Galaxy formation and evolution

Abstract:

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 10¹¹M⊙ dark matter halo at 2<z<4. To this end, we post-process the simulations with ionizing radiative transfer and perform Monte Carlo Lyman-α (Lyα) transfer calculations. CR feedback reduces H_I column densities around young stars, thereby allowing more Lyα photons to escape and consequently offering a better match to the Lyα luminosities of observed Lyα emitters. Although galaxies with CR-driven outflows have more extended H_I in the circumgalactic medium, two Lyα line properties sensitive to optical depth and gas kinematics - the location of the red peak 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 Lyα photons propagate preferentially along low HI 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 Lyα 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.