REIONIZATION CONSTRAINTS FROM FIVE-YEAR WMAP DATA

We study the constraints on reionization from 5 years of WMAP data, parameterizing the evolution of the average fraction of ionized hydrogen with principal components that provide a complete basis for describing the effects of reionization on large-scale E-mode polarization. Using Markov Chain Monte Carlo methods, we find that the resulting model-independent estimate of the total optical depth is nearly twice as well determined as the estimate from 3 year WMAP data, in agreement with simpler analyses that assume instantaneous reionization. The mean value of the optical depth from principal components is slightly larger than the instantaneous value; we find tau = 0.097 +/- 0.017 using only large-scale polarization, and tau = 0.101 +/- 0.019 when temperature data is included. Scale-invariant n(s) = 1 spectra are allowed by WMAP for exotic ionization histories with large, sudden changes in the ionized fraction at high redshift, but more realistic models still favor a red spectral tilt. Higher moments of the ionization history show less improvement in the 5 year data than the optical depth. By plotting the distribution of polarization power for models from the MCMC analysis, we show that extracting most of the remaining information about the shape of the reionization history from the CMB requires better measurements of E-mode polarization on scales of l similar to 10-20. Conversely, the quadrupole and octopole polarization power is already predicted to better than cosmic variance given any allowed ionization history at so that more z < 30 precise measurements will test the Lambda CDM paradigm.