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

Abstract

We present new upper limits on the volume-weighted neutral hydrogen fraction, xHI, 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 xHI, 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 xHI derived from dark pixel analysis to xHI ≤ 0.06+0.05(1σ) at z = 5.9 and xHI ≤ 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.