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Harvard ADS: Chasing the beginning of reionization in the JWST era

Paper abstract: Recent JWST observations at z > 6 may imply galactic ionizing photon production in excess of prior expectations. Under observationally motivated assumptions about escape fractions, these suggest a z ~ 8-9 end to reionization, in strong tension with the z < 6 end required by the Ly\alpha forest. 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 (at z ~ 8) alongside two more realistic scenarios that end late at z ~ 5: one that starts late (z ~ 9) and another that starts 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 recent measurements of \xi_{\rm ion} by JWST, which indicate little evolution. We also study how these models compare to recent measurements of the Ly\alpha forest opacity, mean free path, IGM thermal history, visibility of z > 8 Ly\alpha emitters, and the patchy kSZ signal from the CMB. We find that neither of the late-ending scenarios is conclusively disfavored by any single data set. However, a majority of these 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 between observables. Observations by multiple experiments (including JWST, Roman, and CMB-S4) in the coming years will either establish a concordance picture of reionization's early stages or reveal systematics in data and/or theoretical modeling.