Spectral shapes of the Lyman-alpha emission from galaxies: I. blueshifted emission and intrinsic invariance with redshift

TL;DR

This study analyzes the evolution of Lyman-alpha emission profiles from star-forming galaxies across redshifts, revealing a significant decrease in blueshifted emission contribution with increasing redshift, primarily due to intergalactic medium absorption.

Contribution

It provides the first comprehensive analysis of blueshifted Lyman-alpha emission evolution with redshift, attributing changes mainly to IGM absorption effects without invoking additional local opacity sources.

Findings

Blueshifted Lyman-alpha emission decreases from ~30% at z~0 to below 3% at z~6.

IGM absorption explains the reduction in blueshifted emission without extra local opacity.

Correlation between Lyα luminosity ratio and equivalent width is suppressed at high redshift due to IGM effects.

Abstract

We demonstrate the redshift-evolution of the spectral profile of H i Lyman-alpha (Ly{\alpha}) emission from star-forming galaxies. In this first study we pay special attention to the contribution of blueshifted emission. At redshift z = 2.9-6.6, we compile spectra of a sample of 229 Ly{\alpha}-selected galaxies identified with the Multi-Unit Spectroscopic Explorer at the Very Large Telescope, while at low-z (< 0.44) we use a sample of 74 ultraviolet-selected galaxies observed with the Cosmic Origin Spectrograph onboard the Hubble Space Telescope. At low-z, where absorption from the intergalactic medium (IGM) is negligible, we show that the ratio of Ly{\alpha} luminosity bluewards and redwards of line center (L_B/R) increases rapidly with increasing equivalent width (W Ly{\alpha}). This correlation does not, however, emerge at z = 3-4, and we use bootstrap simulations to demonstrate that trends in L B/R should be suppressed by variations in IGM absorption. Our main result is that the observed blueshifted contribution evolves rapidly downwards with increasing redshift: L_B/R~30 % at z~0, but drops to 15 % at z~3, and to below 3 % by z~6. Applying further simulations of the IGM absorption to the unabsorbed COS spectrum, we demonstrate that this decrease in the blue-wing contribution can be entirely attributed to the thickening of intervening Ly{\alpha} absorbing systems, with no need for additional H i opacity from local structure, companion galaxies, or cosmic infall. We discuss our results in light of the numerical radiative transfer simulations, the evolving total Ly{\alpha} and ionizing output of galaxies, and the utility of resolved Ly{\alpha} spectra in the reionization epoch.