2.5 Estimates from spectroscopy and photometry of integrated light
In remote galaxies without H II regions, integrated spectroscopy may be used to derive metal abundances. This method have been widely employed e.g. for giant ellipticals. Absorption line features, such as the Mg2 band, are compared to populations of observed or model stars to estimate metallicities (cf. Mould 1978; Worthey 1994). This method is difficult to use since the interplay between stellar population mixes, star formation history and stellar initial mass function (IMF), which influences the spectral shape and the absorption line strengths, is not known a priori; and it has turned out to be non trivial to transform the observed Mg2 strengths into metallicity. In addition, there are indications of nonsolar [Mg/Fe] values (e.g. Worthey et al. 1992). Since, in general, the surface brightnesses of dwarf galaxies decrease with decreasing luminosity, even integrated spectroscopy is difficult and time consuming, and practically impossible for the really faint dwarfs.
Integrated photometry is a poor-man's tool. When no spectroscopic information may be obtained and the galaxy is too distant to resolve the stellar population, some constraints may be put on the metallicity (cf. Sect. 4.2.2) from integrated photometry under certain assumptions. However, as for the case of integrated spectroscopy (but to a much greater extent), the assumed population mix, star formation history, IMF, internal extinction etc., influence the broad-band colours and make the derived metallicity very uncertain. The infamous age-metallicity degeneracy is here acting in its full power. If some of these parameters can be constrained, this method may be used in a statistical sense for large samples.