A very extended reionization epoch ?

The recent observations of cross temperature-polarization power spectra of the cosmic microwave background (CMB) made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite are in better agreement with a high value of the Thomson scattering optical depth tau approximate to 0.17. This value is close to tau= 0.3, which is taken as the upper limit in the parameter extraction analysis made by the WMAP team. However, models with tau similar to 0.3 provide a good fit to current CMB data and are not significantly excluded when combined with large-scale structure data. By making use of a self-consistent reionization model, we verify the astrophysical feasibility of models with tau similar to 0.3. It turns out that current data on various observations related to the thermal and ionization history of the intergalactic medium are not able to rule out tau similar to 0.3. The possibility of a very extended reionization epoch can significantly undermine the WMAP constraints on crucial cosmological parameters such as the Hubble constant, the spectral index of primordial fluctuations and the amplitude of dark matter clustering.