ON THE TREND OF [MG FE] AMONG GIANT ELLIPTIC GALAXIES/

We revisit the problem of the flat slope of the Mg-2 versus < Fe > rel ationship found for nuclei of elliptical galaxies (Faber et al. 1992; Worthey et al. 1992; Carollo et al. 1993; Davies et al. 1993), indicat ing that the Mg/Fe ratio should increase with galactic luminosity and mass. We transform the abundance of Fe, as predicted by classic wind m odels and alternative models for the chemical evolution of elliptical galaxies, into the metallicity indices Mg-2 and < Fe >, by means of th e more recent index calibrations and show that none of the current mod els for the chemical evolution of elliptical galaxies is able to repro duce exactly the observed slope of the < Fe > versus Mg-2 relation, al though the existing spread in the data makes this comparison quite dif ficult. In other words, we can not clearly discriminate between models predicting a decrease (classic wind model) or an increase of such a r atio with galactic mass. The reason for this resides in the fact that the available observations show a large spread due mostly to the error s in the derivation of the < Fe > index. In our opinion this fact prev ents us from drawing any firm conclusion on the behaviour of Mg and Fe in these galaxies. Moreover, as already shown by other authors, one s hould be careful in deriving trends in the real abundances just from t he metallicity indices, since these latter depend also on other physic al parameters than the metallicity. This is an important point since a bundance ratios have been proven to represent strong constraints for g alaxy formation mechanisms.