Cosmic Rays Can Drive Strong Outflows From Gas-Rich High-Redshift Disk Galaxies

Abstract

We present simulations of the magnetized interstellar medium (ISM) in models of massive star forming (40 M ⊙ yr −1 ) disk galaxies with high gas surface densities (Σ gas ∼ 100 M ⊙ pc −2 ) similar to observed star forming high-redshift disks. We assume that type II supernovae deposit 10 per cent of their energy into the ISM as cosmic rays and neglect the additional deposition of thermal energy or momentum. With a typical Galactic diffusion coefficient for CRs (3 · 10 28 cm 2 s −1 ) we demonstrate that this process alone can trigger the local formation of a strong low density galactic wind maintaining vertically open field lines. Driven by the additional pressure gradient of the relativistic fluid the wind speed can exceed 10 3 km s −1 , much higher than the escape velocity of the galaxy. The global mass loading, i.e. the ratio of the gas mass leaving the galactic disk in a wind to the star formation rate becomes of order unity once the system has settled into an equilibrium. We conclude that relativistic particles accelerated in supernova remnants alone provide a natural and efficient mechanism to trigger winds similar to observed mass-loaded galactic winds in high-redshift galaxies. These winds also help explaining the low efficiencies for the conversion of gas into stars in galaxies as well as the early enrichment of the intergalactic medium with metals. This mechanism can be at least of similar importance than the traditionally considered momentum feedback from massive stars and thermal and kinetic feedback from supernova explosions.

Simulating Gamma-Ray Emission in Star-forming Galaxies

“…This could originate from a selfregulated feedback loop and may even suggest that CRs play an active role in shaping galaxies by altering the dynamics and driving galactic winds through their gradient pressure force, as suggested by theoretical works (Breitschwerdt et al 1991;Everett et al 2008) threedimensional simulations of galaxies (Uhlig et al 2012;Hanasz et al 2013;Booth et al 2013;Salem & Bryan 2014;Pakmor et al 2016b) and the interstellar medium (ISM) (Girichidis et al 2016;Simpson et al 2016). This idea can be scrutinized by studying CR-induced radiative processes with the goal to extract the CR pressure from the non-thermal emission at radio and gamma-ray energies.…”

Galactic Winds Driven by Isotropic and Anisotropic Cosmic-Ray Diffusion in Disk Galaxies

“…Cosmic rays (CRs) in galaxies have also been proposed to drive outflows in a number of theoretical works (Ipavich 1975;Breitschwerdt et al 1991;Zirakashvili et al 1996;Ptuskin et al 1997;Breitschwerdt et al 2002;Everett et al 2008Everett et al , 2010Socrates et al 2008;Samui et al 2010;Dorfi & Breitschwerdt 2012) or by using three-dimensional simulations of the ISM (Hanasz et al 2013;Girichidis et al 2016). Polarized radio observations of edge-on galaxies show poloidal field lines at the disk-halo interface (e.g., Tüllmann et al 2000).…”

Galaxy evolution: modelling the role of non-thermal pressure in the interstellar medium

“…We demonstrate that non-thermal pressure components can be instrumental in solving the depletion time discrepancy in two respects: they reduce the quasi-steady state density and the corresponding star formation rates and cooling times, and they stabilize the gas by adding longer relaxation times in cases where star formation flickers on and off. The regulating effect has been shown previously for cosmic rays (Salem & Bryan 2014;Booth et al 2013;Hanasz et al 2013) and turbulence (Ostriker et al 2001;Braun & Schmidt 2012) but the two were not considered together and in any case were not yet formulated in a way which is applicable to large scale cosmological simulations.…”

“…Booth et al (2013); Salem & Bryan (2014) separately implemented a two fluid approximation for cosmic rays and gas for single-galaxy simulations, and propagated the CR as a diffusive component with constant diffusion coefficient. Hanasz et al (2013) used an MHD code to simulate anisotropic diffusion that preferentially diffuses CRs along magnetic fields. In all three implementations the CRs are found to drive winds in some cases.…”