The JWST early galaxy crisis resolved by a reionization degeneracy

Abstract

JWST's discovery of unexpectedly bright $z>10$ galaxies has triggered claims that standard $\Lambda$CDM cannot reproduce their abundances, while estimates of the ionizing escape fraction $f_{\mathrm{esc}}$ at $z>6$ have spanned a factor of four for over a decade. Here we show that both tensions arise from a structural degeneracy in reionization equations: global observables constrain only the product $f_{\mathrm{esc}}\times f_{\star ,0}$ (peak star formation efficiency), not individual parameters. We demonstrate that this degeneracy, previously considered a limitation, provides a precise diagnostic framework. By leveraging JWST UV luminosity function shapes to independently constrain $f_{\star ,0}$, we derive robust bounds on $f_{\mathrm{esc}}$. Joint profile-likelihood analysis across Gaussian, log-normal, and duty-cycle burst scatter models excludes the proposed crisis threshold ($\epsilon >3.5\%$) at $4.5\sigma$ confidence, with stochastic star formation histories strengthening rather than weakening the result. Combining these constraints with constant and evolving $f_{\star ,0}$ measurements yields the first empirical reconstruction of $f_{\mathrm{esc}}(z)$ across $z=7$--$12$. A constant-efficiency scenario ($f_{\mathrm{esc}}\approx 10$--$16\%$) connects smoothly to low-redshift direct detections, whereas an evolving scenario ($f_{\mathrm{esc}}\approx 6\%$ at $z=12$) conflicts with low-metallicity ISM porosity expectations. JWST Cycle 3--4 will distinguish these pathways at $>2\sigma$, transforming a long-standing fundamental inference barrier into a powerful quantitative probe of early-universe physics.

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