Updated dark pixel fraction constraints on reionization's end from the Lyman-series forests of XQR-30

TL;DR

This study updates constraints on the end of reionization by measuring dark pixel fractions in quasar spectra at redshifts 4.85 to 6.25, providing nearly model-independent upper limits on the neutral hydrogen fraction and confirming reionization completion by z>6.

Contribution

It introduces new measurements of dark pixel fractions in the Lyman-series forests from the enlarged XQR-30 quasar sample, refining constraints on the neutral hydrogen fraction during reionization.

Findings

Reionization is mostly complete by z>6.

Upper limits on neutral hydrogen fraction at various redshifts.

Consistent results between model-independent and model-dependent methods.

Abstract

The fraction of "dark pixels" in the Ly$\alpha$ and other Lyman-series forests at $z\sim 5-6$ provides a powerful constraint on the end of the reionization process. Any spectral region showing transmission must be highly ionized, while dark regions could be ionized or neutral, thus the dark pixel fraction provides a (nearly) model independent upper limit to the volume-filling fraction of the neutral intergalactic medium, modulo choices in binning scale and dark pixel definition. Here we provide updated measurements of the 3.3 comoving Mpc dark pixel fraction at $z=4.85-6.25$ in the Ly$\alpha$, Ly$\beta$, and Ly$\gamma$ forests of 34 deep $5.8\lesssim z\lesssim 6.6$ quasar spectra from the (enlarged) XQR-30 sample. Using the negative pixel method to measure the dark pixel fraction, we derive fiducial $1\sigma$ upper limits on the volume-average neutral hydrogen fraction of $\langle x_{\rm{HI}}\rangle \leq \{0.030 + 0.048, 0.095 + 0.037, 0.191 + 0.056, 0.199 + 0.087\}$ at $\bar{z} = \{5.481, 5.654, 5.831, 6.043\}$ from the optimally sensitive combination of the Ly$\beta$ and Ly$\gamma$ forests. We further demonstrate an alternative method that treats the forest flux as a mixture of dark and transparent regions, where the latter are modeled using a physically-motivated parametric form for the intrinsic opacity distribution. The resulting model-dependent upper limits on $\langle x_{\rm{HI}}\rangle$ are similar to those derived from our fiducial model-independent analysis. We confirm that the bulk of reionization must be finished at $z>6$, while leaving room for an extended "soft landing" to the reionization history down to $z\sim 5.4$ suggested by Ly$\alpha$ forest opacity fluctuations.