Synergistic Radiative Transfer Modeling of Mg II and Lyα Emission in Multiphase, Clumpy Galactic Environments: Application to Low-redshift Lyman Continuum Leakers

We conducted systematic radiative transfer (RT) modeling of the Mg ii doublet line profiles for 33 low-redshift Lyman continuum (LyC) leakers, and Ly α modeling for six of these galaxies, using a multiphase, clumpy circumgalactic medium (CGM) model. Our RT models successfully reproduced the Mg ii emission line profiles for all galaxies, revealing a necessary condition for strong LyC leakage: high maximum clump outflow velocity ( v MgII , max ≳ 390 km s − 1 ) and low total Mg ii column density ( N MgII, tot ≲ 10 14.3 cm −2 ). These two parameters have the strongest influence on the emergent Mg ii profile. For the six galaxies with archival HST data, our Ly α modeling shows that their spectral properties do not consistently match conventional LyC leakage criteria. A direct comparison of the best-fit Mg ii and Ly α model parameters reveals no clear correlations in clump outflow velocities, column densities, or volume filling factors, suggesting that H i and Mg ii may not be kinematically or spatially coherent. The inferred LyC escape fraction from Ly α modeling is primarily governed by the number of optically thick H i clumps per sightline ( f cl ). Intriguingly, two galaxies with low observed LyC leakage exhibited the highest RT-inferred LyC escape fractions, due to having the lowest f cl values, driven by strong Ly α blue peaks. Future spatially resolved observations will be crucial for resolving this puzzle. Our results support a “picket fence” CGM geometry over a “density-bounded” scenario, where high Mg ii outflow velocities and low Mg ii column densities trace low-density H i channels that facilitate LyC escape.