Complex Lyα Profiles in Redshift 6.6 Ultraluminous Lyα Emitters* †

Abstract

We report on a search for ultraluminous Ly α -emitting galaxies (LAEs) at z  = 6.6 using the NB921 filter on the Hyper Suprime-Cam on the Subaru telescope. We searched a 30 deg 2 area around the north ecliptic pole, which we observed in broadband g ′, r ′, i ′, z ′, and y ′ and narrowband NB816 and NB921, for sources with NB921 < 23.5 and z ′-NB921 > 1.3. This corresponds to a selection of log L (Ly α ) > 43.5 erg s −1 . We followed up seven candidate LAEs (out of 13) with the Keck DEIMOS spectrograph and confirmed five z  = 6.6 LAEs, one z  = 6.6 AGN with a broad Ly α line and a strong red continuum, and one low-redshift ([O iii ] 5007) galaxy. The five ultraluminous LAEs have wider line profiles than lower-luminosity LAEs, and one source, NEPLA4, has a complex line profile similar to that of COLA1. In combination with previous results, we show that the line profiles of the z  = 6.6 ultraluminous LAEs are systematically different from those of lower-luminosity LAEs at this redshift. This result suggests that ultraluminous LAEs generate highly ionized regions of the intergalactic medium in their vicinity that allow the full Ly α profile of the galaxy—including any blue wings—to be visible. If this interpretation is correct, then ultraluminous LAEs offer a unique opportunity to determine the properties of the ionized zones around them, which will help in understanding the ionization of the z  ∼ 7 intergalactic medium. A simple calculation gives a very rough estimate of 0.015 for the escape fraction of ionizing photons, but more sophisticated calculations are needed to fully characterize the uncertainties.