Models for the interpretation of CaT and the blue Spectral Indices in Elliptical Nuclei

Abstract

We present a grid of theoretical models where the calculation of absorption line spectral indices in both the blue and red wavelength ranges is done with the same evolutionary synthesis code. We have computed some of these indices: CaT, NaI, MgI in the near infrared and Mgb, Mg2, Fe52, Fe53, NaD and Hbeta, in the blue-visible range, for Single Stellar Population (SSP) of 6 different metallicities, (Z=0.0004,0.001, 0.004, 0.008, 0.02 and 0.05), and ages from 4 Myr to 20 Gyr. From the comparison of these evolutionary synthesis models with a compilation of elliptical galaxy data from the literature, we find that the observed CaT index follows the blue index rather than Mg2 as the models predict. If this implies an over-abundance [Mg/Ca] and we take into account the masses of stars which produce Mg and Ca, these stars could form in a time scale shorter than 5 Myr from the beginning of the star formation process. Alternatively, an IMF biased towards very massive stars (M > 40 Msun) at the early epoch of star formation in elliptical nuclei has to be assumed. We also suggest to revise the calculation of the nucleosynthesis yield of Magnesium. By using the diagnostic diagram CaT-Hbeta to disentangle age and metallicity in such populations, we obtain around solar abundances and a sequence of ages between 4 and 16 Gyr for the galaxy sample.