Where have all the low-metallicity galaxies gone? Tracing evolution in the mass-metallicity plane since a redshift of 0.7

Even over relatively recent epochs, galaxies have evolved significantly in their location in the mass-metallicity plane, which must be telling us something about the latter stages of galaxy evolution. In this paper, we analyse data from the LEGA-C survey using semi-analytic spectral and photometric fitting to determine these galaxies' evolution up to their observed epoch at z similar to 0.7. We confirm that, at z similar to 0.7, many objects already lie on the present-day mass-metallicity relation, but with a significant tail of high-mass low-metallicity galaxies that is not seen in the nearby Universe. Similar modelling of the evolution of galaxies in the nearby MaNGA survey allows us to reconstruct their properties at z similar to 0.7. Once selection criteria similar to those of LEGA-C are applied, we reassuringly find that the MaNGA galaxies populate the mass-metallicity plane in the same way at z similar to 0.7. Matching the LEGA-C sample to their mass-metallicity 'twins' in MaNGA at this redshift, we can explore the likely subsequent evolution of individual LEGA-C galaxies. Galaxies already on the present-day mass-metallicity relation form few more stars and their discs fade, so they become smaller and more bulge-like. By contrast, the high-mass low-metallicity galaxies grow their discs through late star formation, and evolve rapidly to higher metallicities due to a cut-off in their wind-driven mass-loss. There are significant indications that this late cut-off is associated with the belated end of strong active galactic nucleus activity in these objects.