UC Davis

Understanding galaxy formation and evolution requires characterizing elemental abundance distributions in galaxies. Chemical tagging is a useful tool to understand the evolutionary history of the Milky Way (MW) because it takes advantage of the fact that stellar abundances at present-day are identical to the abundances with which stars formed. Thus, stellar elemental abundances provide an observable that can, in principle, be used to determine the birth location of a star.

Elemental abundance observations of gas and stars in the MW and nearby galaxies show that the median elemental abundance typically decreases with increasing radius with little scatter about the mean at each radius. These observations are robust for nearby galaxies, however they are less certain at high redshift because of the observational difficulties associated with obtaining spatially resolved spectra of high redshift galaxies. As a result, many galactic elemental evolution models rely on assumptions that observed properties of the MW and nearby external galaxies, crucially the lack of azimuthal scatter in abundances, are a time-independent property. Alternatively, some researchers use physical models of galaxy evolution to which they fit a multitude of free parameters such that their model recreates the present-day observed properties of the MW and external galaxies. However, these models typically rely on overly-simplified assumptions of physical processes and include multiple free functions unconstrained by physical models.

Cosmological zoom-in hydrodynamic simulations can help to precisely characterize the spatial distribution of stellar elemental abundances in MW-mass galaxies across cosmic time. Our results challenge the status quo of galactic elemental evolution models. We find that the minimal azimuthal abundance variations in MW-mass galaxies are not an intrinsic property, rather galaxies evolve from an epoch of extreme azimuthal abundance variation to their present-day state. Additionally, radial abundance gradients in galaxies were nearly non-existent at sufficiently high redshifts, despite being relatively strong at present-day. These results suggest a higher degree of difficulty in chemically tagging stars than previously assumed. Thus, chemical tagging techniques may only be able to loosely constrain birth locations of older stellar populations. To help address this, we characterize the scale of radial redistribution of stars in simulated MW-mass galaxies as a function of both stellar age and stellar location. Accounting for stellar radial redistribution as a function of present-day radial location can help break degeneracies of birth location for older stellar populations. Our results on stellar radial redistribution suggest that inferring a time-dependent radial abundance gradient of the MW from present-day observations is non-trivial; at present-day, old stellar populations are often at very different radii than the radii at which they formed. Therefore, more emphasis must be placed on models derived from cosmological simulations.