The imprint of cosmic ray driven outflow on Lyman-alpha spectra

TL;DR

Cosmic ray driven outflows significantly influence Lyman-alpha spectra, producing features consistent with observations, unlike thermal-only models which fail to match observed spectral profiles.

Contribution

This study demonstrates that cosmic rays create smoother, colder outflows that shape Lyman-alpha spectra in ways that align with real observations, a novel insight into ISM feedback mechanisms.

Findings

Cosmic ray feedback produces spectra with enhanced red peaks.

Spectra with cosmic rays show strong absorption at line center.

Thermal-only models do not match observed spectra.

Abstract

Recent magneto-hydrodynamic simulations of the star-forming interstellar medium (ISM) with parsec scale resolution indicate that relativistic cosmic rays support the launching of galactic outflows on scales of a few kpc. If these fundamental constituents of the ISM are injected at the sites of supernova (SN) explosions, the outflows are smoother, colder, and denser than the highly structured, hot-phase driven outflows forming, e.g., by thermal SN energy injection alone. In this Letter we present computations of resonant Lyman-$\alpha$ (Ly$\alpha$) radiation transfer through snapshots of a suite of stratified disk simulations from the SILCC project. For a range of thermal, radiative, and kinetic feedback models only simulations including non-thermal cosmic rays produce Ly$\alpha$ spectra with enhanced red peaks and strong absorption at line center -- similar to observed systems. The absence of cosmic ray feedback leads to spectra incompatible with observations. We attribute this to the smoother neutral gas distribution of cosmic ray supported outflows within a few kpc from the disk midplane.