Reionization and galaxy inference from the high-redshift Lyα forest

The transmission of Lyman alpha (Ly alpha) in the spectra of distant quasars depends on the density, temperature, and ionization state of the intergalactic medium. Therefore, high-redshift (z > 5) Ly alpha forests could be invaluable in studying the late stages of the epoch of reionization (EoR), as well as properties of the sources that drive it. Indeed, high-quality quasar spectra have now firmly established the existence of large-scale opacity fluctuations at z > 5, whose physical origins are still debated. Here, we introduce a Bayesian framework capable of constraining the EoR and galaxy properties by forward-modelling the high-z Ly alpha forest. Using priors from galaxy and cosmic microwave background observations, we demonstrate that the final overlap stages of the EoR (when >95 percent of the volume was ionized) should occur at z < 5.6, in order to reproduce the large-scale opacity fluctuations seen in forest spectra. However, it is the combination of patchy reionization and the inhomogeneous ultraviolet background that produces the longest Gunn-Peterson troughs. Ly alpha forest observations tighten existing constraints on the characteristic ionizing escape fraction of galaxies, with the combined observations suggesting f(esc) approximate to 7(-3)(+4) percent, and disfavouring a strong evolution with the galaxy's halo (or stellar) mass.