The Spin of M87 as Measured from the Rotation of its Globular Clusters

We revisit the kinematical data for 204 globular clusters in the halo of M87. Beyond 3reff along the major axis of the galaxy light, these globular clusters exhibit substantial rotation (≃300 ± 70 km s-1) that translates into an equally substantial spin (λ ≃ 0.18). The present appearance of M87 is most likely the product of a single major merger, since this event is best able to account for so sizable a spin. A rotation this large makes improbable any significant accretion of material after this merger, since that would have diluted the rotation signature. We see weak evidence for a difference between the kinematics of the metal-poor and metal-rich population, in the sense that the metal-poor globular clusters appear to dominate the rotation. If, as we suspect, the last major merger event of M87 was mainly dissipationless and did not trigger the formation of a large number of globular clusters, the kinematic difference between the two could reflect their orbital properties in the progenitor galaxies; these differences would be compatible with these progenitors having formed in dissipational mergers. However, to put strong kinematic constraints on the origin of the globular clusters themselves is difficult, given the complex history of the galaxy and its last dominant merger event.