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.