The Evolution of Lyα Emitter Line Widths from z = 5.7 to z = 6.6

Abstract

Recent evidence suggests that high-redshift Ly α emitting galaxies (LAEs) with log L ( Ly α ) > 43.5 erg s − 1 , referred to as ultraluminous LAEs (ULLAEs), may show less evolution than lower-luminosity LAEs in the redshift range z = 5.7–6.6. Here we explore the redshift evolution of the velocity widths of the Ly α emission lines in LAEs over this redshift interval. We use new wide-field, narrowband observations from Subaru/Hyper Suprime-Cam to provide a sample of 24 z = 6.6 and 12 z = 5.7 LAEs with log L ( Ly α ) > 43 erg s − 1 , all of which have follow-up spectroscopy from Keck/DEIMOS. Combining with archival lower-luminosity data, we find a significant narrowing of the Ly α lines in LAEs at log L ( Ly α ) < 43.25 erg s − 1 —somewhat lower than the usual ULLAE definition—at z = 6.6 relative to those at z = 5.7, but we do not see this in higher-luminosity LAEs. As we move to higher redshifts, the increasing neutrality of the intergalactic medium should increase the scattering of the Ly α lines, making them narrower. The absence of this effect in the higher-luminosity LAEs suggests they may lie in more highly ionized regions, self-shielding from the scattering effects of the intergalactic medium.