Integrated spectroscopy of bulge globular clusters and fields - II. Implications for population synthesis models and elliptical galaxies

An empirical calibration is presented for the synthetic Lick indices (e.g. Mg-2, [Fe], Hbeta, etc.) of Simple Stellar Population (SSP) models that for the first time extends up to solar metallicity. This is accomplished by means of a sample of Milky Way globular clusters (GCs) whose metallicities range from similar toZ./30 to Z similar to Z, thanks to the inclusion of several metal rich clusters belonging to the Galactic bulge (e.g., NGC 6553 and NGC 6528). This metallicity range approaches the regime that is relevant for the interpretation of the integrated spectra of elliptical galaxies. It is shown that the spectra of both the globular clusters and the Galactic bulge follow the same correlation between magnesium and iron indices that extends to elliptical galaxies, showing weaker iron indices at given magnesium indices with respect to the predictions of models that assume solar-scaled abundances. This similarity provides robust empirical evidence for enhanced [alpha/Fe] ratios in the stellar populations of elliptical galaxies, since the globular clusters are independently known to have enhanced [alpha/Fe] ratios from spectroscopy of individual stars. We check the uniqueness of this alpha-overabundance solution by exploring the whole range of model ingredients and parameters, i.e. fitting functions, stellar tracks, and the initial mass function (IMF). We argue that the standard models (meant for solar abundance ratios) succeed in reproducing the Mg-Fe correlation at low metallicities ([Z/H]less than or similar to - 0.7) because the stellar templates used in the synthesis are Galactic halo stars that actually are alpha-enhanced. The same models, however, fail to predict the observed Mg-Fe pattern at higher metallicities ([Z/H]; - 0.7) (i.e., for bulge clusters and ellipticals alike) because the high-metallicity templates are disk stars that are not alpha-enhanced. We show that the new set of SSP models which incorporates the dependence on the [alpha/Fe] ratio (Thomas et al. 2003) is able to reproduce the Mg and Fe indices of GCs at all metallicities, with an alpha-enhancement [alpha/Fe] = +0.3, in agreement with the available spectroscopic determinations. The Hbeta index and the higher-order Balmer indices are well calibrated, provided the appropriate morphology of the Horizontal Branch is taken into account. In particular, the Balmer line indices of the two metal rich clusters NGC 6388 and NGC 6441, which are known to exhibit a tail of warm Horizontal Branch stars, are well reproduced. Finally, we note that the Mg indices of very metal-poor ([Z/H]less than or similar to - 1.8) populations are dominated by the contribution of the lower Main Sequence, hence are strongly affected by the present-day mass function of individual globular clusters, which is known to vary from cluster to cluster due to dynamical effects.