The gas metallicity gradient and the star formation activity of disc galaxies

We study oxygen abundance profiles of the gaseous disc components in simulated galaxies in a hierarchical universe. We analyse the disc metallicity gradients in relation to the stellar masses and star formation rates of the simulated galaxies. We find a trend that galaxies with low stellar masses have steeper metallicity gradients than galaxies with high stellar masses at z ∼ 0. We also detect that the gas-phase metallicity slopes and the specific star formation rate (sSFR) of our simulated disc galaxies are consistent with recently reported observations at z ∼ 0. Simulated galaxies with high stellar masses reproduce the observed relationship at all analysed red shifts and have an increasing contribution of discs with positive metallicity slopes with increasing red shift. Simulated galaxies with low stellar masses have a larger fraction of negative metallicity gradients with increasing red shift. Simulated galaxies with positive or very negative metallicity slopes exhibit disturbed morphologies and/or have a close neighbour. We analyse the evolution of the slope of the oxygen profile and sSFR for a gas-rich galaxy–galaxy encounter, finding that this kind of event could generate either positive or negative gas-phase oxygen profiles depending on their state of evolution. Our results support claims that the determination of reliable metallicity gradients as a function of red shift is a key piece of information in understanding galaxy formation and setting constraints on the subgrid physics.