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, which is a phenomenon that was not foreseen in existing models of Galactic chemical evolution (GCE). If this phenomenon is confirmed in future studies, we can show that this effect, at least in part, is due to large-scale winds that once enriched the disk. These set up a steep abundance gradient in the inner disk (R-GC less than or similar to 14 kpc). At the close of the wind phase, chemical enrichment through accretion of metal-poor material from the halo to the disk gradually reduced the metallicity of the inner region, whereas a slight increase in the metallicity proceeded beyond the solar circle. Our "wind+infall" model accounts for a flattening of the abundance gradient in the inner disk, in agreement with observations. Accordingly, we propose that enrichment by large-scale winds is a crucial factor in chemical evolution in the disk. We anticipate that rapid flattening of the abundance gradient is the hallmark of disk galaxies with significant central bulges.