Metallicity Gradients in Modern Cosmological Simulations I: Tension Between Smooth Stellar Feedback Models and Observations

The metallicity of galaxies, and its variation with galactocentric radius, provides key insights into the formation histories of galaxies and the physical processes driving their evolution. In this work, we analyze the radial metallicity gradients of star forming galaxies in the EAGLE, Illustris, IllustrisTNG, and SIMBA cosmological simulations across a broad mass ($10^{8.0}M_\odot\leq M_\star \lesssim10^{12.0}M_\odot$) and redshift ($0\leq z\leq8$) range. We find that all simulations predict strong negative (i.e., radially decreasing) metallicity gradients at early cosmic times, likely due to their similar treatments of relatively smooth stellar feedback not providing sufficient mixing to quickly flatten gradients. The strongest redshift evolution occurs in galaxies with stellar masses of $10^{10.0}-10^{11.0}M_\odot$, while galaxies with stellar masses $< 10^{10}M_\odot$ and $>10^{11}M_\odot$ exhibit weaker redshift evolution. Our results of negative gradients at high-redshift contrast with the many positive and flat gradients in the $1<z<4$ observational literature. At $z>6$, the negative gradients observed with JWST and ALMA are flatter than those in simulations, albeit with closer agreement than at lower redshift. Overall, we suggest that these smooth stellar feedback galaxy simulations may not sufficiently mix their metal content radially, and that either stronger stellar feedback or additional subgrid turbulent metal diffusion models may be required to better reproduce observed metallicity gradients.