On the Decreasing Fraction of Strong Lyα Emitters around z ∼ 6-7

Abstract

Abstract The fraction of galaxies with strong Lyα emission has been observed to decrease rapidly with redshift at z ≳ 6, after a gradual increase at z < 6. This has been interpreted as being a trace of the reionization of the intergalactic medium (IGM): the emitted Lyα photons would be scattered by an increasingly neutral IGM at z > 6. We study this effect by modeling the ionization and Lyα radiative transfer in the infall region and the IGM around a Lyα emitting galaxy (LAE), for a spherical halo model with the mean density and radial velocity profiles in the standard ΛCDM cosmological scenario. We find that the expected fast increase of the ionizing background intensity toward the end of the reionization epoch implies a rapid evolution of halo infall regions from being self-shielded against the external ionizing background to being mostly ionized. Whereas self-shielded infall regions can scatter the Lyα photons over a much larger area than the commonly used apertures for observing LAEs, the same infalling gas is no longer optically thick to the Lyα emission line after it is ionized by the external background, making the Lyα emission more compact and brighter within the observed apertures. Based on this simple model, we show that the observed drop in the abundance of LAEs at z > 6 does not imply a rapid increase with redshift of the fraction of the whole IGM volume that is atomic, but is accounted for by a rapid increase of the neutral fraction in the infall regions around galaxy host halos.