SIMBAD references

New calibrations of spectrophotometric indices of elliptical galaxies as functions of spectrophotometric indices are presented, permitting estimates of mean stellar population ages and metallicities. These calibrations are based on evolutionary models including a two-phase interstellar medium, infall and a galactic wind. Free parameters were fixed by requiring that models reproduce the mean trend of data in the colour-magnitude diagram as well as in the plane of indices Hβ-Mg₂ and Mg₂-<Fe>. To improve the location of faint ellipticals (M_B> -20) in the Hβ-Mg₂diagram, downsizing was introduced. An application of our calibrations to a sample of ellipticals and a comparison with results derived from single stellar population models are given. Our models indicate that mean population ages span an interval of 7-12 Gyr and are correlated with metallicities, which range from approximately half up to three times solar.