Color-metallicity relations of GCs. X. M87 spec.

The fundamental properties of GCs such as age, metallicity, and {alpha}-element abundance are investigated by using simple stellar population models. It is confirmed that the majority of M87 GCs are as old as, more metal-rich than, and more enhanced in {alpha}-elements than the Milky Way GCs.

Our high-quality, homogeneous data set enables us to test the theoretical prediction of inflected color-metallicity relations (CMRs). The nonlinear-CMR hypothesis entails an alternative explanation for the widely observed GC color bimodality, in which even a unimodal metallicity spread yields a bimodal color distribution by virtue of nonlinear metallicity-to-color conversion. The newly derived CMRs of old, high-signal-to-noise-ratio GCs in M87 (the V-I CMR of 83 GCs and the M-T2 CMR of 78 GCs) corroborate the presence of the significant inflection.