The phase structure of cosmic ray driven outflows in stream fed disc galaxies

Abstract

ABSTRACT Feeding with gas in streams is well established to be an important galaxy growth mechanism. Using an idealized set-up of an isolated galaxy, we study the impact of stream feeding (with 107 M⊙ Myr−1 rate) on the star formation and outflows of disc galaxies with ∼1011 M⊙ baryonic mass. The magnetohydrodynamical simulations are carried out with the piernik code and include star formation, feedback from supernova, and cosmic ray advection and diffusion, on a uniform grid with 195 pc spatial resolution. We find that the introduction of a cold gas stream accreted by the disc enhances galactic star formation. Lower angular momentum streams result in more compact discs, higher star formation rates and stronger outflows. In agreement with previous studies, models including cosmic rays launch stronger outflows travelling much further into the galactic halo. Cosmic ray supported outflows are also cooler than supernova only driven outflows. With cosmic rays, the star formation is suppressed and the thermal pressure is reduced. We find evidence for two distinct outflow phases. The warm outflows have high angular momentum and stay close to the galactic disc, while the hot outflow phase has low angular momentum and escapes from the centre deep into the halo. Cosmic rays can therefore have a strong impact on galaxy evolution by removing low angular momentum, possibly metal enriched gas from the disc and injecting it into the circumgalactic medium.