The SLUGGS Survey: NGC 3115, A Critical Test Case for Metallicity Bimodality in Globular Cluster Systems

TL;DR

This study confirms that the bimodal color distribution of globular clusters in NGC 3115 reflects a true bimodality in their metallicity, supporting the idea that such bimodality is common and linked to galaxy formation history.

Contribution

It provides robust evidence that color bimodality in globular clusters corresponds to metallicity bimodality using combined photometry and spectroscopy, validating the use of CaT as a metallicity indicator.

Findings

NGC 3115 globular clusters show bimodality in both color and CaT index.

CaT index is unaffected by horizontal branch morphology variations.

Metallicity bimodality is common in globular cluster systems.

Abstract

Due to its proximity (9 Mpc) and the strongly bimodal color distribution of its spectroscopically well-sampled globular cluster (GC) system, the early-type galaxy NGC 3115 provides one of the best available tests of whether the color bimodality widely observed in GC systems generally reflects a true metallicity bimodality. Color bimodality has alternatively been attributed to a strongly nonlinear color--metallicity relation reflecting the influence of hot horizontal branch stars. Here we couple Subaru Suprime-Cam gi photometry with Keck/DEIMOS spectroscopy to accurately measure GC colors and a CaT index that measures the CaII triplet. We find the NGC 3115 GC system to be unambiguously bimodal in both color and the CaT index. Using simple stellar population models, we show that the CaT index is essentially unaffected by variations in horizontal branch morphology over the range of metallicities relevant to GC systems (and is thus a robust indicator of metallicity) and confirm bimodality in the metallicity distribution. We assess the existing evidence for and against multiple metallicity subpopulations in early and late-type galaxies and conclude that metallicity bi/multimodality is common. We briefly discuss how this fundamental characteristic links directly to the star formation and assembly histories of galaxies.