Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra

We present a new investigation of the intergalactic medium near reionization using dark gaps in the Ly beta forest. With its lower optical depth, Ly beta offers a potentially more sensitive probe to any remaining neutral gas compared to the commonly used Ly alpha line. We identify dark gaps in the Ly beta 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 similar or equal to 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 Ly alpha 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 (tau (eff) greater than or similar to 6) gap persisting down to z similar or equal to 5.5 in the Ly beta 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) greater than or similar to 5% by z = 5.6. Finally, we infer constraints on x (H I) over 5.5 less than or similar to z less than or similar to 6.0 based on the observed Ly beta 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 similar or equal to 5.55, 5.75, and 5.95, respectively. These constraints are consistent with models where reionization ends significantly later than z = 6.