The Reionization History at High Redshifts I: Physical Models and New Constraints from CMB Polarization

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Abstract
The recent discovery of a high optical depth tau to Thomson scattering from the WMAP data implies that significant reionization took place at redshifts z~15. This discovery has important implications for the sources of reionization, and allows, for the first time, constraints to be placed on physical reionization scenarios out to redshift z~20. Using a new suite of semi-analytic reionization models, we show that the high value of tau requires a surprisingly high efficiency epsilon of the first generation of UV sources for injecting ionizing photons into the intergalactic medium. We find that no simple reionization model can be consistent with the combination of the WMAP result with data from the z10^4K decreased by a factor of ~ 30 between (z~20) and (z~6). We discuss the relevant physical issues to produce either scenario, and argue that both options are viable, and allowed by current data. In detailed models of the reionization history, we find that the evolution of the ionized fractions in the two scenarios have distinctive features that Planck can distinguish at 3 sigma significance. At the high WMAP value for tau, Planck will also be able to provide tight statistical constraints on reionization model parameters, and elucidate much of the physics at the end of the Dark Ages. The sources responsible for the high optical depth discovered by WMAP should be directly detectable out to z~15 by the James Webb Space Telescope.

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