The Strong Rotation of M5 (NGC 5904) as Seen from the MIKiS Survey of Galactic Globular Clusters

Abstract

In the context of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of\nGalactic globular clusters, we present the line-of-sight rotation curve and\nvelocity dispersion profile of M5 (NGC 5904), as determined from the radial\nvelocity of more than 800 individual stars observed out to 700" (~ 5 half-mass\nradii) from the center. We find one of the cleanest and most coherent rotation\npatterns ever observed for globular clusters, with a very stable rotation axis\n(having constant position angle of 145^o at all surveyed radii) and a\nwell-defined rotation curve. The density distribution turns out to be flattened\nin the direction perpendicular to the rotation axis, with a maximum ellipticity\nof 0.15. The rotation velocity peak (~3 km/s in projection) is observed at ~0.6\nhalf-mass radii, and its ratio with respect to the central velocity dispersion\n(~0.3-0.4 at 4 projected half-mass radii) indicates that ordered motions play a\nsignificant dynamical role. This result strengthens the growing empirical\nevidence of the kinematic complexity of Galactic globular clusters and\nmotivates the need of fundamental investigations of the role of angular\nmomentum in collisional stellar dynamics.\n