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

Abstract

ABSTRACT 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 ∼ 0.7. We confirm that, at z ∼ 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 ∼ 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 ∼ 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.