Chasing the beginning of reionization in the JWST era

Abstract

Recent JWST observations at $z > 6$ may imply galactic ionizing photon production above prior expectations. Under observationally motivated assumptions about escape fractions, these suggest a $z \sim 8-9$ end to reionization, in 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 ($z_{\rm end} \approx 8$) alongside two more realistic scenarios with $z_{\rm end} \approx 5$: one starting late ($z \sim 9$) and another early ($z \sim 13$). 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 of $\xi_{\rm ion}$ by JWST, which indicate little evolution. We 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 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.