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

Abstract

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 reionization. However, low data quality and systematic uncertainties complicate the use of Ly-α transmission as a precision probe of reionization’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 reionization-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 reionization.

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

“…A comparison between the observed Lyβ dark gap statistics for the whole sample of 42 lines of sight and predictions from multiple reionization models (Bolton et al 2017;Keating et al 2020a;Nasir & D'Aloisio 2020) confirms that evidence of reionization in the form of neutral islands and/or a fluctuating UV background persists down to at least z ∼ 5.5. This supports the conclusions in Paper I and Bosman et al (2022). In Paper I we noted a possible tension between Lyα gap statistics and a model wherein reionization ends by z < 6 but has a relatively early midpoint of z = 8.4 (Keating et al 2020a).…”

“…the homogeneous-UVB model from the Sherwood Simulation Suite(Bolton et al 2017), which uses a homogeneousHaardt & Madau (2012) UV background; 2. late reionization models wherein reionization ends at z  5.3, including K20-low-τ CMB , K20-low-τ CMBhot, and K20-high-τ CMB models fromKeating et al (2020a) and ND20-late-longmfp and ND20late-shortmfp models from Nasir & D'Aloisio (2020); and 3. an early reionization model wherein the IGM is mostly ionized by z = 6 but large-scale fluctuations in the UVB, which are amplified by a short mean free path of ionizing photons (z = 5.6), persist down to lower redshifts (ND20-early-shortmfp; Nasir & D'Aloisio 2020).These models were chosen, in part, because they reproduce at least some other observations of the Lyα forest. The homogeneous-UVB model agrees well with observations at z < 5 including the IGM temperature and flux power spectra(Bolton et al 2017), although it fails to predict the Lyα opacity distribution at z > 5 (e.g.,Bosman et al 2022). The late reionization and fluctuating UVB models are broadly consistent with observations of IGM temperature, mean Lyα transmission, and fluctuations in Lyα opacity over the redshift range we…”

“…We follow a similar method to that described in Paper I to fit and predict the continuum. Briefly, we implement and apply the log-PCA method of Davies et al (2018c) in the Lyα and Lyβ forest portion of the spectrum following Bosman et al (2022). The continuity between the Lyα forest and the Lyβ forest continuum is broken intentionally to correct for the effect of overlapping Lyα absorption in the Lyβ forest in the PCA training sample.…”

“…Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. large-scale fluctuations in the measured Lyα effective optical depth, t = -á ñ F ln eff , where F is the continuum-normalized transmission flux (e.g., Fan et al 2006;Becker et al 2015;Eilers et al 2018;Yang et al 2020a;Bosman et al 2022), together with long troughs extending to or below z ; 5.5 in the Lyα forest (e.g., Becker et al 2015;Zhu et al 2021), potentially indicate the existence of large neutral IGM islands (e.g., Kulkarni et al 2019;Keating et al 2020b;Nasir & D'Aloisio 2020). The underdensities around long dark gaps traced by Lyα emitters (LAEs) are also consistent with a late reionization model wherein reionization ends at z < 6 (Becker et al 2018;Kashino et al 2020;Christenson et al 2021).…”

The effect of ionizing background fluctuations on the spatial correlations of high redshift Ly$α$-emitting galaxies

“…The distributions of the measured Lyα effective optical depths τα at z > 5 are unexpectedly broad compared with the predictions of numerical simulations of the IGM. The breadths of the distributions increase with redshift, with patches of high attenuation extending over 100 h −1 comoving Mpc appearing along some lines of sight at z > 6 (Becker et al 2015(Becker et al , 2018Bosman et al 2018;Yang et al 2020;Bosman et al 2022). It has been suggested that late reionization may account for the breadths of the Lyα effective optical depth distributions, with the EoR ending as late as z 5.2 − 5.5 (Kulkarni et al 2019a;Keating et al 2020;Nasir & D'Aloisio 2020).…”