The ESO-VLT MIKiS survey reloaded: velocity dispersion profile and rotation curve of NGC 1904

Abstract

We present an investigation of the internal kinematic properties of M79 (NGC 1904). Our study is based on radial velocity measurements obtained from the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) of Galactic globular clusters for more than 1700 individual stars distributed between $\sim 0.3^{\prime \prime }$ and $770^{\prime \prime }$ ($\sim 14$ three-dimensional half-mass radii), from the center. Our analysis reveals the presence of ordered line-of-sight rotation with a rotation axis almost aligned along the East-West direction and a velocity peak of $1.5$ km s${}^{-1}$ at $\sim 70^{\prime \prime }$ from the rotation axis. The velocity dispersion profile is well described by the same King model that best fits the projected density distribution, with a constant central plateau at ${\sigma }_0\sim 6$ km s${}^{-1}$. To investigate the cluster rotation in the plane of the sky, we have analyzed the proper motions provided by the Gaia EDR3, finding a signature of rotation with a maximum amplitude of $\sim 2.0$ km s${}^{-1}$ at $\sim 80^{\prime \prime }$ from the cluster center. Analyzing the three-dimensional velocity distribution, for a sub-sample of 130 stars, we confirm the presence of systemic rotation and find a rotation axis inclination angle of $37$° with respect to the line-of-sight. As a final result, the comparison of the observed rotation curves with the results of a representative N-body simulation of a rotating star cluster shows that the present-day kinematic properties of NGC 1904 are consistent with those of a dynamically old system that has lost a significant fraction of its initial angular momentum.

30 pages, 20 figures, 8 tables, accepted for publication in ApJ

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