Beacons into the Cosmic Dark Ages: Boosted transmission of Ly$\alpha$
  from UV bright galaxies at $z \gtrsim 7$
  Recent detections of Lyman alpha (Ly$\alpha$) emission from $z>7.5$ galaxies
were somewhat unexpected given a dearth of previous non-detections in this era
when the intergalactic medium (IGM) is still highly neutral. But these
detections were from UV bright galaxies, which preferentially live in
overdensities which reionize early, and have significantly Doppler-shifted
Ly$\alpha$ line profiles emerging from their interstellar media (ISM), making
them less affected by the global IGM state. Using a combination of reionization
simulations and empirical ISM models we show, as a result of these two effects,
UV bright galaxies in overdensities have $>2\times$ higher transmission through
the $z\sim7$ IGM than typical field galaxies, and this boosted transmission is
enhanced as the neutral fraction increases. The boosted transmission is not
sufficient to explain the observed high Ly$\alpha$ fraction of $M_\mathrm{UV}
\lesssim -22$ galaxies (Stark et al. 2017), suggesting Ly$\alpha$ emitted by
these galaxies must be stronger than expected due to enhanced production and/or
selection effects. Despite the bias of UV bright galaxies to reside in
overdensities we show Ly$\alpha$ observations of such galaxies can accurately
measure the global neutral hydrogen fraction, particularly when Ly$\alpha$ from
UV faint galaxies is extinguished, making them ideal candidates for
spectroscopic follow-up into the cosmic Dark Ages.
