Optical Line Emission from z ∼ 6.8 Sources with Deep Constraints on Lyα Visibility

Abstract

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 Lyα line. Deep IRAC observations at 3.6 and 4.5 μm are used to determine the strength of optical emission lines affecting these bands at z ∼ 6.5–6.9 in order to (1) investigate possible physical differences between Lyα 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]+Hβ emission in the average (stacked) SEDs of galaxies both with and without Lyα emission. The blue IRAC [3.6]–[4.5] color of the stack with detected Lyα line can be converted into a rest-frame equivalent width EW([O iii]+Hβ) = Å assuming a flat intrinsic stellar continuum. This strong optical line emission enables a first estimate of on the escape fraction of ionizing photons from Lyα detected objects. The objects with no Lyα line show less extreme EW([O iii]+Hβ) = Å, suggesting different physical conditions of the H ii regions with respect to Lyα-emitting ones, or a larger . The latter case is consistent with a combined evolution of and the neutral hydrogen fraction as an explanation of the lack of bright Lyα emission at z > 6. A lower limit on the specific star formation rate, SSFR > 9.1 Gyr−1 for galaxies at these redshifts can be derived from the spectroscopically confirmed sample.