3. The average metallicity of a composite stellar population
In order to compare model results with observations we should first calculate the average abundances of Mg and Fe of the composite stellar population of the given galaxies. The average metallicity or abundance in general which should be compared with the indices should be averaged on the visual light, namely:
where is the number of stars in the abundance interval and luminosity . On the other hand, the real average abundance should be the mass-averaged one, namely:
where the subscript 1 refers to the specific time (the present time) and is the total mass of stars ever born. Here we will use Eq. (5) in order to compare model results with indices and the reason is that we want to compare our results with previous ones (Matteucci 1994; MG95), and because for giant ellipticals the difference between the mass-averaged metallicity and the luminosity-averaged one is negligible (Yoshii and Arimoto, 1987; Gibson 1997). On the other hand, in smaller systems the difference between the two abundances is not negligible due to the contribution to the light of low metallicity red giants (Greggio 1996). We will show in Sect. 4 that in the cases studied here .
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998