Improved Constraints on The Neutral Intergalactic Hydrogen Surrounding   Quasars at Redshifts z>6
  We analyze the evolution of HII regions around the seven known SDSS quasars at z>6. The comparison between observed and model radii of the HII regions generated by these quasars individually, suggests that the surrounding intergalactic hydrogen is significantly neutral. When all constraints are combined, the existing quasar sample implies a volume averaged neutral fraction that is larger than 10% at z>6. This limited sample permits a preliminary analysis of the correlations between the quasar parameters, the sizes of their HII regions, and the associated constraints on the neutral hydrogen fraction. We find no evidence in these correlations to contradict the interpretation that the red side of the Gunn-Peterson trough corresponds to the boundary between an HII region and a partially neutral IGM. 
Stuart Wyithe, Abraham Loeb & Chris Carilli