Ages and metallicities in elliptical galaxies from the H_beta, {Fe}, and Mg_2 diagnostics

Systematic variations in the line strength indices H-beta, Mg-2, and [Fe] are observed across elliptical galaxies and limited to the central regions passing from one object to another. Furthermore, since the gradients in Mg-2 and [Fe] have often different slopes arguments are given for an enhancement of Mg (alpha-elements in general) with respect to Fe toward the center of these galaxies. Finally, the inferred degree of enhancement seems to increase passing from dwarfs to massive ellipticals. In this study we have investigated the ability of the H-beta, Mg-2 and [Fe] diagnostics to assess the metallicity, [Mg/Fe] ratios, and ages of elliptical galaxies. To this aim, first we derive basic calibrations for the variations delta H-beta, delta Mg-2 and delta[Fe] as a function of variation in age Delta log(t), metallicity Delta log(Z/Z.), and Delta[Mg/Fe]. Second, examining the gradients observed in a small sample of galaxies, we analyze how the difference delta H-beta, delta Mg-2, and delta[Fe] between the external and central values of each index translates into Delta [Mg/Fe], Delta log(Z/Z.), and Delta log(t). We find that out of six galaxies under examination, four have the nuclear region more metal-rich, more enhanced in alpha-elements, and younger (i.e. containing a significant fraction of stars of relatively young age) than the external regions. In contrast the remaining two galaxies have the nuclear region more metal-rich, more enhanced in alpha-elements but marginally older than the external zones. Third, we explore the variation from galaxy to galaxy of the nuclear values of H-beta, Mg-2 and [Fe] limited to a sub-sample of the Gonzales (1993) list. The differences delta H-beta, delta Mg-2, and delta [Fe] are converted into the differences Delta log(t), Delta log(Z/Z.), and Delta[Mg/Fe]. Various correlations among the age, metallicity, and enhancement variations are explored. In particular we thoroughly examine the relationships Delta log(t) - M-V, Delta log(Z/Z.) - M-V, and Delta[Mg/Fe] - M-V. It is found that a sort of age limit is likely to exist in the Delta log(t) - M-V plane, traced by galaxies with mild or no sign of rejuvenation. In these objects, the duration of the star forming activity is likely to have increased at decreasing galactic mass. Limited to these galaxies, the mass-metallicity sequence implied by the color-magnitude relation is recovered, likewise for the alpha-enhancement-luminosity relation suggested by the gradients in Mg-2 and [Fe]. For the remaining galaxies the situation is more intrigued: sporadic episodes of star formations are Likely to have occurred scattering the galaxies in the space of age (H-beta), metallicity, and [Mg/Fe]. The results are discussed in regard to predictions from the merger and isolation models of galaxy formation and evolution highlighting points of difficulty with each scheme. Finally, the suggestion is advanced that models with an IMF that at the early epochs favors higher mass stars in massive ellipticals galaxies, and lower mass stars in low-mass ellipticals, might be able to alleviate some of the difficulties encountered by the standard SN-driven galactic wind model and lead to a coherent interpretation of the data.