2021ApJ...908...36H

We demonstrate the redshift evolution of the spectral profile of H I Lyα 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 Lyα-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 on board the Hubble Space Telescope. At low z, where absorption from the intergalactic medium (IGM) is negligible, we show that the ratio of Lyα luminosity blueward and redward of line center (L_B/R) increases rapidly with increasing equivalent width (W_Lyα). 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 downward with increasing redshift: L_B/R ≃ 30% at z ≃ 0, but dropping 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 Lyα 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 Lyα and ionizing output of galaxies, and the utility of resolved Lyα spectra in the reionization epoch.