Next Generation Virgo Cluster Survey (NGVS). XXXII:~Search for a Globular Cluster Substructure in the Virgo Galaxy Cluster Core

Substructure in globular cluster (GC) populations around large galaxies is expected in galaxy formation scenarios that involve accretion or merger events, and it has been searched for using direct associations between GCs and structure in the diffuse galaxy light, or with GC kinematics. Here, we present a search for candidate substructures in the GC population around the Virgo cD galaxy M87 through the analysis of the spatial distribution of the GC colors.~The study is based on a sample of $\sim\!1800$ bright GCs with high-quality $u,g,r,i,z,K_s$ photometry, selected to ensure a low contamination by foreground stars or background galaxies.~The spectral energy distributions of the GCs are associated with formal estimates of age and metallicity, which are representative of its position in a 4-D color-space relative to standard single stellar population models.~Dividing the sample into broad bins based on the relative formal ages, we observe inhomogeneities which reveal signatures of GC substructures.~The most significant of these is a spatial overdensity of GCs with relatively young age labels, of diameter $\sim\!0.1$\,deg ($\sim\!30\,$kpc), located to the south of M87.~The significance of this detection is larger than about 5$σ$ after accounting for estimates of random and systematic errors.~Surprisingly, no large Virgo galaxy is present in this area, that could potentially host these GCs.~But candidate substructures in the M87 halo with equally elusive hosts have been described based on kinematic studies in the past.~The number of GC spectra available around M87 is currently insufficient to clarify the nature of the new candidate substructure.