Metallicities in the Outer Regions of Spiral Galaxies
  The analysis of the chemical composition of galaxies provides fundamental
insights into their evolution. This holds true also in the case of the outer
regions of spiral galaxies. This Chapter presents the observational data,
accumulated in the past few years mostly from the analysis of HII region
spectra, concerning the metallicity of the outer disks of spirals that are
characterized by extended HI envelopes and low star formation rates. I present
evidence from the literature that the metal radial distribution flattens at
large galactocentric distances, with levels of enrichment that exceed those
expected given the large gas mass fractions and the weak star formation
activity. The interpretation of these results leads to speculations regarding
mechanisms of metal mixing in galactic disks and the possibility that
metal-enriched gas infall plays a role in determining the chemical evolution of
the outskirts of spirals.
