Scaling relations and baryonic cycling in local star-forming galaxies.
  III. Outflows, effective yields and metal loading factors
  Gas accretion and stellar feedback processes link metal content, star
formation, and gas and stellar mass (and the potential depth) in star-forming
galaxies. Constraining this hypersurface has been challenging because of the
need for measurements of HI and HII gas masses spanning a broad parameter
space. A recent step forward has been achieved through the "Metallicity And Gas
for Mass Assembly" (MAGMA) sample of local star-forming galaxies, which
consists of homogeneously-determined parameters and a significant quantity of
dwarf galaxies, with stellar masses as low as $\sim 10^5 - 10^{6}\, M_{\odot}$.
Here, we adopt a "standard" galactic chemical evolution model, with which we
can quantify stellar-driven outflows. In particular, we constrain the
difference between the mass-loading in accretion and outflows and the wind
metal-loading factor. The resulting model reproduces very well the local
mass-metallicity relation, and the observed trends of metallicity with gas
fraction. Although the difference in mass loading between accreted and expelled
gas is extremely difficult to constrain, we find indications that, on average,
the amount of gas acquired through accretion is roughly the same as the gas
lost through bulk stellar outflows, corresponding to a "gas equilibrium"
scenario. In agreement with previous work, the wind metal-loading factor shows
a steep increase toward lower mass and circular velocity, indicating that
low-mass galaxies are more efficient at expelling metals, thus shaping the
mass-metallicity relation. Effective yields are found to increase with mass up
to an inflection mass threshold, with a mild decline at larger masses and
circular velocities. A comparison of our results for metal loading in outflows
with the expectations for their mass loading favors momentum-driven winds at
low masses, rather than energy-driven ones. (abridged)
