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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 (zend ≈ 8) alongside two more realistic scenarios withzend ≈ 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 ofξionby 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.
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This content will become publicly available on July 3, 2026
The Cosmic Microwave Background Optical Depth Constrains the Duration of Reionization
Abstract Recently, it was pointed out that invoking a large value of the cosmic microwave background (CMB) optical depth,τCMB = 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 ofτCMB = 0.058 measured from the Planck low-ℓpolarization data. Traditionally,τCMBis 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, rendersτCMBa sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values ofτCMBto upper limits on the patchy kinematic Sunyaev–Zel'dovich (pkSZ) effect, another CMB observable that constrains reionization’s duration, and find that a value ofτCMB = 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.
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- Award ID(s):
- 2045600
- PAR ID:
- 10616025
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 987
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L29
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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