The ESO-VLT MIKiS survey reloaded: exploring the internal kinematics of NGC 6440

TL;DR

This study presents a detailed velocity dispersion and rotation profile of globular cluster NGC 6440, revealing a flat dispersion profile consistent with a King model and evidence of central rotation and flattening.

Contribution

First detailed kinematic analysis of NGC 6440 combining multi-instrument data, revealing its velocity dispersion, rotation, and shape characteristics.

Findings

Velocity dispersion profile fits King model with a constant core velocity dispersion.

Detected ordered rotation in the innermost 5 arcseconds of the cluster.

Observed central flattening consistent with rotation signature.

Abstract

In the context of the ESO-VLT Multi-Instrument Kinematic Survey (MIKiS) 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 $\sim 1800$ individual stars distributed over the entire cluster extension, from $\sim$0.1$"$ 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 ${\sigma}_0 \sim $ 12 km s$^{-1}$) and no evidence of a central cusp or other significant deviations. Our data allowed 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 $\sim$132 $\pm$ 2{\deg} and an amplitude of $\sim$3 km s$^{-1}$ (corresponding to Vrot/${\sigma}_0 \sim$ 0.3). Also, a flattening of the system qualitatively consistent with the rotation signal has been detected in the central region.