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 < MUV < -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 3.7+ _1.7-^{0 1.1} 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 1 upper limit on the fraction of UV-bright galaxies at z ≃ 7 that display EW0 > 50 A, which we estimate to be <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.