Dark-ages reionization and galaxy formation simulation XX. The Ly α IGM transmission properties and environment of bright galaxies during the epoch of reionization. Issue 3 (24th December 2021) - Qin, Yuxiang
ABSTRACT: 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 ( H 160 ∼ 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 ( H 160 ≲ 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 < H 160 < 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.