Model-independent evidence in favour of an end to reionization by z ≈ 6

We present new upper limits on the volume-weighted neutral hydrogen fraction, xH I , at z 5-6 derived from spectroscopy of bright quasars. The fraction of the Ly and Ly forests that is 'dark' (with zero flux) provides the only model-independent upper limit on xH 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 xH I derived from dark pixel analysis to xH I 0.06+0.05(1 ) at z = 5.9 and xH I 0.04+0.05(1 ) 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.