Internal Rotation in the Globular Cluster M53

Abstract

We present an analysis of the internal bulk rotation in the metal-poor globular cluster (GC) NGC 5024 (M53) using radial velocities (RVs) of individual cluster members. We use RV measurements from a previous abundance study of M53 done using the Hydra multi-object spectrograph on the WIYN 3.5 m telescope. The Hydra sample greatly increases the number of RVs available in the central regions of the cluster where the internal rotation is the strongest. The sample of cluster members is further increased through two previous kinematic studies of M53. The combined total sample contains 245 cluster members. With our sample, we are able to create a velocity dispersion profile of the cluster and derive a central velocity dispersion ${\sigma }_{0}=4.0\pm 0.3\ \mathrm{km}\,{{\rm{s}}}^{-1};$ we find that M53 inner regions are characterized by a peak amplitude of rotation equal to $1.4\pm 0.1\ \mathrm{km}\,{{\rm{s}}}^{-1}$ corresponding to a relatively high value of the ratio of the rotation speed to central velocity dispersion (${V}_{\mathrm{rot}}/{\sigma }_{0}=0.35\pm 0.04$). Our data also reveal a radial variation in the orientation of the projected rotation axis suggesting complex internal kinematics.