The duration of reionization constrains the ionizing sources
  We investigate how the nature of the galaxies that reionized the Universe
affects the duration of reionization. We contrast two sets of models: one in
which galaxies on the faint side of the luminosity function dominate the
ionizing emissivity, and a second in which the galaxies on the bright side of
the luminosity function dominate. The faint-end of the luminosity function
evolves slowly, therefore the transition from mostly neutral to mostly ionized
state takes a much longer time in the first set of models compared to the
second. Existing observational constraints on the duration of this transition
are relatively weak, but taken at face value prefer the model in which galaxies
on the bright side play a major role. Measurements of the kinetic Sunyaev
Zeldovich effect in the cosmic microwave background from the epoch of
reionization also point in the same direction.
