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

We analyse the stellar abundances of massive galaxies (log  M */M⊙ > 10.5) at redshift, 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 anticorrelated 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 time-scale 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. [ABSTRACT FROM AUTHOR]