Spectral Shapes of the Lyα Emission from Galaxies. I. Blueshifted Emission and Intrinsic Invariance with Redshift*

Abstract

We demonstrate the redshift evolution of the spectral profile of H i Ly α 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 α -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 Ly α luminosity blueward and redward of line center ( L B/R ) increases rapidly with increasing equivalent width ( W Ly α ). 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 Ly α 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 α and ionizing output of galaxies, and the utility of resolved Ly α spectra in the reionization epoch.