A new constraint on the lyα fraction of uv very bright galaxies at redshift 7

International audienceWe study the extent to which very bright (-23.0 \textless M-UV \textless -21.75) Lyman-break-selected galaxies at redshifts z similar or equal to 7 display detectable Ly alpha emission. To explore this issue, we obtained follow-up optical spectroscopy of 9 z similar or equal to 7 galaxies from a parent sample of 24 z similar or equal to 7 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 alpha at z=7.168 with a rest-frame equivalent width EW0 = 3.7(-1.1)(+1.7) angstrom. The relative weakness of this line, combined with our failure to detect Ly alpha emission from the other spectroscopic targets, allows us to place a new upper limit on the prevalence of strong Ly alpha emission at these redshifts. For conservative calculation and to facilitate comparison with previous studies at lower redshifts, we derive a 1 sigma upper limit on the fraction of UV-bright galaxies at z similar or equal to 7 that display EW0 \textgreater 50 angstrom, which we estimate to be \textless 0.23. This result may indicate a weak trend where the fraction of strong Ly alpha emitters ceases to rise, and possibly falls between z similar or equal to 6 and z similar or equal to 7. Our results also leave open the possibility that strong Ly alpha 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 similar or equal to 7 based on the Ly alpha fraction in the bright galaxies