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