The SAMI Galaxy Survey: the drivers of gas and stellar metallicity
  differences in galaxies
  The combination of gas-phase oxygen abundances and stellar metallicities can
provide us with unique insights into the metal enrichment histories of
galaxies. In this work, we compare the stellar and gas-phase metallicities
measured within a 1$R_{e}$ aperture for a representative sample of 472
star-forming galaxies extracted from the SAMI Galaxy Survey. We confirm that
the stellar and interstellar medium (ISM) metallicities are strongly
correlated, with scatter $\sim$3 times smaller than that found in previous
works, and that integrated stellar populations are generally more metal-poor
than the ISM, especially in low-mass galaxies. The ratio between the two
metallicities strongly correlates with several integrated galaxy properties
including stellar mass, specific star formation rate, and a gravitational
potential proxy. However, we show that these trends are primarily a consequence
of: (a) the different star formation and metal enrichment histories of the
galaxies, and (b) the fact that while stellar metallicities trace primarily
iron enrichment, gas-phase metallicity indicators are calibrated to the
enrichment of oxygen in the ISM. Indeed, once both metallicities are converted
to the same `element base' all of our trends become significantly weaker.
Interestingly, the ratio of gas to stellar metallicity is always below the
value expected for a simple closed-box model, which requires that outflows and
inflows play an important role in the enrichment history across our entire
stellar mass range. This work highlights the complex interplay between stellar
and gas-phase metallicities and shows how care must be taken in comparing them
to constrain models of galaxy formation and evolution.
