Impact of Cosmic Ray-driven Outflows on Lyα Emission in Cosmological Simulations

Cosmic ray (CR) feedback has been proposed as a powerful mechanism for driving warm gas outflows in galaxies. We use cosmological magnetohydrodynamic simulations to investigate the impact of CR feedback on neutral hydrogen (H i ) in a 10 11 M ⊙ dark matter halo at 2 < z < 4. To this end, we postprocess the simulations with ionizing radiative transfer and perform Monte Carlo Lyman- α (Ly α ) transfer calculations. CR feedback reduces H i column densities around young stars, thereby allowing more Ly α photons to escape and consequently offering a better match to the Ly α luminosities of observed Ly α emitters. Although galaxies with CR-driven outflows have more extended H i in the circumgalactic medium, two Ly α line properties sensitive to optical depth and gas kinematic—the location of the red peak relative to the Ly α line center in velocity space ( v red ) and relative strength of the blue-to-red peaks ( B / R )—cannot distinguish between the CR-driven and non-CR simulations. This is because Ly α photons propagate preferentially along low H i density channels created by the ionizing radiation, thereby limiting the scattering with volume-filling H i . In contrast, the observed low flux ratios between the valley and peak and the surface brightness profiles are better reproduced in the model with CR-driven outflows because the Ly α photons interact more before escaping, rather than being destroyed by dust as is the case in the non-CR simulation. We discuss the potential cause of the paucity of sightlines in simulations that exhibit prominent red peaks and large v red , which may require the presence of more volume-filling H i .