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The spectra of high-redshift ( z ≳ 6) quasars contain valuable information on the progression of the Epoch of Reionization. At redshifts z < 6, the observed Lyman-series forest shows that the intergalactic medium is nearly ionized, while at z > 7 the observed quasar damping wings indicate high neutral gas fractions. However, there remains a gap in neutral gas fraction constraints at 6 ≲ z ≲ 7 where the Lyman-series forest becomes saturated but damping wings have yet to fully emerge. In this work, we use a sample of 18 quasar spectra at redshifts 6.0 < z < 7.1 to close this gap. We apply neural networks to reconstruct the quasars’ continuum emission around the partially absorbed Ly α line to normalize their spectra, and stack these continuum-normalized spectra in three redshift bins. To increase the robustness of our results, we compare the stacks to a grid of models from two hydrodynamical simulations, ATON and CROC, and we measure the volume-averaged neutral gas fraction, ${\bar{x}}_{\mathrm{HI}}$ , while jointly fitting for the mean quasar lifetime, t _Q , for each stacked spectrum. We chronicle the evolution of neutral gas fraction using the ATON (CROC) models as follows: ${\bar{x}}_{\mathrm{HI}}={0.21}_{-0.07}^{+0.17}$ ( ${\bar{x}}_{\mathrm{HI}}={0.10}_{\lt {10}^{-4}}^{+0.73}$ ) at 〈 z 〉 = 6.10, ${\bar{x}}_{\mathrm{HI}}={0.21}_{-0.07}^{+0.33}$ ( ${\bar{x}}_{\mathrm{HI}}={0.57}_{-0.47}^{+0.26}$ ) at 〈 z 〉 = 6.46, and ${\bar{x}}_{\mathrm{HI}}={0.37}_{-0.17}^{+0.17}$ ( ${\bar{x}}_{\mathrm{HI}}={0.57}_{-0.21}^{+0.26}$ ) at 〈 z 〉 = 6.87. At the same time, we constrain the average quasar lifetime to be t _Q ≲ 7 Myr across all redshift bins, in good agreement with previous studies.

The variations in Ly α forest opacity observed at z > 5.3 between lines of sight to different background quasars are too strong to be caused by fluctuations in the density field alone. The leading hypothesis for the cause of this excess variance is a late, ongoing reionization process at redshifts below six. Another model proposes strong ionizing background fluctuations coupled to a short, spatially varying mean free path of ionizing photons, without explicitly invoking incomplete reionization. With recent observations suggesting a short mean free path at z ∼ 6, and a dramatic improvement in z > 5 Ly α forest data quality, we revisit this latter possibility. Here, we apply the likelihood-free inference technique of approximate Bayesian computation (ABC) to jointly constrain the hydrogen photoionization rate Γ _HI and the mean free path of ionizing photons λ _mfp from the effective optical depth distributions at z = 5.0–6.1 from XQR-30. We find that the observations are well-described by fluctuating mean free path models with average mean free paths that are consistent with the steep trend implied by independent measurements at z ∼ 5–6, with a concomitant rapid evolution of the photoionization rate.

We present a new upper limit on the cosmic molecular gas density at z = 2.4–3.4 obtained using the first year of observations from the CO Mapping Array Project (COMAP). COMAP data cubes are stacked on the 3D positions of 243 quasars selected from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) catalog, yielding a 95% upper limit for flux from CO(1–0) line emission of 0.129 Jy km s ^−1 . Depending on the balance of the emission between the quasar host and its environment, this value can be interpreted as an average CO line luminosity ${L}_{\mathrm{CO}}^{{\prime} }$ of eBOSS quasars of ≤1.26 × 10 ^11 K km pc ^2 s ^−1 , or an average molecular gas density ${\rho }_{{{\rm{H}}}_{2}}$ in regions of the Universe containing a quasar of ≤1.52 × 10 ^8 M _⊙ cMpc ^−3 . The ${L}_{\mathrm{CO}}^{{\prime} }$ upper limit falls among CO line luminosities obtained from individually targeted quasars in the COMAP redshift range, and the ${\rho }_{{{\rm{H}}}_{2}}$ value is comparable to upper limits obtained from other line intensity mapping (LIM) surveys and their joint analyses. Further, we forecast the values obtainable with the COMAP/eBOSS stack after the full 5 yr COMAP Pathfinder survey. We predict that a detection is probable with this method, depending on the CO properties of the quasar sample. Based on the achieved sensitivity, we believe that this technique of stacking LIM data on the positions of traditional galaxy or quasar catalogs is extremely promising, both as a technique for investigating large galaxy catalogs efficiently at high redshift and as a technique for bolstering the sensitivity of LIM experiments, even with a fraction of their total expected survey data.