On the minimum mass of reionization sources

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 circle dot fail at reproducing the Gunn-Peterson and electron scattering optical depths simultaneously, as they contribute too few (many) photons at high (low, z approximate to 6) redshift. Marginally acceptable solutions require haloes with M approximate to 5 x 10(7) M circle dot at z approximate to 10, corresponding to virial temperatures (similar to 10(4) K) for which cooling can be ensured by atomic transitions. However, a much better match to the data is obtained if minihaloes (M similar to 10(6) M circle dot) 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.