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 (LB/R) increases rapidly with increasing equivalent width (WLyα). This correlation does not, however, emerge at z = 3-4, and we use bootstrap simulations to demonstrate that trends in LB/R should be suppressed by variations in IGM absorption. Our main result is that the observed blueshifted contribution evolves rapidly downward with increasing redshift: LB/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.
Bibliographical noteFunding Information:
We are grateful to the core members of the MUSE-WIDE team - Lutz Wisotzki, Tanya Urrutia, and E. Christian Herenz - for making the extracted spectra available and enabling this project. We also thank Lutz Wisotzki and Tanya Urrutia for useful discussions on the MUSE data, repeatability, and pointing out a bug in the original manuscript. We also thank Peter Laursen for providing useful insights into the CGM and evolution of the IGM, and Anne Verhamme for helping us understand the systematics involved in systemic redshift estimation. M.H. acknowledges the support of the Swedish Research Council, Vetenskapsr?det, and the Swedish National Space Agency (SNSA) and is a Fellow of the Knut and Alice Wallenberg Foundation. M.G. was supported by NASA through the NASA Hubble Fellowship grant HSTHF2- 51409 and acknowledges support from HST grants HSTGO- 15643.017-A and HST-AR-15039.003-A and XSEDE grant TG-AST180036.
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