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Abstract

We investigate the impact of gas accretion in streams on the evolution of disc galaxies, using magnetohydrodynamic simulations including advection and anisotropic diffusion of cosmic rays (CRs) generated by supernovae as the only source of feedback. Stream accretion has been suggested as an important galaxy growth mechanism in cosmological simulations and we vary their orientation and angular momentum in idealized setups. We find that accretion streams trigger the formation of galactic rings and enhanced star formation. The star formation rates and consequently the CR-driven outflow rates are higher for low angular momentum accretion streams, which also result in more compact, lower angular momentum discs. The CR generated outflows show a characteristic structure. At low outflow velocities (<50 km s⁻¹), the angular momentum distribution is similar to the disc and the gas is in a fountain flow. Gas at high outflow velocities (>200 km s⁻¹), penetrating deep into the halo, has close to zero angular momentum, and originates from the centre of the galaxies. As the mass loading factors of the CR-driven outflows are of the order of unity and higher, we conclude that this process is important for the removal of low angular momentum gas from evolving disc galaxies and the transport of, potentially metal enriched, material from galactic centres far into the galactic haloes