James Johnson (Carnegie Science Observatories): Inflows, Outflows, and Radial Flows: The Puppet Masters of Chemical Evolution in the Milky Way

Thursday 3 Apr 2025, 14:00 → 15:00 Europe/Stockholm

90103Å

Title: Inflows, Outflows, and Radial Flows: The Puppet Masters of Chemical Evolution in the Milky Way
Speaker: James Johnson
Affiliation: Carnegie Science Observatories
Time: Thursday 3 April 2025, 1400 to 1500
Location: 90103Å

Abstract:

Measurements of the ages of stars have revolutionized our understanding of chemical enrichment in the Milky Way. Multiple methods of measurement tell a consistent story that old stellar populations are more metal-rich than expected based on recent models of Galactic chemical evolution. Across much of the disk, no substantial decline in stellar population metallicity (up to about ~0.1 dex) is observed up to ages of at least ~5-6 Gyr. In this talk, I will demonstrate that these results favor a scenario in which the gas-phase metallicity reaches a chemical quasi-equilibrium, at which abundances relative to hydrogen vary with radius in the disk but evolve slowly in time. The empirical evidence for this equilibrium is compelling, but its origin in physically motivated models is uncertain. Two possibilities are outflows ejecting matter from the interstellar medium to the circumgalactic medium and radial gas flows transferring matter toward the Galactic center. With only outflows, models consistent with the observations have a higher ejection efficiency in the outer disk than the inner disk. With only radial gas flows, models consistent with the observations experience minimal acceleration and have relatively flat velocity profiles with speeds of order -1 km/s. In both cases, the radial metallicity gradient reflects a decline in the number of stars formed per unit mass accreted toward the outer disk. The chemical equilibrium state is reached quickly if the depletion time due to star formation, outflows, and radial gas flows is no more than ~4-5 Gyr.