Signatures of reionization on Lyα emitters

We use a semi-analytic model of Lyα emitters (LAEs) to constrain the reionization history. By considering two physically motivated scenarios in which reionization ends either early [early reionization model (ERM), zi≈ 7] or late [late reionization model (LRM), zi≈ 6], we fix the global value of the intergalactic medium neutral fraction (e.g. χH i= 3 × 10−4, 0.15 at z= 6.56 for the ERM and LRM, respectively) leaving only the star formation efficiency and the effective escape fraction of Lyα photons as free parameters. The ERM fits the observed LAE luminosity function (LF) at z= 5.7 and 6.56 requiring no redshift evolution or mass dependence of the star formation efficiency, and LAE star formation rates (SFR) of ⁠, contributing ≈8 per cent of the cosmic SFR density at z= 5.7. The LRM requires a physically uncomfortable drop of ≈4.5 times in the SFR of the emitters from z= 6.5 to 5.7. Thus, the data seem to imply that the Universe was already highly ionized at z= 6.56. The mass-dependent Lyα transmissivity is 0.36 ≲Tα≲ 0.51 (ERM) and Tα≲ 0.26 (LRM) at z= 6.56. The LF data at z= 4.5 imply an extra Lyα line damping factor of ≈ 0.25 possibly due to dust; the presence of a (clumpy) dust component with E(B−V) ≲ 0.28 is also required to reproduce the observed large Lyα equivalent widths at the same redshift. Additional useful information can be extracted from the line profile (weighted) skewness, found to be SW= 10–17 Å for the two reionization models, which shows an interesting Lα−χH i anti-correlation, holding under the model assumptions. The shortcomings of the model and strategies to overcome them are discussed.