2022ApJ...935...52S

Recent evidence suggests that high-redshift Lyα emitting galaxies (LAEs) with $\mathrm{log}L(\mathrm{Ly}\alpha )\gt 43.5\,\mathrm{erg}\ {{\rm{s}}}^{-1}$, 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 Lyα 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 $\mathrm{log}L(\mathrm{Ly}\alpha )\gt 43\,\mathrm{erg}\ {{\rm{s}}}^{-1}$, all of which have follow-up spectroscopy from Keck/DEIMOS. Combining with archival lower-luminosity data, we find a significant narrowing of the Lyα lines in LAEs at $\mathrm{log}L(\mathrm{Ly}\alpha )\lt 43.25\,\mathrm{erg}\ {{\rm{s}}}^{-1}$-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 Lyα 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.