Evidence of Early Enrichment of the Galactic Disk by Large-Scale Winds
  Large-scale homogeneous surveys of Galactic stars may indicate that the
elemental abundance gradient evolves with cosmic time, a phenomenon that was
not foreseen in existing models of Galactic chemical evolution (GCE). If the
phenomenon is confirmed in future studies, we show that this effect, at least
in part, is due to large-scale winds that once enriched the disk. These set up
the steep abundance gradient in the inner disk (R <14 kpc). At the close of the
wind phase, chemical enrichment through accretion of metal-poor material from
the halo onto the disk gradually reduced the metallicity of the inner region,
whereas a slow increase in the metallicity proceeded beyond the solar circle.
Our "wind+infall" model accounts for flattening of the abundance gradient in
the inner disk, in good agreement with observations. Accordingly, we propose
that enrichment by large-scale winds is a crucial factor for chemical evolution
in the disk. We anticipate that rapid flattening of the abundance gradient is
the hallmarks of disk galaxies with significant central bulges.
