The stellar chemical abundances of simulated massive galaxies at z = 2

We analyze the stellar abundances of massive galaxies (log M_*/M_⊙ > 10.5) at z = 2 in the IllustrisTNG simulation with the goal of guiding the interpretation of current and future observations, particularly from JWST. We find that the effective size, R_e, of galaxies strongly affects the abundance measurements: both [Mg/H] and [Fe/H] are anti-correlated with R_e, while the relative abundance [Mg/Fe] slightly increases with R_e. The α enhancement as tracked by [Mg/Fe] traces the formation timescale of a galaxy weakly, and mostly depends on R_e. Aperture effects are important: measuring the stellar abundances within 1 kpc instead of within R_e can make a large difference. These results are all due to a nearly universal, steeply declining stellar abundance profile that does not scale with galaxy size - small galaxies appear metal-rich because their stars live in the inner part of the profile where abundances are high. The slope of this profile is mostly set by the gas-phase abundance profile and not substantially modified by stellar age gradients. The gas-phase abundance profile, in turn, is determined by the strong radial dependence of the gas fraction and star formation efficiency. We develop a simple model to describe the chemical enrichment, in which each radial bin of a galaxy is treated as an independent closed-box system. This model reproduces the gas-phase abundance profile of simulated galaxies, but not the detailed distribution of their stellar abundances, for which gas and/or metal transport are likely needed.