Two modes of LyC escape from bursty star formation: implications for [C II] deficits and the sources of reionization

We use the SPHINX²⁰ cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR₁₀) or 100 Myr (SFR₁₀₀). Both have $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ and experienced an extreme burst of star formation, but Bursty Leakers have SFR₁₀ > SFR₁₀₀, while Remnant Leakers have SFR₁₀ < SFR₁₀₀. The maximum SFRs in these bursts were typically ~100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H II region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H II region densities. Both types of leaker exhibit [C II]_158μm deficits on the [C II]-SFR₁₀₀ relation, while only Bursty Leakers show deficits when SFR₁₀ is used. We predict that [C II] luminosity and SFR indicators such as H α and M$_{\rm 1500\mathring{\rm A}}$ can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C II] observations of known z = 3-4 LyC leakers. Finally, we show that leakers with $f_{\rm esc}\gt 20{{\ \rm per\ cent}}$ dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with $f_{\rm esc}\lt 5{{\ \rm per\ cent}}$ becomes significant at the tail-end of reionization.