James Webb Space Telescope Feed Post

Harvard ADS: The great escape: understanding the connection between Ly α emission and LyC escape in simulated JWST analogues

Paper abstract: 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\, \alpha emission line correlate with the inferred LyC escape fraction (f_{\rm esc}^{\rm LyC}) of low-redshift galaxies. Using a data set of 9600 mock Ly\, \alpha spectra of star-forming galaxies at 4.64 \le z \le 6 from the SPHINX^{20} cosmological radiation hydrodynamical simulation, we study the physics controlling the escape of Ly\, \alpha and LyC photons. We find that our mock Ly\, \alpha observations are representative of high-redshift observations and that typical observational methods tend to overpredict the Ly\, \alpha escape fraction (f_{\rm esc}^{\rm Ly\, \alpha }) by as much as 2 dex. We investigate the correlations between f_{\rm esc}^{\rm LyC} and f_{\rm esc}^{\rm Ly\, \alpha }, Ly\, \alpha equivalent width (W_{\lambda }({\rm Ly\, \alpha })), peak separation (v_{\rm sep}), central escape fraction (f_{\rm cen}), and red peak asymmetry (A_f^{\rm red}). We find that f_{\rm esc}^{\rm Ly\, \alpha } and f_{\rm 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_{\lambda }({\rm Ly\, \alpha }) and v_{\rm sep} are found to be necessary but insufficient diagnostics, while A_f^{\rm red} carries little information. Finally, we use stacks of Ly\, \alpha, H\, \alpha, and F150W mock surface brightness profiles to find that galaxies with high f_{\rm esc}^{\rm LyC} tend to have less extended Ly\, \alpha and F150W haloes but larger H\, \alpha haloes than their non-leaking counterparts. This confirms that Ly\, \alpha spectral profiles and surface brightness morphology can be used to better understand the escape of LyC photons from galaxies during the epoch of reionization.