2.3 Photometry of Resolved Stellar Populations
Colour-magnitude diagrams (CMDs) provide a photometric estimate of the stellar abundances, usually assumed to be represented by iron, from the colour of the red giant branch (RGB). Stars more massive than approximately 2 solar masses never ascend it and thus stars on the RGB have ages from approximately 1 to 15 Gyr, and their abundance reflects the ISM abundance of Fe when the stars were born. The width of the RGB provides a measure of the metallicity spread, of course convolved with the broadening from age dispersion and the photometric error function (see e.g. Lee et al. 1993).
The calibration of this method rests upon the comparison of the RGBs with those of old Galactic globular clusters with metallicity estimated from integrated spectroscopy or spectroscopy of individual giant stars (see Da Costa and Armandroff 1990). Therefore the RGB colour of an old population measures an average metallicity and is unable to reveal abundance ratios. Moreover, comparison with old Galactic globular clusters rests upon some degree of similarity between the two kinds of objects. However since it is now evident that the bulk of the stellar population in many dEs is younger than that of Galactic globular clusters, and has a considerable abundance spread, one must be aware that physically different properties of globular clusters and galaxies may possibly cause systematic errors.
Other metallicity indicators include the colour and morphology of the horizontal branch (HB) in the CMD (Grebel 1998), although this cannot be uniquely translated into metallicities due to poorly understood ``second parameter effects'', also known from the study of HBs in globular clusters. Intermediate in character between photometric and spectroscopic abundance determinations is the fraction of carbon (C) stars among the late type giants, and the number of carbon rich (WC) to nitrogen rich (WN) Wolf-Rayet (WR) stars. Such indicators give only very crude guesses of the the metallicity. Moreover, each population sample stellar populations of different age and are not directly comparable.
A major drawback in using CMDs for abundance determinations is that it is limited to nearby galaxies, basically the Local Group and its immediate surroundings. Photometric abundances can also be derived using Strömgren photometry. However the relatively narrow filters employed, limit the usefulness of Strömgren photometry for extragalactic objects, a situation that could change with the new generation of large telescopes. There are also other photometric systems designed for metallicity sensitivity, e.g. the intermediate wide Washington system.