CLASSY. XIII. Cutting through the Clouds—Comparing Indirect Tracers of Ionizing Photon Escape

Abstract The Epoch of Reionization (EoR) provides critical insights into the role of early galaxies in shaping the ionization state of the Universe. However, because of the opacity of the intergalactic medium, it is often not possible to make direct measurements of the ionizing photon escape fraction ( f esc LyC ) of high-redshift ( z ≳ 4) galaxies. To explore the agreement and systematics of common indirect approaches, we applied six empirically calibrated diagnostics to predict f esc LyC for the 45 nearby star-forming galaxies from the COS Legacy Spectroscopic SurveY. These methods—based on ultraviolet (UV) absorption lines, the UV continuum slope, Ly α kinematics, a multivariate model, radiation-hydrodynamic simulations, and nebular emission-line ratios—enable us to explore systematic differences between predictions and assess how galactic properties influence inferred LyC escape. Despite significant variations in method predictions, there is broad consistency in the resulting weak and strong LyC leaker classifications, with approximately half exhibiting predicted escape fractions >1%. We find evidence for two different pathways of LyC escape in nearby star-forming galaxies: (1) an early escape model driven by very young stellar populations, and (2) a delayed escape model that is consistent with supernova-driven outflows and time-dependent interstellar medium clearing. The early escape model is favored among galaxies with a single, intense burst of recent star formation. In contrast, the delayed escape model is common among galaxies with more extended starburst histories. To interpret ionizing photon escape during the EoR, it will be necessary to recognize and understand this diversity in LyC escape mechanisms.