The role of environment in the mass–metallicity relation

Using a sample of 57 377 star-forming galaxies drawn from the Sloan Digital Sky Survey, we study the relationship between gas-phase oxygen abundance and environment in the local Universe. We find that there is a strong relationship between metallicity and environment such that more metal-rich galaxies favour regions of higher overdensity. Furthermore, this metallicity–density relation is comparable in strength to the colour–density relation along the blue cloud. After removing the mean dependence of environment on colour and luminosity, we find a weak, though significant, residual trend between metallicity and environment which is largely driven by galaxies in high-density regions, such as groups and clusters. We discuss the potential source of this relationship between metallicity and local galaxy density in the context of feedback models, with special attention paid to quantifying the impact of environment on the scatter in the mass–metallicity relation. We find that environment is a non-negligible source of scatter in this fundamental relation, with ≳15 per cent of the measured scatter correlated with environment.