A New Constraint on the Lyα Fraction of UV Very Bright Galaxies at Redshift 7

We study the extent to which very bright (-23.0\lt {M}{UV}\lt -21.75) Lyman-break-selected galaxies at redshifts z≃ 7 display detectable Lyα emission. To explore this issue, we obtained follow-up optical spectroscopy of 9 z≃ 7 galaxies from a parent sample of 24 z≃ 7 galaxy candidates selected from the 1.65 deg2 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 Lyα at z = 7.168 with a rest-frame equivalent width {{EW}}0 = {3.7}-1.1+1.7 \mathringA . The relative weakness of this line, combined with our failure to detect Lyα emission from the other spectroscopic targets, allows us to place a new upper limit on the prevalence of strong Lyα emission at these redshifts. For conservative calculation and to facilitate comparison with previous studies at lower redshifts, we derive a 1σ upper limit on the fraction of UV-bright galaxies at z≃ 7 that display {{EW}}0\gt 50 \mathringA , which we estimate to be \lt 0.23. This result may indicate a weak trend where the fraction of strong Lyα emitters ceases to rise, and possibly falls between z ≃ 6 and z≃ 7. Our results also leave open the possibility that strong Lyα 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 z∼ 7 based on the Lyα fraction in the bright galaxies.