Dark-ages reionization and galaxy formation simulation XX. The Ly α IGM transmission properties and environment of bright galaxies during the epoch of reionization

The highly neutral intergalactic medium (IGM) during the epoch of reionization (EoR) is expected to suppress Ly α emission with damping-wing absorption, causing nearly no Ly α detection from star-forming galaxies at z ∼8. However, spectroscopic observations of the four brightest galaxies (H160 ∼25 mag) at these redshifts do reveal prominent Ly α line, suggesting locally ionized IGM. In this paper, we explore the Ly α IGM transmission and environment of bright galaxies during the EoR using the Meraxes semi-analytic model. We find brighter galaxies to be less affected by damping-wing absorption as they are effective at ionizing surrounding neutral hydrogen. Specifically, the brightest sources (H160 25.5 mag) lie in the largest ionized regions in our simulation, and have low attenuation of their Ly α from the IGM (optical depth <1). Fainter galaxies (25.5 mag < H160 < 27.5 mag) have transmission that depends on UV luminosity, leading to a lower incidence of Ly α detection at fainter magnitudes. This luminosity-dependent attenuation explains why Ly α has only been observed in the brightest galaxies at z ∼8. Follow-up observations have revealed counterparts in the vicinity of these confirmed z ∼8 Ly α emitters. The environments of our modelled analogues agree with these observations in the number of nearby galaxies, which is a good indicator of whether Ly α can be detected among fainter galaxies. At the current observational limit, galaxies with ≥2-5 neighbours within 2 arcmin × 2 arcmin are ∼2-3 times more likely to show Ly α emission. JWST will discover an order of magnitude more neighbours, revealing ≲50 galaxies in the largest ionizing bubbles and facilitating direct study of reionization morphology.