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

TL;DR

The paper models the ionization and Ly$ extalpha$ radiative transfer around galaxies to explain the observed decrease in strong Ly$ extalpha$ emitters at high redshift, attributing it to local infall region ionization rather than global IGM neutrality.

Contribution

It introduces a spherical halo model to connect the evolution of infall regions with the observed Ly$ extalpha$ emitter fraction at high redshift, challenging previous interpretations.

Findings

Infall regions transition from self-shielded to ionized, affecting Ly$ extalpha$ visibility.

The drop in LAE fraction at z>6 is due to local infall region ionization, not global IGM neutrality.

Ly$ extalpha$ emission becomes more compact and brighter as infall regions ionize.

Abstract

The fraction of galaxies with strong Ly$\alpha$ 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$\alpha$ photons would be scattered by an increasingly neutral IGM at $z>6$. We study this effect by modeling the ionization and Ly$\alpha$ radiative transfer in the infall region and the IGM around a Ly$\alpha$ emitting galaxy (LAE), for a spherical halo model with the mean density and radial velocity profiles in the standard $\Lambda$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$\alpha$ 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$\alpha$ emission line after it is ionized by the external background, making the Ly$\alpha$ 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.