  We use thirty-eight high-resolution simulations of galaxy formation between
redshift 10 and 5 to study the impact of a 3 keV warm dark matter (WDM)
candidate on the high-redshift Universe. We focus our attention on the stellar
mass function and the global star formation rate and consider the consequences
for reionization, namely the neutral hydrogen fraction evolution and the
electron scattering optical depth. We find that three different effects
contribute to differentiate warm and cold dark matter (CDM) predictions: WDM
suppresses the number of haloes with mass less than few $10^9$ M$_{\odot}$; at
a fixed halo mass, WDM produces fewer stars than CDM; and finally at halo
masses below $10^9$ M$_{\odot}$, WDM has a larger fraction of dark haloes than
CDM post-reionization. These three effects combine to produce a lower stellar
mass function in WDM for galaxies with stellar masses at and below $\sim 10^7$
M$_{\odot}$. For $z > 7$, the global star formation density is lower by a
factor of two in the WDM scenario, and for a fixed escape fraction, the
fraction of neutral hydrogen is higher by 0.3 at $z \sim 6$. This latter
quantity can be partially reconciled with CDM and observations only by
increasing the escape fraction from 23 per cent to 34 per cent. Overall, our
study shows that galaxy formation simulations at high redshift are a key tool
to differentiate between dark matter candidates given a model for baryonic
physics.
