2025ApJ...991...49L

We conducted systematic radiative transfer (RT) modeling of the Mg II doublet line profiles for 33 low-redshift Lyman continuum (LyC) leakers, and Lyα 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 (${v}_{{\rm{MgII}},{\rm{\max }}}\gtrsim 390\,{\rm{km}}\,{{\rm{s}}}^{-1}$) 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 Lyα modeling shows that their spectral properties do not consistently match conventional LyC leakage criteria. A direct comparison of the best-fit Mg II and Lyα 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 Lyα 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 Lyα 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.