VLT/FORS2 view at z ∼ 6: Lyman-α emitter fraction and galaxy physical properties at the edge of the epoch of cosmic reionization

The fraction of Lyman-alpha emitters (LAEs) among the galaxy population has been found to increase from z similar to 0 to z similar to 6 and drop dramatically at z > 6. This drop has been interpreted as an effect of an increasingly neutral intergalactic medium (IGM) with increasing redshift, while a Lyman continuum escape fraction evolving with redshift and/or a sudden change of galaxy physical properties can also contribute to the decreasing LAE fraction. We report the result of a large VLT/FORS2 program aiming to confirm spectro-scopically a large galaxy sample at z >= 6 that has been selected in several independent fields through the Lyman break technique. Combining those data with archival data, we create a large and homogeneous sample of z similar to 6 galaxies (N = 127), complete in terms of Ly alpha detection at > 95% for Ly alpha equivalent width EW(Ly alpha) >= 25 angstrom. We use this sample to derive a new measurement of the LAE fraction at z similar to 6 and derive the physical properties of these galaxies through spectral energy distribution (SED) fitting. We find a median LAE fraction at z similar to 6 lower than in previous studies, while our sample exhibits typical properties for z similar to 6 galaxies in terms of UV luminosity and UV beta slope. The comparison of galaxy physical properties between LAEs and non-LAEs is comparable to results at lower redshift: LAEs with the largest EW(Ly alpha) exhibit bluer UV slopes, are slightly less massive and less star-forming. The main difference between LAEs and non-LAEs is that the latter are significantly dustier. Using predictions of our SED fitting code accounting for nebular emission, we find an effective Ly alpha escape fraction f(esc)(eff) (Ly alpha) = 0.23(-0.17)(+0.36) 0 : 17 remarkably consistent with the value derived by comparing UV luminosity function with Ly alpha luminosity function. We conclude that the drop in the LAE fraction from z similar to 6 to z > 6 is less dramatic than previously found and the effect of an increasing IGM neutral fraction is possibly observed at 5 < z < 6. The processes driving the escape of Ly alpha photons at z similar to 6 are similar to those at lower redshifts and based on our derived f(esc)(eff) (Ly alpha), we find that the IGM has a relatively small impact on Ly alpha photon visibility at z similar to 6, with a lower limit for the IGM transmission to Ly alpha photons, T-IGM greater than or similar to 0.20, likely due to the presence of outflows.