The Great Escape: Understanding the Connection Between Lyα Emission and LyC Escape in Simulated JWST Analogues

Constraining the escape fraction of Lyman Continuum (LyC) photons from high-redshift galaxies is crucial to understanding reionization. Recent observations have demonstrated that various characteristics of the Lyα emission line correlate with the inferred LyC escape fraction (f_ esc^ LyC) of low-redshift galaxies. Using a data-set of 9,600 mock Lyα spectra of star-forming galaxies at 4.64 ≤ z ≤ 6 from the SPHINX^20 cosmological radiation hydrodynamical simulation, we study the escape of Lyα and LyC photons. We find that our mock Lyα observations are representative of high-redshift observations and that typical observational methods tend to over-predict the Lyα escape fraction (f_ esc^ Lyα) by as much as two dex. We investigate the correlations between f_ esc^ LyC and f_ esc^ Lyα, Lyα equivalent width (W_λ( Lyα)), peak separation (v_ sep), central escape fraction (f_ cen), and red peak asymmetry (A_f^ red). We find that f_ esc^ Lyα and f_ cen are good diagnostics for LyC leakage, selecting for galaxies with lower neutral gas densities and less UV attenuation that have recently experienced supernova feedback. In contrast, W_λ( Lyα) and v_ sep are found to be necessary but insufficient diagnostics, while A_f^ red carries little information. Finally, we use stacks of Lyα, Hα, and F150W mock surface brightness profiles to find that galaxies with high f_ esc^ LyC have less extended Lyα and F150W but larger Hα haloes than their non-leaking counterparts. This confirms that Lyα spectral profiles and surface brightness morphology can be used to better understand the escape of LyC photons from galaxies during the Epoch of Reionization.