Can galaxies' gas halos reveal when the universe reionized?

Simulations show that gas halos around galaxies reionize later than the space between them, creating an "inside-out" pattern during the universe's reionization epoch. The thickness of neutral hydrogen clouds observed in front of distant galaxies depends mainly on halo mass, making them potential tracers of reionization progress if we know the galaxy's mass. This opens a new window into mapping how and when the early universe transformed from opaque to transparent.