The Physics of Indirect Estimators of Lyman Continuum Escape and their Application to High-Redshift JWST Galaxies

Reliable indirect diagnostics of LyC photon escape from galaxies are required to understand which sources were the dominant contributors to reionization. While multiple LyC escape fraction (f_ esc) indicators have been proposed to trace favourable conditions for LyC leakage from the interstellar medium of low-redshift ”analog” galaxies, it remains unclear whether these are applicable at high redshifts where LyC emission cannot be directly observed. Using a library of 14,120 mock spectra of star-forming galaxies with redshifts 4.64 ≤ z ≤ 10 from the SPHINX^20 cosmological radiation hydrodynamics simulation, we develop a framework for the physics that leads to high f_ esc. We investigate LyC leakage from our galaxies based on the criteria that successful LyC escape diagnostics must i) track a high specific star formation rate, ii) be sensitive to stellar population age in the range 3.5-10Myr representing the times when supernova first explode to when LyC production significantly drops, and iii) include a proxy for neutral gas content and gas density in the interstellar medium. O_32, Σ_ SFR, M_ UV, and Hβ equivalent width select for one or fewer of our criteria, rendering them either necessary but insufficient or generally poor diagnostics. In contrast, UV slope (β), and E(B-V) match two or more of our criteria, rendering them good f_ esc diagnostics (albeit with significant scatter). Using our library, we build a quantitative model for predicting f_ esc based on direct observables. When applied to bright z > 6 Lyα emitters observed with JWST, we find that the majority of them have f_ esc≲ 10%.