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Abstract Observations of quasar absorption spectra provide strong evidence that reionization extended belowz= 6. The relationship between Lyαforest opacity and local galaxy density (the opacity-density relation) is a key observational test of this scenario. Using narrow-band surveys ofz≈ 5.7 Lyαemitters (LAEs) centered on quasar sight lines, ref. [1] showed that two of the most transmissive Lyαforest segments at this redshift intersect under-densities in the galaxy distribution. This result is in tension with models of a strongly fluctuating ionizing background, including some models of late reionization, which predict that the vast majority of these segments should intersect over-densities where the ionizing intensity is strongest. In this paper, we use radiative transfer simulations to explore in more detail the opacity-density relation predicted by late reionization models. We find that fields like the one toward quasar PSO J359-06 — the more under-dense of the two transmissive sight lines in ref. [1] — are typically associated with recently reionized gas inside of cosmic voids where the hotter temperatures and rarefied densities enhance Lyαtransmission. The opacity-density relation's transmissive end is sensitive to the amount of neutral gas in the voids, as well as its morphology, set by the clustering of reionization sources. These effects are, however, largely degenerate with each other. We demonstrate that models with very different source clustering can nonetheless yield nearly identical opacity-density relations when their reionization histories are calibrated to match Lyα forest mean flux measurements atz< 6. In models with fixed source clustering, a lower neutral fraction increases the likelihood of intersecting hot, recently reionized gas in the voids, increasing the likelihood of observing fields like PSO J359-06. For instance, the probability of observing this field is 15% in a model with neutral fractionxHI= 5% atz= 5.7, three times more likely than in a model withxHI= 15%. The opacity-density relation may thus provide a complementary probe of reionization's tail end.
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This content will become publicly available on February 5, 2026
Chasing the Beginning of Reionization in the JWST Era
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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- Award ID(s):
- 2045600
- PAR ID:
- 10616023
- Publisher / Repository:
- The Astrophysical Journal
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 980
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 83
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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