Turning Points in the Age-Metallicity Relations -- Created by Late
  Satellite Infall and Enhanced by Radial Migration
  The present-day Age-Metallicity Relation (AMR) is a record of the star
formation history of the Galaxy, as this traces the chemical enrichment of the
gas over time. We use a zoomed-in cosmological simulation that reproduces key
signatures of the Milky Way (MW), g2.79e12 from the NIHAO-UHD project, to
examine how stellar migration and satellite infall shape the AMR across the
disk. We find in the simulation, similar to the MW, the AMR in small spatial
regions (R, z) shows turning points that connect changes in the direction of
the relations. The turning points in the AMR in the simulation, are a signature
of late satellite infall. This satellite infall has a mass radio similar as
that of the Sagittarius dwarf to the MW (~ 0.001). Stars in the apex of the
turning points are young and have nearly not migrated. The late satellite
infall creates the turning points via depositing metal-poor gas in the disk,
triggering star formation of stars in a narrow metallicity range compared to
the overall AMR. The main effect of radial migration on the AMR turning points
is to widen the metallicity range of the apex. This can happen when radial
migration brings stars born from the infallen gas in other spatial bins, with
slightly different metallicities, into the spatial bin of interest. These
results indicate that it is possible that the passage of the Sagittarius dwarf
galaxy played a role in creating the turning points that we see in the AMR in
the Milky Way.
