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 3 r_eff along the major axis of the galaxy light, these globularclusters exhibit substantial rotation (~ 300 +/- 70 km/s) that translates intoan equally substantial spin (lambda ~ 0.18). The present appearance of M87 ismost likely the product of a single major merger, since this event is best ableto account for so sizable a spin. A rotation this large makes improbable anysignificant accretion of material after this merger, since that would havediluted the rotation signature. We see weak evidence for a difference betweenthe kinematics of the metal-poor and metal-rich population, in the sense thatthe metal-poor globular clusters appear to dominate the rotation. If, as wesuspect, the last major merger event of M87 was mainly dissipationless and didnot trigger the formation of a large number of globular clusters, the kinematicdifference between the two could reflect their orbital properties in theprogenitor galaxies; these differences would be compatible with theseprogenitors having formed in dissipational mergers. However, to put strongkinematic constraints on the origin of the globular clusters themselves isdifficult, given the complex history of the galaxy and its last dominant mergerevent.