CosmoCoffee

One of the main improvements in parameter constraints in year 5 seems to be the significantly better constraint on the optical depth. Reassuringly \tau~0.09 is fully consistent with the previous result, though moves around by ~0.01 in the alternative low-l polarization analysis presented (but not used) in this paper, indicating a non-negligible (but not large) systematic error.

Having constrained the optical depth, you can then try to infer the reionization redshift; they quote z_{re} ~ 11.0 \pm 1.4 assuming reionization is sharp. They also claim to rule out sharp reionization at z=6-7 at ~3 sigma which is an interesting result.

Now my question is this: what happens with helium reionization? If helium singly ionized at the same time as hydrogen, this potentially shifts \tau by ~ 10% (or equivalently z_{re} by ~ 6%). In the reionization literature it seems people often assume that this was the case (due to the relatively closeness of the ionization energies); does anyone know any evidence? More practically, should we all be assuming x_e ~1.08 if we are going to assume sharp reionization?

(the effect of the second reionization of helium at z~3-4 corresponds to only an extra \tau ~ 0.001)

To a good approximation helium does become singly-ionized at the same time and places as hydrogen. This is not just an assumption, but is shown by simulations. See e.g.:

http://adsabs.harvard.edu/abs/2004MNRAS.348..753S

This paper has a different goals, but it does show this point as well (I am
sure there are many other examples). See e.g. Figs. 7 and 8 - the H II and He II ionization fronts are in all cases at very similar positions and their ionization levels are similar, as well. There are, however, some minor differences between the two, which are also dependent on the ionizing source spectrum.