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We investigate archaeologically how the metallicity in both stellar andgaseous components of spiral galaxies of differing masses evolve with time,using data from the SDSS-IV MaNGA survey. For the stellar component, we canmeasure this evolution directly by decomposing the galaxy absorption-linespectra into populations of different ages and determining their metallicities.For the gaseous component, we can only measure the present-day metallicitydirectly from emission lines. However, there is a well-established relationshipbetween gas metallicity, stellar mass and star formation rate which does notevolve significantly with redshift; since the latter two quantities can bedetermined directly for any epoch from the decomposition of the absorption-linespectra, we can use this relationship to infer the variation in gas metallicityover cosmic time. Comparison of present-day values derived in this way withthose obtained directly from the emission lines confirms the validity of themethod. Application of this approach to a sample of 1619 spiral galaxiesreveals how the metallicity of these systems has changed over the last 10billion years since cosmic noon. For lower-mass galaxies, both stellar andgaseous metallicity increase together, as one might expect in well-mixed fairlyisolated systems. In higher-mass systems, the average stellar metallicity hasnot increased in step with the inferred gas metallicity, and actually decreaseswith time. Such disjoint behaviour is what one might expect if these moremassive systems have accreted significant amounts of largely pristine gas overtheir lifetimes, and this material has not been well mixed into the galaxies.

SDSS-IV MaNGA: the chemical co-evolution of gas and stars in spiral galaxies