The Lyman Continuum Escape Survey: Connecting Time-dependent [O iii] and [O ii] Line Emission with Lyman Continuum Escape Fraction in Simulations of Galaxy Formation

Escaping Lyman continuum photons from galaxies likely reionized the intergalactic medium at redshifts z ≳ 6. However, the Lyman continuum is not directly observable at these redshifts and secondary indicators of Lyman continuum escape must be used to estimate the budget of ionizing photons. Observationally, at redshifts z ∼ 2–3 where the Lyman continuum is observationally accessible, surveys have established that many objects that show appreciable Lyman continuum escape fractions fesc also show enhanced [O iii]/[O ii] (O32) emission line ratios. Here, we use radiative transfer analyses of cosmological zoom-in simulations of galaxy formation to study the physical connection between fesc and O32. Like the observations, we find that the largest fesc values occur at elevated O32 ∼ 3–10 and that the combination of high fesc and low O32 is extremely rare. While high fesc and O32 often are observable concurrently, the timescales of the physical origin for the processes are very different. Large O32 values fluctuate on short (∼1 Myr) timescales during the Wolf–Rayet-powered phase after the formation of star clusters, while channels of low absorption are established over tens of megayears by collections of supernovae. We find that while there is no direct causal relation between fesc and O32, high fesc most often occurs after continuous input from star formation-related feedback events that have corresponding excursions to large O32 emission. These calculations are in agreement with interpretations of observations that large fesc tends to occur when O32 is large, but large O32 does not necessarily imply efficient Lyman continuum escape.