3.2. External Galaxies
Spectroscopy of individual stars in local group galaxies M31 and M33 has become possible in recent years for supergiants, typically A type. Like B main-sequence stars, A supergiants suffer from serious non-LTE effects in the outer photosphere, but lines can be chosen that form deep in the photosphere, and a partial non-LTE analysis can be attempted for other interesting lines. The resultant accuracy can be ± 0.2 dex (Venn 1995, 1999). For M31, the [O/H] gradient obtained from A supergiants is consistent within the errors with that obtained from nebular studies (McCarthy et al. 1998). For M33, based on four B supergiants, Monteverde et al. (1997) obtain an [O/H] gradient of -0.16 ± 0.06 dex kpc-1, which is also similar to nebular results.
Most stellar abundance work in external galaxies relies on the colors of red giant stars from older populations. After spectroscopic abundance work in globular clusters showed a wide range of metallicities among clusters, it was obvious from published color-magnitude diagrams (CMDs) that the red giant branches are redder at progressively higher metallicities. This finding can be used as an abundance indicator, especially in the HST era where the tip of the red giant branch can be seen at distances of ~ 10 Mpc. A younger age population has a somewhat bluer giant branch, but this effect is fairly minimal and in some cases negligible when the age is already known. Crowding of stars excludes near-nuclear regions from CMD analysis.
The halo of M31 has been examined by Durrell, Harris, & Pritchet (1994) and Rich, Mighell, & Neill (1996) from HST optical colors, with the conclusion that, like the Milky Way, no abundance gradient is apparent. Unlike the Milky Way, the average abundance of the stars is [Fe/H] -0.6 (Durrell et al.) or even higher (Rich et al.). Grillmair et al. (1996) derive an abundance distribution (number of stars per interval [Fe/H]) for the outer disk of M31 that is identical within the errors with the abundance distribution of the solar neighborhood. Elliptical galaxies NGC 5128 (Soria et al. 1996) and M32 (Grillmair et al. 1996) have also been studied in this fashion, but only at a single radius so far, so we await further data before we can draw conclusions about abundance profiles.