PPT Slide
Why then is the gas so neutral? If you take all the baryons that could be in this volume and distribute them uniformly throughout the cluster volume, the UV background should ionise them to within an inch of their lives.
I originally thought that this might be an unusually shaded region of the early universe, with no strong UV sources bombarding it. The mean free path, however, of UV photons at this redshift is around 500 Mpc, which implies that the UV background should average out extremely well.
So: the only way to make the gas neutral is to increase its recombination rate.