Observing the tail of reionization: neutral islands in the $z=5.5$ Lyman-$\alpha$ Forest

Understand

Previous studies have noted difficulties in modeling the highest opacities of the $z>5.5$ Ly$\alpha$ forest, epitomized by the extreme Ly$\alpha$ trough observed towards quasar ULAS J0148+0600.

• One possibility is that the most opaque regions at these redshifts contain significant amounts of neutral hydrogen.
• This explanation, which abandons the common assumption that reionization ended before $z=6$, also reconciles evidence from independent observations of a significantly neutral Universe at $z=7.5$.
• Here we explore a model in which the neutral fraction is still $\approx 10\%$ at $z=5.5$.