Published in "Outskirts of Galaxies", Astrophysics and Space Science Library, Volume 434. ISBN 978-3-319-56569-9. Springer International Publishing AG, 2017, p. 145.
https://arxiv.org/abs/1612.05278

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Abstract: 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 H ii region spectra, concerning the metallicity of the outer disks of spirals that are characterized by extended H i 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.

Table of Contents

• INTRODUCTION
• MEASURING NEBULAR ABUNDANCES
• CHEMICAL ABUNDANCES OF H II REGIONS IN OUTER DISKS
• Early Work
• M83: a Case Study
• Other Systems
• Results from Galaxy Surveys
• Nitrogen Abundances
• ADDITIONAL CONSIDERATIONS
• Relation Between Metallicity and Surface Brightness Breaks
• An Analogy with Low Surface Brightness Galaxies?
• THE EVOLUTIONARY STATUS OF OUTER DISKS
• Flattening the Gradients
• Bringing Metals to the Outer Disks
• Mixing
• Enriched Infall
• CONCLUSION
• REFERENCES