Globular clusters of NGC 3115 in the near-infrared. Demonstrating the correctness of two opposing scenarios

TL;DR

This study combines near-infrared, optical, and spectroscopic data to analyze the globular cluster system of NGC 3115, providing evidence for metallicity bimodality and addressing the debate on color bimodality origins.

Contribution

It offers a comprehensive multiband analysis confirming metallicity bimodality and discusses the implications for the projection scenario versus nonlinear color-metallicity relations.

Findings

Confirmed metallicity bimodality through index bimodality.

Identified nonlinearity in color-color relations.

Supported the idea that color bimodality can result from nonlinearities.

Abstract

We combined new near-infrared VLT/HAWK-I data of the globular clusters (GCs) in the isolated edge-on S0 galaxy NGC 3115 with optical and spectroscopic ones taken from the literature, with the aim of analyzing the multiband GC color distributions. A recent study from the SLUGGS survey has shown that the GCs in this galaxy follow a bimodal distribution of Ca II triplet indices. Thus, NGC 3115 presents a critical example of a GC system with multiple, distinct, metallicity subpopulations, and this may argue against the "projection" scenario, which posits that the ubiquitous color bimodality mainly results from nonlinearities in the color-metallicity relations. Using optical, NIR, and spectroscopic data, we found strong and consistent evidence of index bimodality, which independently confirms the metallicity bimodality in NGC 3115 GCs. At the same time, we also found evidence for some color-color nonlinearity. Taken in the broader context of previous studies, the multicolor consistency of the GC bimodality in NGC 3115 suggests that in cases where GC systems exhibit clear differences between their optical and optical-NIR color distributions (as in some giant ellipticals), the apparent inconsistencies most likely result from nonlinearities in the color-metallicity relations.