Dark-ages Reionization and Galaxy Formation Simulation XX. The Ly$\alpha$ IGM transmission properties and environment of bright galaxies during the Epoch of Reionization

Abstract

The highly neutral inter-galactic medium (IGM) during the Epoch of Reionization (EoR) is expected to suppress Ly$\alpha$ emission with damping-wing absorption, causing nearly no Ly$\alpha$ detection from star-forming galaxies at $z\sim 8$. However, spectroscopic observations of the 4 brightest galaxies (${\mathrm{H}}_{160}\sim 25$ mag) at these redshifts do reveal prominent Ly$\alpha$ line, suggesting locally ionised IGM. In this paper, we explore the Ly$\alpha$ 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 (${\mathrm{H}}_{160}\lesssim 25.5$ mag) lie in the largest ionized regions in our simulation, and have low attenuation of their Ly$\alpha$ from the IGM (optical depth $<1$). Fainter galaxies (25.5 mag$<{\mathrm{H}}_{160}<27.5$ mag) have transmission that depends on UV luminosity, leading to a lower incidence of Ly$\alpha$ detection at fainter magnitudes. This luminosity-dependent attenuation explains why Ly$\alpha$ has only been observed in the brightest galaxies at $z\sim 8$. Follow-up observations have revealed counterparts in the vicinity of these confirmed $z\sim 8$ Ly$\alpha$ 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$\alpha$ can be detected among fainter galaxies. At the current observational limit, galaxies with $\ge 2$--5 neighbours within $2^{\prime }\times 2^{\prime }$ are $\sim 2$--3 times more likely to show Ly$\alpha$ emission. JWST will discover an order of magnitude more neighbours, revealing $\gtrsim 50$ galaxies in the largest ionizing bubbles and facilitating direct study of reionization morphology.

10 pages, 3 figures, submitted to MNRAS, comments are welcome

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