We examine large-scale fluctuations in the He ii Ly \alpha forest transmission during and after He ii reionization .
We use a simple Monte Carlo model to distribute quasars throughout a large volume and compute the resulting radiation field along one-dimensional skewers .
In agreement with previous studies , we find that the rarity of these sources induces order unity fluctuations in the mean optical depth after reionization , even when averaged over large segments ( \sim 10 – 100 \mbox { Mpc } across ) .
We compare our models to existing data along five He ii Ly \alpha forest lines of sight spanning z \sim 2 – 3.2 .
The large cosmic variance contained in our model plausibly explains many of the observed fluctuations at z \lesssim 2.5 .
But our models can not accommodate the large fluctuations toward high optical depths on \sim 30 \mbox { Mpc } scales observed at z \sim 2.7 – 2.9 , and the measured optical depths ( \tau _ { eff } \gtrsim 4 ) at z > 2.9 are difficult to explain with a smoothly-evolving mean radiation field .
In order to better understand this data , we construct a toy model of He ii reionization , in which we assume that regions with the smallest radiation fields in a post-reionization Universe ( or farthest from strong ionizing sources ) are completely dark during reionization .
The observed fluctuations fit much more comfortably into this model , and we therefore argue that , according to present data , He ii reionization does not complete until z \lesssim 2.9 .