A NEW CONSTRAINT ON THE Lyα FRACTION OF UV VERY BRIGHT GALAXIES AT REDSHIFT 7

We study the extent to which very bright ( ) Lyman-break-selected galaxies at redshifts display detectable Ly α emission. To explore this issue, we obtained follow-up optical spectroscopy of 9 galaxies from a parent sample of 24 galaxy candidates selected from the 1.65 deg 2 COSMOS-UltraVISTA and SXDS-UDS survey fields using the latest near-infrared public survey data, and new ultra-deep Subaru z ′-band imaging (which we also present and describe in this paper). Our spectroscopy yielded only one possible detection of Ly α at z = 7.168 with a rest-frame equivalent width . The relative weakness of this line, combined with our failure to detect Ly α emission from the other spectroscopic targets, allows us to place a new upper limit on the prevalence of strong Ly α emission at these redshifts. For conservative calculation and to facilitate comparison with previous studies at lower redshifts, we derive a 1 σ upper limit on the fraction of UV-bright galaxies at that display , which we estimate to be . This result may indicate a weak trend where the fraction of strong Ly α emitters ceases to rise, and possibly falls between z ≃ 6 and . Our results also leave open the possibility that strong Ly α may still be more prevalent in the brightest galaxies in the reionization era than their fainter counterparts. A larger spectroscopic sample of galaxies is required to derive a more reliable constraint on the neutral hydrogen fraction at based on the Ly α fraction in the bright galaxies.