Abstract

We analyze a sample of z-dropout galaxies in the CANDELS GOODS South and UDS fields that have been targeted by a dedicated spectroscopic campaign aimed at detecting their Ly alpha line. Deep IRAC observations at 3.6 and 4.5 mu m are used to determine the strength of optical emission lines affecting these bands at z similar to 6.5-6.9 in order to (1) investigate possible physical differences between Ly alpha emitting and non-emitting sources; (2) constrain the escape fraction of ionizing photons; and. (3) provide an estimate of the specific star formation rate at high redshifts. We find evidence of strong [O III]+H beta emission in the average (stacked) SEDs of galaxies both with and without Lya emission. The blue IRAC [3.6]-[4.5] color of the stack with detected Ly alpha line can be converted into a rest-frame equivalent width EW([O III]+ H beta) = 1500(-440) (+530) angstrom assuming a flat intrinsic stellar continuum. This strong optical line emission enables a first estimate of f(esc) less than or similar to 20% on the escape fraction of ionizing photons from Ly alpha detected objects. The objects with no Ly alpha line show less extreme EW([O III]+ H beta) = 520(-150)(+170) angstrom suggesting different physical conditions of the H II regions with respect to Ly alpha-emitting ones, or a larger f(esc). The latter case is consistent with a combined evolution of f(esc) and the. neutral hydrogen fraction as an. explanation of the lack of bright Lya emission at z > 6. A lower limit on the specific star formation rate, SSFR > 9.1 Gyr(-1) for M-star = 2 x 10(9) M-circle dot galaxies at these redshifts can be derived from the spectroscopically confirmed sample.