  The Mg II 2796, 2803 doublet has been suggested to be a useful indirect
indicator for the escape of Ly-alpha and Lyman continuum (LyC) photons in local
star-forming galaxies. However, studies to date have focused on small samples
of galaxies with strong Mg II or strong LyC emission. Here we present the first
study of Mg II probing a large dynamic range of galaxy properties, using newly
obtained high signal-to-noise, moderate-resolution spectra of Mg II for a
sample of 34 galaxies selected from the Low-redshift Lyman Continuum Survey. We
show that the galaxies in our sample have Mg II profiles ranging from strong
emission to P-Cygni profiles, and to pure absorption. We find there is a
significant trend (with a possibility of spurious correlations of ~ 2%) that
galaxies detected as strong LyC Emitters (LCEs) also show larger equivalent
widths of Mg II emission, and non-LCEs tend to show evidence of more scattering
and absorption features in Mg II We then find Mg II strongly correlates with
Ly-alpha in both equivalent width and escape fraction, regardless of whether
the emission or absorption dominates the Mg II profiles. Furthermore, we
present that, for galaxies categorized as Mg II emitters (MgE), one can adopt
the information of Mg II, metallicity, and dust to estimate the escape fraction
of LyC within a factor of 3. These findings confirm that Mg II lines can be
used as a tool to select galaxies as LCEs and to serve as an indirect indicator
for the escape of Ly-alpha and LyC.
