The ACS Virgo Cluster Survey. IX. The Color Distributions of Globular Cluster Systems in Early‐Type Galaxies

Abstract

We present the color distributions of globular cluster (GC) systems for 100 Virgo cluster earlytype galaxies observed in the ACS Virgo Cluster Survey, the deepest and most homogeneous survey of this kind to date. While the color distributions of individual GC systems can show significant variations from one another, their general properties are consistent with continuous trends across galaxy luminosity, color, and stellar mass. On average, galaxies at all luminosities in our study (−22 < M B < −15) appear to have bimodal or asymmetric GC color distributions. Almost all galaxies possess a component of metal-poor GCs, with the average fraction of metal-rich GCs ranging from 15 to 60%. The colors of both subpopulations correlate with host galaxy luminosity and color, with the red GCs having a steeper slope. The steeper correlation seen in the mean color of the entire GC system is driven by the increasing fraction of metal-rich GCs for more luminous galaxies.To convert color to metallicity, we also introduce a preliminary (g-z)-[Fe/H] relation calibrated to Galactic, M49 and M87 GCs. This relation is nonlinear with a steeper slope for [Fe/H] −0.8. As a result, the metallicities of the metal-poor and metal-rich GCs vary similarly with respect to galaxy luminosity and stellar mass, with relations of [Fe/H] MP ∝ L 0.16±0.04 ∝ M 0.17±0.04 ⋆ and [Fe/H] MR ∝ L 0.26±0.03 ∝ M 0.22±0.03 ⋆, respectively. Although these relations are shallower than the mass-metallicity relation predicted by wind models and observed for dwarf galaxies, they are very similar to the mass-metallicity relation for star forming galaxies in the same mass range. The offset between the two GC populations varies slowly (∝ M 0.05 ⋆ ) and is approximately 1 dex across three orders of magnitude in mass, suggesting a nearly universal amount of enrichment between the formation of the two populations of GCs. We also find that although the metal-rich GCs show a larger dispersion in color, it is the metal-poor GCs that have an equal or larger dispersion in metallicity. The similarity in the M ⋆ -[Fe/H] relations for the two populations, implies that the conditions of GC formation for metal-poor and metal-rich GCs could not have been too different. Like the color-magnitude relation, these relations derived from globular clusters present stringent constraints on the formation and evolution of early-type galaxies.

The SLUGGS survey: combining stellar and globular cluster metallicities in the outer regions of early-type galaxies

“…A weak correlation between bGC average colour/metallicity and galaxy mass has been confirmed by several studies (e.g. Larsen et al 2001;Strader, Brodie & Forbes 2004;Strader et al 2006;Peng et al 2006). However, in the cases where a colour/metallicity gradient exists (e.g.…”

“…If the main sources of ex-situ GCs are minor mergers, one may expect a lower number of accreted rGCs than accreted bGCs. This because low-mass accreted galaxies likely had low numbers of rGCs, hence their contribution to the host galaxy's rGC subpopulation should be small (Forbes, Brodie & Grillmair 1997;Strader et al 2006;Peng et al 2006;Forbes et al 2011). On the other hand, major mergers will contribute with a more similar number of rGCs and bGCs.…”

“…However, in the cases where a colour/metallicity gradient exists (e.g. Peng et al 2006) this result may be simply a consequence of an aperture bias (i.e. a GC system average will lead to measuring the colours/metallicities of mostly the innermost bGCs in massive galaxies).…”

“…Almost all the GC systems of galaxies with M 10 10 M present a bimodality in colour and metallicity, with blue GCs (bGCs) showing lower metallicities than red GCs (rGCs) Peng et al 2006, and references therein). The origin of this bimodality is still unclear, but it is commonly believed that it is intrinsically linked with the host galaxy's formation history (e.g.…”

The Next Generation Virgo Cluster Survey Xvi: The Angular Momentum of Dwarf Early-Type Galaxies From Globular Cluster Satellites

“…The GCs targeted are selected from the Next Generation Virgo Cluster Survey (NGVS; Ferrarese et al 2012) and from the ACS Virgo Cluster Survey (ACSVCS; Peng et al 2006;Jordán et al 2009). The NGVS is a ∼900 hr program carried out at the Canada-France-Hawaii Telescope (CFHT) between the years 2008 and 2013.…”