L-GALAXIES 2020: The evolution of radial metallicity profiles and global
  metallicities in disc galaxies
  We present a modified version of the L-GALAXIES 2020 semi-analytic model of
galaxy evolution, which includes significantly increased direct metal
enrichment of the circumgalactic medium (CGM) by supernovae (SNe). These more
metal-rich outflows do not require increased mass-loading factors, in contrast
to some other galaxy evolution models. This modified L-GALAXIES 2020 model is
able to simultaneously reproduce the gas-phase metallicity $(Z_{\rm g})$ and
stellar metallicity $(Z_{*})$ radial profiles observed in nearby disc galaxies
by MaNGA and MUSE, as well as the observed mass - metallicity relations for gas
and stars at $z=0$ and their evolution back to $z\sim{}2-3$. A direct CGM
enrichment fraction of $\sim{}90\%$ for SNe-II is preferred. We find that
massive disc galaxies have slightly flatter $Z_{\rm g}$ profiles than their
lower-mass counterparts in L-GALAXIES 2020, due to more efficient enrichment of
their outskirts via inside-out growth and metal-rich accretion. Such a weak,
positive correlation between stellar mass and $Z_{\rm g}$ profile slope is also
seen in our MaNGA-DR15 sample of 571 star-forming disc galaxies. Although,
below ${\rm log}(M_{*}/{\rm M}_{\odot})\sim{}10.0$ this observational result is
strongly dependent on the metallicity diagnostic and morphological selection
chosen. In addition, a lowered maximum SN-II progenitor mass of $25{\rm
M}_{\odot}$, reflecting recent theoretical and observational estimates, can
also provide a good match to observed metallicity profiles at $z=0$ in
L-GALAXIES 2020. However, this model version fails to reproduce an evolution in
$Z_{\rm g}$ at fixed mass over cosmic time, or the magnesium abundances
observed in the intracluster medium (ICM).
