SIMBAD references

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 α-element, is depressed with respect to the other α-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 α/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 α/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 Caii triplet absorption of early-type galaxies at 8600 Å 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.