The Stellar Chemical Abundances of Simulated Massive Galaxies at z=2

Stellar metallicities reflect the relative abundances of elements heavier than hydrogen and helium enclosed in stars in galaxies. Abundance ratios, such as [Fe/H] and [Mg/Fe], can characterize galaxy evolution timescales and trace the history of a galaxy. Such ratios have been observed in many neighboring galaxies. For high-redshift galaxies, however, it is much more difficult to survey metals. The James Webb Space Telescope will soon enable the first high-redshift measurements of stellar metallicities. Here we analyze the stellar abundances of massive galaxies (log M_∗/M_⊙>10.5) at 0 ≤ z ≤ 2 in the IllustrisTNG simulation with the goal of guiding the interpretation of current and future observations, particularly from the James Webb Space Telescope. 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 only weakly, and mostly depends on the effective size. 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 the existence of a nearly universal stellar abundance profile that is steeply declining and does not scale with galaxy sizes, so that small galaxies appear metal-rich because their stars live in the inner part of the profile where the abundances are high. We investigate the physical origin of this profile, and conclude that it is set by the gas-phase abundance profile at the time when most of the stars were formed. The gas-phase abundance profile, in turn, is determined by the strong radial dependence of the gas fraction and star formation efficiency, with only a minor role played by outflows and other processes. 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 is able to reproduce 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. The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. AST-2050813, and by the Smithsonian Institution.