Nonlinear Color-Metallicity Relations of Globular Clusters. X. Subaru/FOCAS Multiobject Spectroscopy of M87 Globular Clusters

Authors: Sooyoung Kim, Suk-Jin Yoon, Sang-Yoon Lee, Chul Chung, Sangmo Tony Sohn

The Astrophysical Journal Supplement Series, Volume 256, Issue 2, id.29, 17 pp.

We obtained spectra of some 140 globular clusters (GCs) associated with the Virgo central cD galaxy M87 with the Subaru/FOCAS MOS mode. The fundamental properties of GCs such as age, metallicity and α-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 α-elements than the Milky Way GCs. Our high-quality, homogeneous dataset 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. Furthermore, from a combined catalog with the previous study on M87 GC spectroscopy, we find that a total of 185 old GCs exhibit a broad, unimodal metallicity distribution. The results corroborate the nonlinear-CMR interpretation of the GC color bimodality, shedding further light on theories of galaxy formation.

https://arxiv.org/abs/2109.08167