Model independent approaches to reionization in the analysis of upcoming CMB data

On large angular scales, CMB polarization depends mostly on the evolution of the ionization level of the IGM during reionization. In order to avoid biasing parameter estimates, an accurate and model independent approach to reionization is needed when analyzing high precision data, like those expected from the Planck experiment. In this paper, we consider two recently proposed methods of fitting for reionization and we discuss their respective advantages. We test both methods by performing a MonteCarlo Markov Chain analysis of simulated Planck data, assuming different fiducial reionization histories. We take into account both temperature and polarization data up to high multipoles, and we fit for both reionization and non-reionization parameters. We find that while a wrong assumption on reionization may bias tau(e), A(s) and r by 1-3 standard deviations, other parameters, in particular n(s), are not significantly biased. The additional reionization parameters introduced by considering the model independent methods do not affect the accuracy of the estimates of the main cosmological parameters, the biggest degradation being of order similar to 15% for tau(e). Finally, we show that neglecting helium contribution in the analysis increase the bias on tau(e), r and A(s) even when a general fitting approach to reionization is assumed. (C) 2008 Published by Elsevier B.V.