Two Modes of LyC Escape From Bursty Star Formation: Implications for [C II] Deficits and the Sources of Reionization

We use the SPHINX$^{20}$ 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 (SFR$_{10}$) or 100 Myr (SFR$_{100}$). Both have $f_{\rm esc}>20\%$ and experienced an extreme burst of star formation, but Bursty Leakers have ${\rm SFR_{10}>SFR_{100}}$, while Remnant Leakers have ${\rm SFR_{10}<SFR_{100}}$. The maximum SFRs in these bursts were typically $\sim100$ 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 HII region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower HII region densities. Both types of leaker exhibit [CII]$_{\rm 158μm}$ deficits on the [CII]-SFR$_{100}$ relation, while only Bursty Leakers show deficits when SFR$_{10}$ is used. We predict that [CII] luminosity and SFR indicators such as H$α$ and M$_{\rm 1500Å}$ can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [CII] observations of known $z=3-4$ LyC leakers. Finally, we show that leakers with $f_{\rm esc}>20\%$ dominate the ionizing photon budget at $z\gtrsim7.5$ but the contribution from galaxies with $f_{\rm esc}<5\%$ becomes significant at the tail-end of reionization.