Using current models of spectrophotometric properties of single age, single
metallicity stellar populations I have computed the Mg2, Hbeta, Fe52 and Fe53
line strengths for stellar populations with a metallicity spread. The
comparison of these models with the nuclear indices of early type galaxies
yield the following major conclusions. The metallicity distribution of the
closed box, simple model for the chemical evolution of galaxies is not able to
account for Mg2 and Fe52, Fe53 values in excess of 0.27,3 and 2.7,
respectively, which are observed in the nuclei of a large fraction of
Ellipticals. To reproduce the line strengths in these galaxies high average
metallicities, small metallicity dispersion and old ages are required. In
particular, Mg2 values of 0.3 are reproduced only with a metallicity
distribution ranging from 0.5\Zsun to 3\Zsun, and 15 Gyr old stellar
populations. I interpret the data as indicating that the gas out of which the
nuclei of ellipticals formed was pre-enriched, to larger metallicities for
increasing \Mgtw.The presence of a metallicity dispersion does not alter the
relation between Mg2 and Iron indices with respect to the SSP models. Thus, the
need for a Mg/Fe overabundance in the strongest lined galaxies is confirmed,
and I present a simple way to estimate the [Mg/Fe] ratio on the basis of
existing models with solar abundance ratios.Comment: 16 pages, Latex File+mnrasl.sty, 8 Postscript figures, to appear in
MNRA
