelled, with a mass loss rate reaching a factor of 5 more than the star formation rate.…”

“…Also, there are a few examples of galactic outflows which occupy a region of the parameter space of SFR and galactic rotation speed, as in , that are neither explained by thermal or radiation pressure adequately, with very low SFR ( 0.1 M yr −1 ) and low galactic rotation speed ( 60 km s −1 ). Interestingly, the examples simulated by Uhlig et al (2012) fall in this range of parameters, and it would be curious to study such cases with more detailed simulations in the future.…”