Mapping the Metals in Spiral Galaxies with the Hubble and Roman Space Telescopes

Metals encode the evolutionary histories of galaxies, tracing past accretion events and gaseous outflows. The spatial distribution of metals in galaxies is therefore a powerful constraint for galaxy formation models. We present a spatially resolved accounting of metals in the nearby massive spiral M31, where resolved-star photometry across ~1/3 of the disk was obtained by the Panchromatic Hubble Andromeda Treasury (PHAT) survey. Using the star formation and enrichment histories inferred from the PHAT data, we calculate the total stellar mass formed, the history of metal production by those stars, and the present-day metal mass locked into stars. Combining the stellar metals with the gas-phase metallicity gradient and maps of dust and atomic hydrogen, we conduct a census of metals present in the PHAT footprint, and compare to the total metal mass produced. We find that 62% of the metal mass has been lost via outflows, and that net metal loss is required under any model assumptions. We further show that metals produced in the last 1.5 Gyr have been redistributed within M31's disk. The Roman Space Telescope's wide field of view will obtain resolved-star photometry over large areas of other nearby spirals, mapping stellar metallicity and mass extending into the stellar halos. These data will expand spatially resolved analyses of spiral galaxies' metal content beyond M31, enabling new constraints on the impact of galaxy assembly histories on chemical evolution processes.