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Hydrogen reionization ends by z = 5.3: Lyman-α optical depth measured by the XQR-30 sample

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The presence of excess scatter in the Ly-α forest at z∼5.5, together with the existence of sporadic extended opaque Gunn-Peterson troughs, has started to provide robust evidence for a late end of hydrogen reionisation. However, low data quality and systematic uncertainties complicate the use of Ly-α transmission as a precision probe of reionisation's end stages. In this paper, we assemble a sample of 67 quasar sightlines at z>5.5 with high signal-to-noise ratios of >10 per ≤15 km s−1 spectral pixel, relying largely on the new XQR-30 quasar sample. XQR-30 is a large program on VLT/X-Shooter which obtained deep (SNR >20 per pixel) spectra of 30 quasars at z>5.7. We carefully account for systematics in continuum reconstruction, instrumentation, and contamination by damped Ly-α systems. We present improved measurements of the mean Ly-α transmission over 4.9<z<6.1. Using all known systematics in a forward modelling analysis, we find excellent agreement between the observed Ly-α transmission distributions and the homogeneous-UVB simulations Sherwood and Nyx up to z≤5.2 (<1σ), and mild tension (∼2.5σ) at z=5.3. Homogeneous UVB models are ruled out by excess Ly-α transmission scatter at z≥5.4 with high confidence (>3.5σ). Our results indicate that reionisation-related fluctuations, whether in the UVB, residual neutral hydrogen fraction, and/or IGM temperature, persist in the intergalactic medium until at least z=5.3 (t=1.1 Gyr after the Big Bang). This is further evidence for a late end to reionisation.



intergalactic medium, quasars: absorption lines, dark ages, reionization, first stars, large-scale structure of Universe

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Monthly Notices of the Royal Astronomical Society

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Oxford University Press (OUP)
Science and Technology Facilities Council (ST/S000623/1)
Science and Technology Facilities Council (ST/H008586/1)
Science and Technology Facilities Council (ST/J005673/1)
Science and Technology Facilities Council (ST/K00333X/1)
STFC (ST/M007073/1)