Abstract
We investigate the properties of the galaxies that reionized the Universe and
the history of cosmic reionization using the "Evolution and Assembly of
GaLaxies and their environments" (EAGLE) cosmological hydrodynamical
simulations. We obtain the evolution of the escape fraction of ionizing photons
in galaxies assuming that galactic winds create channels through which
20~percent of photons escape when the local surface density of star formation
is greater than $0.1$ M$_ødot$ yr$^-1$ kpc$^-2$. Such threshold behaviour
for the generation of winds is observed, and the rare local objects which have
such high star formation surface densities exhibit high escape fractions. In
our model the luminosity-weighted mean escape fraction increases with redshift
as $f_esc=0.045~((1+z)/4)^1.1$ at $z>3$, and the galaxy number
weighted mean as $f_esc \rangle=2.2\times10^-3~((1+z)/4)^4$,
and becomes constant $\approx0.2$ at redshift $z>10$. The escape fraction
evolves as an increasingly large fraction of stars forms above the critical
surface density of star formation at earlier times. This evolution of the
escape fraction, combined with that of the star formation rate density from
\eagle, reproduces the inferred evolution of the filling factor of ionized
regions during the reionization epoch ($6<z<8$), the evolution of the
post-reionization ($0z<6$) hydrogen photoionization rate, and the optical
depth due to Thomson scattering of the cosmic microwave background photons
measured by the Planck satellite.
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