Mg II Emission: A new Tracer for Lyman Alpha and Lyman Continuum

In studies of the intergalactic medium (IGM) reionization, the Lyman Alpha (LyA) emission from galaxies is a heavily used diagnostic. This spectral feature is frequently used to infer the IGM neutral hydrogen fraction, and is also thought to predict the emission of hydrogen-ionizing Lyman Continuum (LyC) photons. Nevertheless, LyA is difficult to observe in a neutral IGM, and its evolution in the reionization epoch may be partly due to galaxy evolution. Therefore, we propose a new way forward: emission from the Mg II 2796, 2803 doublet. Recently, Henry et al. (2018) showed that Mg II appears in emission, rather than absorption, in nearby analogs to reionization-epoch galaxies. Since the strength of Mg II correlates tightly with that of LyA in these galaxies, it is plausible that observations of Mg II from z>6 galaxies could provide valuable constraints on reionization. Still, only seven reionization-epoch analogs have observations in LyA, LyC, and Mg II, and this existing sample misses most of the strongest Mg II emitters. Therefore, we propose new COS observations for eight star-forming galaxies with the highest Mg II equivalent widths. These objects may reveal some of the highest LyC escape fractions seen nearby. These observations will quantify how LyC in this sample relates to the intensity of Mg II and LyA emission, as well as the velocity structure of the emission line profiles. Finally, we will carry out new multi-ion radiation transport modeling, in order to obtain stringent constraints on the scattering gas that regulates the escape of LyC, LyA, and Mg II.