  By means of carefully calibrated semi-analytical reionization models, we
estimate the minimum mass of star-forming haloes required to match the current
data. Models which do not include haloes of total mass M < 10^9 M_sun fail at
reproducing the Gunn-Peterson and electron scattering optical depths
simultaneously, as they contribute too few (many) photons at high (low, z
\approx 6) redshift. Marginally acceptable solutions require haloes with M
\approx 5 \times 10^7 M_sun at z \approx 10, corresponding to virial
temperatures (\sim 10^4K) for which cooling can be ensured by atomic
transitions. However, a much better match to the data is obtained if minihaloes
(M \sim 10^6 M_sun) are included in the analysis. We have critically examined
the assumptions made in our model and conclude that reionization in the
large-galaxies-only scenario can remain viable only if metal-free stars and/or
some other exotic sources at z > 6 are included.
