Galactic metallicity

Galactic metallicity measures the abundance of elements heavier than helium in a galaxy. Gas and stars retain different time-weighted records of enrichment.

Gas-phase metallicity is the heavy-element abundance of interstellar gas. In star-forming galaxies it is commonly inferred from nebular emission lines in H II regions, often through oxygen abundance $12+{\log }_{10}(\mathrm{O}/\mathrm{H})$.

Stellar metallicity is the heavy-element abundance locked in stars. Integrated-light measurements are luminosity weighted unless a stellar-population reconstruction converts them to a mass-weighted quantity.

In a well-mixed closed-box model of galactic chemical evolution, gas consumed into locked stellar mass supplies a natural cumulative mass coordinate $\delta$. The mean stellar metallicity is ${\overline{Z}}_{\ast }(\delta )={\delta }^{-1}{\int }_0^{\delta }{Z}_{\mathrm{birth}}(s)ds$, giving the displayed derivative relation. If the birth abundance increases linearly, $Z_{\mathrm{birth}}=b\delta$, its cumulative mean is half the current maximum. More generally the maximum of any nonnegative abundance distribution cannot lie below its mean. A fixed initial mass function and the same survival selection are required to identify this mass-weighted distribution with a star-count distribution.

The galaxy mass--metallicity relation is the empirical tendency for galaxies with greater galaxy stellar mass to have higher gas-phase metallicity and stellar metallicity. Mass-dependent metal loss, gas inflow, star-formation efficiency, and recycling all contribute.