[0803.0586] Five-Year Wilkinson Microwave Anisotropy Probe
Posted: March 11 2008
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)
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)