Mg II in the JWST era: a probe of Lyman continuum escape?

Limited constraints on the evolution of the Lyman Continuum (LyC) escape fraction represent one of the primary uncertainties in the theoretical determination of the reionization history. Due to the intervening intergalactic medium, the possibility of observing LyC photons directly in the epoch of reionization is highly unlikely. For this reason, multiple indirect probes of LyC escape have been identified, some of which are used to identify low-redshift LyC leakers (e.g. O32), while others are primarily useful at z > 6 (e.g. [O III]/[C III] far-infrared emission). The flux ratio of the resonant Mg it doublet emission at 2796 angstrom and 2803 angstrom as well as the Mg II optical depth have recently been proposed as ideal diagnostics of LyC leakage that can be employed at z > 6 with JWST. Using state-of-the-art cosmological radiation hydrodynamics simulations post-processed with CLOUDY and resonant-line radiative transfer, we test whether Mg H is indeed a useful probe of LyC leakage. Our simulations indicate that the majority of bright, star-forming galaxies with high LyC escape fractions are expected to be Mg A emitters rather than absorbers at z = 6. However, we find that the Mg H doublet flux ratio is a more sensitive indicator of dust rather than neutral hydrogen, limiting its use as a LyC leakage indicator to only galaxies in the optically thin regime. Given its resonant nature, we show that Mg ii will be an exciting probe of the complex kinematics in high-redshift galaxies in upcoming JWST observations.