Impact of Cosmic Ray-driven Outflows on Lyman-$\alpha$ Emission in Cosmological Simulations

TL;DR

Cosmic ray feedback influences Lyman-alpha emission in galaxy simulations by reducing HI density around stars, enabling more photon escape and better matching observed luminosities, though some line properties remain unaffected.

Contribution

This study demonstrates the impact of cosmic ray feedback on Lyman-alpha emission and gas distribution in cosmological simulations, providing insights into galaxy outflows and photon escape mechanisms.

Findings

CR feedback reduces HI column densities around young stars.

CR-driven outflows produce more extended HI in the circumgalactic medium.

Models with CR outflows better reproduce observed Lyman-alpha surface brightness profiles.

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 (HI) in a $10^{11}\,M_\odot$ dark matter halo at $2<z<4$. To this end, we post-process the simulations with ionizing radiative transfer and perform Monte Carlo Lyman-$\alpha$ (Lya) transfer calculations. CR feedback reduces HI column densities around young stars, thereby allowing more Lya photons to escape and consequently offering a better match to the Lya luminosities of observed Lya emitters. Although galaxies with CR-driven outflows have more extended HI in the circumgalactic medium, two Lya line properties sensitive to optical depth and gas kinematics - the location of the red peak in velocity space ($v_\mathrm{red}$) and relative strength of the blue-to-red peaks ($B/R$) - cannot distinguish between the CR-driven and non-CR simulations. This is because Lya photons propagate preferentially along low HI density channels created by the ionizing radiation, thereby limiting the scattering with volume-filling HI. 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 Lya 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 $v_\mathrm{red}$, which may require the presence of more volume-filling HI.