The origin of correlations between mass, metallicity and morphology in galaxies from the EAGLE simulation

Observed and simulated galaxies exhibit correlations between stellar mass, metallicity and morphology. We use the EAGLE cosmological simulation to examine the origin of these correlations for galaxies in the stellar mass range $10^9~\rm{M_\odot} \leqslant\ M_\star \leqslant 10^{10}~\rm{M_\odot}$, and the extent to which they contribute to the scatter in the mass-metallicity relation. We find that rotationally supported disc galaxies have lower metallicity than dispersion supported spheroidal galaxies at a given mass, in agreement with previous findings. In EAGLE, this correlation arises because discs form stars at later times, redshift $z\leqslant 1$, from the accretion of low-metallicity gas, whereas spheroidal galaxies galaxies typically form stars earlier, mainly by consumption of their gas reservoir. The different behaviour reflects the growth of their host dark matter halo: at a given stellar mass, disc galaxies inhabit dark matter haloes with lower mass that formed later compared to the haloes of spheroidal galaxies. Halo concentration plays a secondary role.