Abstract
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 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 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. 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.
Highlights
Globular clusters (GCs) are an old simple stellar population (SSP) with a small internal dispersion in age and chemical composition, and provide powerful clues to help understand the formation histories of their host galaxies
We have examined the fundamental stellar population properties of M87 globular clusters (GCs) and reported spectroscopic metallicity estimates using the homogeneous spectroscopic data
The nonlinear-color−metallicity relations (CMRs) scenario for GC color bimodality has been tested in the previous series of studies (Papers I ∼ IX) using multiple heterogeneous data sets of extragalactic GC systems, both photometric and spectroscopic, and found to be successful in explaining the bimodality phenomenon
Summary
Globular clusters (GCs) are an old simple stellar population (SSP) with a small internal dispersion in age and chemical composition, and provide powerful clues to help understand the formation histories of their host galaxies. In Papers II and IV, the shapes of CMRs according to different colors were examined for the GC systems of M87 and M84, respectively They showed that the u-band-related colors in particular, are useful diagnostics of the underlying metallicity distribution function. The CaT is a wellknown metallicity indicator for stellar populations and is used to derive metallicities of extragalactic GCs assuming a linear relation between CaT and metallicity (e.g., Foster et al 2010; Brodie et al 2012; Usher et al 2015) They showed that the conversion via nonlinear CaT −metallicity from a unimodal metallicity distribution reproduces the observed CaT distribution well. Paper VIII showed that the individual color distributions of 78 GC systems in early-type galaxies from the ACS Virgo and Fornax Cluster Surveys are correctly reproduced based on the nonlinear-CMR scenario.
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