Dark-ages Reionization and Galaxy Formation Simulation XX. The
  Ly$\alpha$ IGM transmission properties and environment of bright galaxies
  during the Epoch of Reionization
  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 (${\rm H}_{160}{\sim}25$ 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
(${\rm H}_{160}{\lesssim}25.5$ 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${<}{\rm H}_{160}{<}27.5$ 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 $2'{\times}2'$ 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.
