The great escape: understanding the connection between Ly α 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 Ly\, α emission line correlate with the inferred LyC escape fraction (fescLyC) of low-redshift galaxies. Using a data set of 9600 mock Ly\, α spectra of star-forming galaxies at 4.64 ≤ z ≤ 6 from the SPHINX²⁰ cosmological radiation hydrodynamical simulation, we study the physics controlling the escape of Ly\, α and LyC photons. We find that our mock Ly\, α observations are representative of high-redshift observations and that typical observational methods tend to overpredict the Ly\, α escape fraction (fescLy\, α) by as much as 2 dex. We investigate the correlations between fescLyC and fescLy\, α, Ly\, α equivalent width (Wλ( Ly\, α )), peak separation (vₛₑₚ), central escape fraction (fcen), and red peak asymmetry (Afʳᵉᵈ). We find that fescLy\, α and fcen 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, Wλ( Ly\, α ) and vₛₑₚ are found to be necessary but insufficient diagnostics, while Afʳᵉᵈ carries little information. Finally, we use stacks of Ly\, α, H\, α, and F150W mock surface brightness profiles to find that galaxies with high fescLyC tend to have less extended Ly\, α and F150W haloes but larger H\, α haloes than their non-leaking counterparts. This confirms that Ly\, α spectral profiles and surface brightness morphology can be used to better understand the escape of LyC photons from galaxies during the epoch of reionization.