  Compared with the cold dark matter (CDM) model, in the warm dark matter (WDM)
model formation of small scale structure is suppressed. It is often thought
that this would delay the reionization of the intergalactic medium (IGM), as
the star formation rate during the epoch of reionization (EOR) would be
lowered. However, during the later stage of the EOR, a large portion of the
ionizing photons are consumed by recombination inside the minihalos, where the
gas has higher density and recombination rates than the gas in the IGM. The
suppression of small scale structure would therefore reduce the recombination
rate, and could potentially shorten the reionization process. This effect is
investigated here by using the analytical "bubble model" of reionization. We
find that in some cases, though the initiation of the EOR is delayed in the WDM
model, its completion could be even earlier than the CDM case, but the effect
is generally small. We obtain limits on the WDM particles mass for different
reionization redshifts.
