THE GLOBULAR CLUSTER SYSTEM IN M87: A WIDE-FIELD STUDY WITH CFHT/MEGACAM**This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

Canada-France-Hawaii Telescope 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 i′ = 23.0 (roughly equivalent to MI = −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 Rgc ≃ 100 kpc and perhaps further, 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 a 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 ∼ L0.25 ± 0.05 for the luminosity range MI ≲ −10 and the assumption of a constant M/L. However, this relation is also clearly nonlinear: fainter than this level, the sequence is more nearly vertical. This mass/metallicity trend can be understood as the result of self-enrichment within the most massive metal-poor GCs during their formation. The red sequence formally exhibits a negatively sloped MMR, but the numerical solutions and tests show that this red-GC slope is not very significant. 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.