MgII in the JWST Era: a Probe of Lyman Continuum Escape?

TL;DR

This study evaluates the potential of MgII emission lines as indirect diagnostics for Lyman Continuum escape in high-redshift galaxies, using advanced simulations to understand their effectiveness with JWST observations.

Contribution

It demonstrates through simulations that MgII emission is a promising indicator of LyC leakage and explores its limitations related to dust and neutral hydrogen.

Findings

MgII emitters are likely to have high LyC escape fractions at z=6.

MgII flux ratio is more sensitive to dust than neutral hydrogen.

MgII can probe galaxy kinematics in high-redshift observations.

Abstract

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 (IGM), 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. [OIII]/[CII] far infrared emission). The flux ratio of the resonant MgII doublet emission at 2796$\dot{\rm A}$ and 2803$\dot{\rm A}$ as well as the MgII 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 MgII 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 MgII emitters rather than absorbers at $z=6$. However, we find that the MgII 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 MgII will be an exciting probe of the complex kinematics in high-redshift galaxies in upcoming JWST observations.