Metallicities in the Outer Regions of Spiral Galaxies

TL;DR

This paper reviews recent observational data on the metallicity of outer spiral galaxy regions, highlighting flat radial metallicity profiles and discussing potential mechanisms like metal mixing and gas infall influencing chemical evolution.

Contribution

It compiles and analyzes recent observational evidence on metallicity distributions in outer spiral galaxy regions, emphasizing the role of metal mixing and gas infall.

Findings

Metallicity profiles flatten at large galactocentric distances.

Enrichment levels exceed expectations based on gas mass and star formation.

Metal-enriched gas infall may influence outer disk chemical evolution.

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 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.