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Abstract Recently, it was pointed out that invoking a large value of the cosmic microwave background (CMB) optical depth,τ_CMB = 0.09, could help resolve tensions between Dark Energy Survey Instrument DR2 baryon acoustic oscillation data and the CMB. This is larger than the value ofτ_CMB = 0.058 measured from the Planck low-ℓpolarization data. Traditionally,τ_CMBis thought of as a constraint on reionization’s midpoint. However, recent observations and modeling of the Lyαforest of high-zquasars at 5 < z < 6 have tightly constrained the timing of the last 10%–20% of reionization, adding nuance to this interpretation. Here, we point out that fixing reionization’s endpoint, in accordance with the latest Lyαforest constraints, rendersτ_CMBa sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values ofτ_CMBto upper limits on the patchy kinematic Sunyaev–Zel'dovich (pkSZ) effect, another CMB observable that constrains reionization’s duration, and find that a value ofτ_CMB = 0.09 is in ≈2σtension with existing limits on the pkSZ from the South Pole Telescope (SPT). The strength of this tension is sensitive to the choices involved in modeling the other CMB foregrounds in the SPT measurement, and in the modeling of the pkSZ signal itself. 

Abstract Recent JWST observations atz > 6 may imply galactic ionizing photon production above prior expectations. Under observationally motivated assumptions about escape fractions, these suggest az ~ 8–9 end to reionization, in tension with thez < 6 end required by the Lyαforest. In this work, we use radiative transfer simulations to understand what different observations tell us about when reionization ended and when it started. We consider a model that ends too early (z_end ≈ 8) alongside two more realistic scenarios withz_end ≈ 5: one starting late (z ~ 9) and another early (z ~ 13). We find that the latter requires up to an order-of-magnitude evolution in galaxy ionizing properties at 6 < z < 12, perhaps in tension with measurements ofξ_ionby JWST, which indicate little evolution. We study how these models compare to recent measurements of the Lyαforest opacity, mean free path, intergalactic medium thermal history, visibility ofz > 8 Lyαemitters, and the patchy kSZ signal from the cosmic microwave background (CMB). We find that neither of the late-ending scenarios is strongly disfavored by any single data set. However, a majority of observables, spanning several distinct types of observations, prefer a late start. Not all probes agree with this conclusion, hinting at a possible lack of concordance arising from deficiencies in observations and/or theoretical modeling. Observations by multiple experiments (including JWST, Roman, and CMB-S4) in the coming years will establish a concordance picture of reionization's beginning or uncover such deficiencies. 

ABSTRACT Quasar absorption spectra measurements suggest that reionization proceeded rapidly, ended late at z ∼ 5.5, and was followed by a flat ionizing background evolution. Simulations that reproduce this behaviour often rely on a fine-tuned galaxy ionizing emissivity, which peaks at z ∼ 6–7 and drops a factor of 1.5–2.5 by z ∼ 5. This is puzzling since the abundance of galaxies is observed to grow monotonically during this period. Explanations for this include effects such as dust obscuration of ionizing photon escape and feedback from photoheating of the IGM. We explore the possibility that this drop in emissivity is instead an artefact of one or more modelling deficiencies in reionization simulations. These include possibly incorrect assumptions about the ionizing spectrum and/or inaccurate modelling of IGM clumping. Our results suggest that the need for a drop could be alleviated if simulations are underestimating the IGM opacity from massive, star-forming haloes. Other potential modelling issues either have a small effect or require a steeper drop when remedied. We construct an illustrative model in which the emissivity is nearly flat at reionization’s end, evolving only ∼0.05 dex at 5 < z < 7. More realistic scenarios, however, require a ∼0.1–0.3 dex drop. We also study the evolution of the Ly α effective optical depth distribution and compare to recent measurements. We find that models that feature a hard ionizing spectrum and/or are driven by faint, low-bias sources most easily reproduce the mean transmission and optical depth distribution of the forest simultaneously.