THE REVISED MG-2 INDEX AS A METALLICITY INDICATOR FOR STELLAR-SYSTEMS- GIANT ELLIPTIC GALAXIES AND BULGES

We provide a new calibration of the metallicity dependence of the Mg-2 index in stellar systems. It is based on new isochrones by the Padova group and the latest stellar spectral libraries. Since the Mg-2 line is one of the strongest absorption lines in the optical spectrum, it i s often used to determine metallicities of composite stellar systems a ssuming a certain age. This paper allows one to do this, with an uncer tainty of about 0.1 in log Z, for an age range between 8 and 17 Gyr. W e estimate that the uncertainties of the current Mg, calibrations in t he literature amount to another 0.15 in log Z. Using our calibration w e find, under conservative assumptions, that the large majority of ell iptical galaxies, and also many bulges of spiral galaxies, must have M g abundances larger than solar; this conclusion depends very little on their star formation history. If [Mg/Fe] = 0, the same can be said fo r total metallicities. If we assume that [Mg/Fe] = 0.45, only about 38 % of the ellipticals must have metallicities (Z) larger than solar.