New clues on the calcium underabundance in early-type galaxies

We use our new stellar population models, which include effects from variable element abundance ratios, to model the Ca4227 absorption-line indices of early-type galaxies. and to derive calcium element abundances. We find that calcium, although being an alpha-element, is depressed with respect to the other alpha-elements by up to a factor of 2. This confirms quantitatively earlier speculations that early-type galaxies are calcium-underabundant. We find a clear correlation between the alpha/Ca ratio and central velocity dispersion, which implies that more massive galaxies are more calcium-underabundant. Interestingly, this correlation extends down to the dwarf spheroidal galaxies of the Local Group for which alpha/Ca ratios have been measured from high-resolution spectroscopy of individual stars. The increase of calcium underabundance with galaxy mass balances the higher total metallicities of more massive galaxies, so that calcium abundance in early-type galaxies is fairly constant and, in particular, does not increase with increasing galaxy mass. This result may be the key to understanding why the Ca II triplet absorption of early-type galaxies at 8600 Angstrom is constant to within 5 per cent over a large range of velocity dispersions. The origin of the calcium underabundance in early-type galaxies remains to be understood. We argue that formation time-scales are disfavoured to produce calcium underabundance, and that the option of metallicity-dependent supernova yields may be the most promising track to follow.