2.3. Stellar Photometry and Color-Magnitude Diagrams
Color-magnitude diagrams of galaxy populations can provide some information on the metallicity (or metallicity spread) of a stellar population, since features in the CMD, such as the color of the red giant branch, can vary with metallicity. Unfortunately, these features also vary with age of the populations, so there is a degeneracy between age and metallicity in the CMD (and in composite colors). Systematic uncertainties may also be introduced by variations in element abundance ratios and by reddening. Thus, color-magnitude diagrams are at best indicative of metallicities.
2.4. Spectrum Synthesis of Stellar Populations
Spectrum synthesis for deriving metallicities has been applied mostly to elliptical galaxies. Since I am not an expert on this, I refer the reader to Guy Worthey's review in Henry & Worthey (1999) and references therein for details. Needless to say, spectrum synthesis is an intricate and uncertain art; the results depend on the choices of spectral templates, element abundance ratios, and star formation histories. Worthey points out that 25 spectral indices are available to derive metallicities and ages for old stellar populations. (In 1986 there were 11 indices in the Lick spectral index system [Burstein, Faber, & González 1986].) Unfortunately, most of these vary in a degenerate way with age and metallicity of the stellar population. The best indices for breaking this degeneracy are (1) H or a higher Balmer line, which are more sensitive to age; and (2) Fe4668 (which is actually a C2 feature), more sensitive to metallicity. In addition, the Mg index (covering Mg b and Mg2 features between 5150 and 5200 Å) provides information on Mg/Fe.