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

摘要

The fraction of Lyman- α emitters (LAEs) among the galaxy population has been found to increase from z ~ 0 to z ~ 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 spectroscopically 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 ~ 6 galaxies ( N = 127 ), complete in terms of Ly α detection at > 95% for Ly α equivalent width EW (Ly α ) ≥ 25 ? . We use this sample to derive a new measurement of the LAE fraction at z ~ 6 and derive the physical properties of these galaxies through spectral energy distribution (SED) fitting. We find a median LAE fraction at z ~ 6 lower than in previous studies, while our sample exhibits typical properties for z ~ 6 galaxies in terms of UV luminosity and UV β 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 α ) 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 α escape fraction f _(esc)~(eff)(Ly α ) = 0.23_(-0.17)~(+0.36) remarkably consistent with the value derived by comparing UV luminosity function with Ly α luminosity function. We conclude that the drop in the LAE fraction from z ~ 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 α photons at z ~ 6 are similar to those at lower redshifts and based on our derived f _(esc)~(eff)(Ly α ) , we find that the IGM has a relatively small impact on Ly α photon visibility at z ~ 6 , with a lower limit for the IGM transmission to Ly α photons, T _(IGM)? 0.20 , likely due to the presence of outflows.