The Globular Cluster System in M87: A Wide-Field Study with CFHT/Megacam

TL;DR

This wide-field study of M87's globular cluster system reveals bimodal metallicity distributions, spatial extensions, and a mass/metallicity relation, providing insights into their formation and evolution.

Contribution

The study provides the first deep, wide-field photometric analysis of M87's GCS, revealing detailed spatial and metallicity structures and a nonlinear mass/metallicity relation.

Findings

Blue GCs are more spatially extended than red GCs.

Both GC subpopulations show radial metallicity gradients.

A significant mass/metallicity relation exists for blue GCs.

Abstract

CFHT Megacam data in (g',r',i') are used to obtain deep, wide-field photometry of the globular cluster system (GCS) around M87. A total of 6200 GCs brighter than M_I = -8.5 are included in the study, essentially containing almost the entire bright half of the total GC population in the galaxy. The classic bimodal metal-poor and metal-rich sequences of GCs show up clearly; while the spatial distribution of the GCs can be traced detectably outward to more than 100 kpc, the blue, metal-poor subpopulation is very much more spatially extended than the red subpopulation. Both the red and blue GC subsystems have radial metallicity gradients, where mean heavy-element abundance scales with projected galactocentric distance as Z ~ R^{-0.12} (blue) and R^{-0.17} (red). The blue sequence exhibits a strongly significant mass/metallicity relation (MMR) in which the mean metallicity gradually increases with cluster luminosity as Z ~ L^{0.25 \pm 0.05} for the luminosity range M_I < -10. However, this relation is also clearly nonlinear, becoming vertical at fainter levels. The MMR can be understood as the result of self-enrichment within the most massive metal-poor GCs during their formation. In giant elliptical galaxies, the red GCs are likely to represent a broad composite population formed during several major starbursts. If so, the red sequence might display a population-based MMR that could in principle be either positive or negative.