A self-consistent 3D model of fluctuations in the helium-ionizing
  background
  Large variations in the effective optical depth of the He II Ly$\alpha$
forest have been observed at $z\gtrsim2.7$, but the physical nature of these
variations is uncertain: either the Universe is still undergoing the process of
He II reionization, or the Universe is highly ionized but the He II-ionizing
background fluctuates significantly on large scales. In an effort to build upon
our understanding of the latter scenario, we present a novel model for the
evolution of ionizing background fluctuations. Previous models have assumed the
mean free path of ionizing photons to be spatially uniform, ignoring the
dependence of that scale on the local ionization state of the intergalactic
medium (IGM). This assumption is reasonable when the mean free path is large
compared to the average distance between the primary sources of He II-ionizing
photons, $\gtrsim L_\star$ quasars. However, when this is no longer the case,
the background fluctuations become more severe, and an accurate description of
the average propagation of ionizing photons through the IGM requires
additionally accounting for the fluctuations in opacity. We demonstrate the
importance of this effect by constructing 3D semi-analytic models of the helium
ionizing background from $z=2.5$-$3.5$ that explicitly include a spatially
varying mean free path of ionizing photons. The resulting distribution of
effective optical depths at large scales in the He II Ly$\alpha$ forest is very
similar to the latest observations with HST/COS at $2.5 \lesssim z \lesssim
3.5$.
