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

TL;DR

This study uses Subaru/FOCAS spectroscopy to analyze M87 globular clusters, confirming that nonlinear color-metallicity relations can explain observed color bimodality despite a unimodal metallicity distribution, impacting galaxy formation theories.

Contribution

It provides new empirical evidence for inflected color-metallicity relations in GCs, supporting the nonlinear-CMR hypothesis as an explanation for color bimodality.

Findings

Most M87 GCs are old, metal-rich, and alpha-enhanced.

The derived CMRs show significant inflection consistent with nonlinearity.

A broad, unimodal metallicity distribution explains the color bimodality.

Abstract

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 $\alpha$-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 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.