Can galaxy evolution mimic cosmic reionization?
  Lyman-$\alpha$ (Ly$\alpha$) emitting galaxies are powerful tools to probe the
late stages of cosmic reionization. The observed sudden drop in Ly$\alpha$
fraction at $z>6$ is often interpreted as a sign of reionization, since the
intergalactic medium (IGM) is more neutral and opaque to Ly$\alpha$ photons.
Crucially, this interpretation of the observations is only valid under the
assumption that galaxies themselves experience a minimal evolution at these
epochs. By modelling Ly$\alpha$ radiative transfer effects in and around
galaxies, we examine whether a change in the galactic properties can reproduce
the observed drop in the Ly$\alpha$ fraction. We find that an increase in the
galactic neutral hydrogen content or a reduction in the outflow velocity toward
higher redshift both lead to a lower Ly$\alpha$ escape fraction, and can thus
mimic an increasing neutral fraction of the IGM. We furthermore find that this
change in galactic properties leads to systematically different Ly$\alpha$
spectra which can be used to differentiate the two competing effects. Using the
CANDELSz7 survey measurements which indicate slightly broader lines at $z\sim
6$, we find that the scenario of a mere increase in the galactic column density
towards higher $z$ is highly unlikely. We also show that a decrease in outflow
velocity is not ruled out by existing data but leads to more prominent blue
peaks at $z>6$. Our results caution the use of Ly$\alpha$ observations to
estimate the IGM neutral fraction without accounting for the potential change
in the galactic properties, e.g., by mapping out the evolution of Ly$\alpha$
spectral characteristics.
