New Constraints on Early-type Galaxy Assembly from Spectroscopic Metallicities of Globular Clusters in M87

Abstract

The observed characteristics of globular cluster (GC) systems, such as metallicity distributions, are commonly used to place constraints on galaxy formation models. However, obtaining reliable metallicity values is particularly difficult because of our limited means to obtain high quality spectroscopy of extragalactic GCs. Often, "color-metallicity relations" are invoked to convert easier-to-obtain photometric measurements into metallicities, but there is no consensus on what form these relations should take. In this paper we make use of multiple photometric datasets and iron metallicity values derived from applying full-spectrum stellar population synthesis models to deep Keck/LRIS spectra of 177 GCs centrally located around M87 to obtain a new color-metallicity relation. Our new relation differs substantially from previous relations in the blue, and we present evidence that the M87 relation differs from that of the Milky Way GCs, suggesting environmental dependence of GC properties. We use our color-metallicity relation to derive a new GC metallicity-host galaxy luminosity relation for red and blue GCs and find a shallower relation for the blue GCs than what previous work has found and that the metal-poor GCs are more enriched than what was previously found. This could indicate that the progenitor satellite galaxies that now make up the stellar halos of early-type galaxies are more massive and formed later than previously thought, or that the properties of metal-poor GCs are less dependent on their present-day host, indicating a common origin.

The SLUGGS survey: measuring globular cluster ages using both photometry and spectroscopy

“…We note that our samples are ∼ 10× larger than the previous spectroscopic studies of these galaxies and achieved with significantly shorter exposure times (∼ 2 h per mask for our observations compared to 3.5 h for Kuntschner et al 2002 and 7 h for Cenarro et al (2007)). The diversity in GC ages and metallicities both within and between galaxies agrees with the range of colour-CaT relationships observed by Usher et al (2015), the variation in colour-metallicity relationship seen by Villaume et al (2019) and by the variation in GC colour-colour relationship with environment seen by Powalka et al (2016). Further evidence for this diversity in stellar populations is seen with the various age distributions observed in previous spectroscopic studies which find uniformly old ages in some galaxies .…”

The Fornax 3D project: Non-linear colour–metallicity relation of globular clusters

“…Nonetheless, the deviations we found with some literature CZRs could lie in the different galaxy masses that are probed. M87 studied by Villaume et al (2019) is a giant ETG several times more massive than Fig. 9.…”

“…Very recently, Villaume et al (2019) presented a sample of 177 GCs of M87 with spectroscopic metallicities and found a linear relation shown in Fig. 7.…”

“…Their CZR follows the relation of Harris et al (2017) at red colours and shows a deviation from our relation at the bluest GC colours. Villaume et al (2019) discussed that their findings of more metal-rich blue GCs could indicate an environmental effect caused by the assembly history of M87 itself.…”

“…But, it could also indicate that the CZR is not universal and possibly depends on the host galaxy or the environment. For example, Villaume et al (2019) found a lack of metal-poor GCs in M87 compared to other systems such as the MW. Usher et al (2015) found indications 1 We chose CZR as abbreviation to prevent possible confusion with the term "colour-magnitude relation".…”