Variations of Observed Lyman-{\alpha} Spectra Shapes due to the Intergalactic Absorption

TL;DR

This study uses cosmological simulations to analyze how intergalactic medium absorption affects the shapes of observed Lyman-alpha spectra across different redshifts, revealing degeneracies and providing tools for better interpretation.

Contribution

It introduces a detailed analysis of IGM effects on Lyα spectra using IllustrisTNG100, highlighting the limitations of average transmission curves and offering a public catalog for improved modeling.

Findings

Distribution of Lyα peaks and asymmetry varies with environment and redshift.

Degeneracy between intrinsic spectra and IGM absorption can be broken using spectral shape distributions.

Up to 30% of spectra may be triple-peaked at z~3 due to porous HI distribution.

Abstract

Lyman-$\alpha$ (Ly$\alpha$) spectra provide insights into the small-scale structure and kinematics of neutral hydrogen (HI) within galaxies as well as the ionization state of the intergalactic medium (IGM). The former defines the intrinsic spectrum of a galaxy, which is modified by the latter. These two effects are degenerate. Using the IllustrisTNG100 simulation, we study the impact of the IGM on Ly$\alpha$ spectral shapes between $z\sim 0$ and $5$. We compute the distribution of the expected Ly$\alpha$ peaks and of the peak asymmetry for different intrinsic spectra, redshifts and large-scale environments. We find that the commonly used averaged transmission curves give an incorrect perception of the observed spectral properties. We show that the distributions of peak counts and asymmetry can lift the degeneracy between the intrinsic spectrum and the IGM absorption. For example, we expect a significant number of triple peaked Ly$\alpha$ spectra (up to 30% at $z\sim 3$) if the galaxies' HI distribution become more porous at higher redshift as predicted by cosmological simulations. We provide a public catalog of transmission curves to be used in future simulations and observations to allow a more realistic IGM treatment.