On the decreasing fraction of Strong Ly$α$ Emitters around $z$ $\sim$ $6$-$7$

Abstract

The fraction of galaxies with strong Ly$α$ emission has been observed to decrease rapidly with redshift at $z \ge 6$, after a gradual increase at $z< 6$. This has been interpreted as a hint of the reionization of the intergalactic medium (IGM): the emitted Ly$α$ photons would be scattered by an increasingly neutral IGM at $z>6$. We study this effect by modeling the ionization and Ly$α$ radiative transfer in the infall region and the IGM around a Ly$α$ emitting galaxy (LAE), for a spherical halo model with the mean density and radial velocity profiles in the standard $Λ$CDM 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 Ly$α$ 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 Ly$α$ emission line after it is ionized by the external background, making the Ly$α$ 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.

10 pages, 5 figures, submitted to ApJ