The angular momentum structure of cosmic ray driven galactic outflows triggered by stream accretion

TL;DR

This study uses magneto-hydrodynamic simulations to explore how cosmic ray-driven outflows, influenced by stream accretion, affect galaxy evolution, particularly in removing low angular momentum gas and shaping outflow structures.

Contribution

It introduces a detailed simulation analysis of cosmic ray-driven outflows in the context of stream accretion, highlighting their role in galaxy evolution and angular momentum redistribution.

Findings

Low angular momentum streams lead to higher star formation and outflow rates.

Outflows have a dual structure with fountain flows at low velocities and central, zero angular momentum gas at high velocities.

Cosmic ray-driven outflows effectively remove low angular momentum gas and transport enriched material into galactic haloes.

Abstract

We investigate the impact of gas accretion in streams on the evolution of disc galaxies, using magneto-hydrodynamic simulations including advection and anisotropic diffusion of cosmic rays 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 idealised setups. We find that accretion streams trigger the formation of galactic rings and enhanced star formation. The star formation rates and consequently the cosmic ray driven outflow rates are higher for low angular momentum accretion streams, which also result in more compact, lower angular momentum discs. The cosmic ray generated outflows show a characteristic structure. At low outflow velocities (< 50 km/s) the angular momentum distribution is similar to the disk and the gas is in a fountain flow. Gas at high outflow velocities (> 200 km/s), 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 cosmic ray driven outflows are of order unity and higher, we conclude that this process is important for the removal of low angular momentum gas from evolving disk galaxies and the transport of, potentially metal enriched, material from galactic centres far into the galactic haloes.