Updating reionization scenarios after recent data

The recent release of data on (i) high-redshift source counts from the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Hubble Ultra Deep Field (HUDF) and (ii) electron scattering optical depth from the Wilkinson Microwave Anisotropy Probe (WMAP) requires a re-examination of reionization scenarios. Using an improved self-consistent reionization model, based on Choudhury & Ferrara, we determine the range of reionization histories which can match a wide variety of data sets simultaneously. The updated constraints from our analysis imply that hydrogen reionization starts at around z≈ 15, driven by the metal-free Population III (PopIII) stars (with normal Salpeter-like initial mass functions), and it is 90 per cent complete by z≈ 10. The photoionizing power of the PopIII stars fades for z≲ 10 because of concomitant action radiative and chemical feedbacks, which causes the reionization process to stretch considerably and to end only by z≈ 6. The combination of different data sets still favours a non-zero contribution from metal-free stars, with star-forming efficiencies larger than 2 per cent.