2022ApJ...932...76Z

We present a new investigation of the intergalactic medium near reionization using dark gaps in the Lyβ forest. With its lower optical depth, Lyβ offers a potentially more sensitive probe to any remaining neutral gas compared to the commonly used Lyα line. We identify dark gaps in the Lyβ forest using spectra of 42 QSOs at z_em > 5.5, including new data from the XQR-30 VLT Large Programme. Approximately 40% of these QSO spectra exhibit dark gaps longer than 10 h^–1 Mpc at z ≃ 5.8. By comparing the results to predictions from simulations, we find that the data are broadly consistent both with models where fluctuations in the Lyα forest are caused solely by ionizing ultraviolet background fluctuations and with models that include large neutral hydrogen patches at z < 6 due to a late end to reionization. Of particular interest is a very long (L = 28 h^–1 Mpc) and dark (τ_eff >= 6) gap persisting down to z ≃ 5.5 in the Lyβ forest of the z = 5.85 QSO PSO J025-11. This gap may support late reionization models with a volume-weighted average neutral hydrogen fraction of <x_H I_> >= 5% by z = 5.6. Finally, we infer constraints on <x_H I_> over 5.5 <= z <= 6.0 based on the observed Lyβ dark gap length distribution and a conservative relationship between gap length and neutral fraction derived from simulations. We find <x_H I_> <= 0.05, 0.17, and 0.29 at z ≃ 5.55, 5.75, and 5.95, respectively. These constraints are consistent with models where reionization ends significantly later than z = 6.