2015MNRAS.447..499M

We present new upper limits on the volume-weighted neutral hydrogen fraction, {bar}x _H i, at z ∼ 5-6 derived from spectroscopy of bright quasars. The fraction of the Lyα and Lyβ forests that is `dark' (with zero flux) provides the only model-independent upper limit on {bar}x _H i, requiring no assumptions about the physical conditions in the intergalactic medium or the quasar's unabsorbed UV continuum. In this work, we update our previous results using a larger sample (22 objects) of medium-depth ( ∼ few hours) spectra of high-redshift quasars obtained with the Magellan, MMT, and VLT. This significantly improves the upper bound on {bar}x _H i derived from dark pixel analysis to {bar}x _H i≤0.06+0.05 (1σ) at z = 5.9 and {bar}x _H i≤0.04+0.05 (1σ) at z = 5.6. These results provide robust constraints for theoretical models of reionization, and provide the strongest available evidence that reionization has completed (or is very nearly complete) by z ~ 6.