The ESO-VLT MIKiS Survey Reloaded: Exploring the Internal Kinematics of NGC 6440

Abstract

In the context of the ESO-VLT Multi-Instrument Kinematic Survey of Galactic globular clusters, here we present the line-of-sight velocity dispersion profile of NGC 6440, a massive globular cluster located in the Galactic bulge. By combining the data acquired with four different spectrographs, we obtained the radial velocity of a sample of ∼1800 individual stars distributed over the entire cluster extension, from ∼0.″1 to 778″ from the center. Using a properly selected sample of member stars with the most reliable radial velocity measures, we derived the velocity dispersion profile up to 250″ from the center. The profile is well described by the same King model that best fits the projected star density distribution, with a constant inner plateau (at σ _0 ∼ 12 km s ^−1 ) and no evidence of a central cusp or other significant deviations. Our data allowed us to study the presence of rotation only in the innermost regions of the cluster ( r < 5″), revealing a well-defined pattern of ordered rotation with a position angle of the rotation axis of ∼132° ± 2° and an amplitude of ∼3 km s ^−1 (corresponding to V _rot / σ _0 ∼ 0.3). In addition, a flattening of the system qualitatively consistent with the rotation signal has been detected in the central region.

Keywords

• Globular star clusters
• Stellar kinematics
• Spectroscopy