Starlight from galaxies plays a pivotal role throughout the process of cosmic reionisation .
We present the statistics of dwarf galaxy properties at z > 7 in haloes with masses up to 10 ^ { 9 } { M _ { \odot } } , using a cosmological radiation hydrodynamics simulation that follows their buildup starting with their Population III progenitors .
We find that metal-enriched star formation is not restricted to atomic cooling ( { T _ { vir } } \geq 10 ^ { 4 } \textrm { K } ) haloes , but can occur in haloes down to masses \sim 10 ^ { 6 } { M _ { \odot } } , especially in neutral regions .
Even though these smallest galaxies only host up to 10 ^ { 4 } { M _ { \odot } } of stars , they provide nearly 30 per cent of the ionising photon budget .
We find that the galaxy luminosity function flattens above M _ { UV } \sim - 12 with a number density that is unchanged at z \la 10 .
The fraction of ionising radiation escaping into the intergalactic medium is inversely dependent on halo mass , decreasing from 50 to 5 per cent in the mass range \log M / { M _ { \odot } } = 7.0 - 8.5 .
Using our galaxy statistics in a semi-analytic reionisation model , we find a Thomson scattering optical depth consistent with the latest Planck results , while still being consistent with the UV emissivity constraints provided by Ly \alpha forest observations at z = 4 - 6 .