  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 z<6.5 universe. Satisfying both constraints requires
either of the following: (i) H_2 molecules form efficiently at z~20, survive
feedback processes, and allow UV sources in halos with virial temperatures
below Tvir=10^4 K to contribute substantially to reionization, or (ii) the
efficiency epsilon in halos with Tvir>10^4K 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.
