To conclude the discussion of the observational constraints on the metallicity of the outer regions of spiral galaxies I include two additional topics that have been addressed by recent work. Future investigations of the chemical abundance properties of the outer disks of spiral galaxies will benefit from the study of the spatially-resolved gas content (both atomic and molecular), which will help to shed light on the interplay between chemical and secular evolution of the outer disks and accretion events or gas flows taking place in the very outskirts of galaxies.
4.1. Relation Between Metallicity and Surface Brightness Breaks
Recent work by Marino et al. (2016) probed into the possible connection between outer disk abundance gradients and surface brightness profile breaks that characterize the disks of spiral galaxies, as discussed elsewhere in this volume. These authors focused on 131 galaxies, extracted from the larger CALIFA sample, displaying either Type II (‘down-bending’) or Type III (‘up-bending’) surface brightness profiles. A correlation was found in the case of Type III galaxies. At lower masses, log(M / M⊙) < 10, a modest flattening in the g′−r′ colour tends to be a common feature, together with a mild flattening of the O/H gradient, while at higher masses both colour and O/H radial profiles display a pronounced flattening. The different behaviour detected for the Type III galaxies is tentatively attributed by Marino et al. (2016) to a downsizing effect, such that the higher-mass systems have already experienced a phase of inside-out growth, while for the smaller systems an enhanced disk buildup phase is more recent or still on-going.
4.2. An Analogy with Low Surface Brightness Galaxies?
It has been pointed out by some authors (Thilker et al. 2007; Bresolin et al. 2009) that the structural parameters and the star-formation properties of outer spiral disks, such as the low mass surface densities and low star formation rates, resemble those observed in low surface brightness (LSB) galaxies. We can then ask the question whether this analogy extends to the chemical abundance properties. In particular, does a low star formation efficiency (Wyder et al. 2009) lead to a flat abundance distribution also in the case of LSB galaxies? The question remained without a clear answer until recently, because very few studies addressed the gas-phase chemical abundance properties of this type of galaxies, and in particular their abundance gradients, in part due to observational challenges. Bresolin and Kennicutt (2015) measured H ii region oxygen abundances for a sample of 10 LSB spiral galaxies, and investigated the presence of radial abundance gradients in this sample. They found that LSB galaxies do display radial abundance gradients that, when normalized by the effective radii, are consistent with those measured for high surface brightness galaxies. Thus, the analogy between LSB galaxies and the outer disks of spiral galaxies does not seem to extend to the chemical abundance properties, despite the similarities outlined above. This result suggests that the chemical evolution of LSB galaxies proceeds in a similar fashion to the high surface brightness galaxies, albeit at a slower pace due to the lower star formation rates, while the outer disks probably follow a different evolutionary path. The latter possibility is addressed in the following section.