On the trend of (Mg/Fe) among giant elliptical galaxies

We revisit the problem of the flat slope of the Mg2 versus relationship found for nuclei of elliptical galax- ies (Faber et al. 1992; Worthey et al. 1992; Carollo et al. 1993; Davies et al. 1993), indicating that the Mg/Fe ratio should in- crease with galactic luminosity and mass. We transform the abundance of Fe, as predicted by classic wind models and alter- native models for the chemical evolution of elliptical galaxies, into the metallicity indices Mg2 and , by means of the more recent index calibrations and show that none of the current models for the chemical evolution of elliptical galaxies is able to reproduce exactly the observed slope of the versus Mg2 relation, although the existing spread in the data makes this comparison quite difficult. In other words, we can not clearly discriminate between models predicting a decrease (classic wind model) or an increase of such a ratio with galactic mass. The reason for this resides in the fact that the available observations show a large spread due mostly to the errors in the derivation of the index. In our opinion this fact prevents us from drawing any firm conclusion on the behaviour of Mg and Fe in these galaxies. Moreover, as already shown by other authors, one should be careful in deriving trends in the real abundances just from the metallicity indices, since these latter depend also on other physical parameters than the metal- licity. This is an important point since abundance ratios have been proven to represent strong constraints for galaxy forma- tion mechanisms.