The M4 Core Project with HST - IV. Internal Kinematics from Accurate Radial Velocities of 2771 Cluster Members

TL;DR

This study provides a detailed analysis of the internal kinematics of globular cluster M 4 using precise radial velocities for thousands of stars, revealing its velocity dispersion profile and confirming minimal rotation.

Contribution

The paper introduces new methods for radial velocity measurement and sky contribution removal, improving the accuracy of internal kinematic studies of globular clusters.

Findings

Average cluster radial velocity is 71.08 km/s.

Radial velocity dispersion decreases from 4.5 km/s near the center outward.

20 new binary candidates identified, totaling 87.

Abstract

We present a detailed study of the internal kinematics of the Galactic Globular Cluster M 4 (NGC 6121), by deriving the radial velocities from 7250 spectra for 2771 stars distributed from the upper part of the Red Giant Branch down to the Main Sequence. We describe new approaches to determine the wavelength solution from day-time calibrations and to determine the radial velocity drifts that can occur between calibration and science observations when observing with the GIRAFFE spectrograph at VLT. Two techniques to determine the radial velocity are compared, after a qualitative description of their advantages with respect to other commonly used algorithm, and a new approach to remove the sky contribution from the spectra obtained with fibre-fed spectrograph and further improve the radial velocity precision is presented. The average radial velocity of the cluster is $\langle v \rangle = 71.08 \pm 0.08$ km s$^{-1}$ with an average dispersion of $\mu_{v_c} = 3.97$ km s$^{-1}$. Using the same dataset and the same statistical approach of previous analyses, 20 additional binary candidates are found, for a total of 87 candidates. A new determination of the internal radial velocity dispersion as a function of cluster distance is presented, resulting in a dispersion of $4.5$ km s$^{-1}$ within 2$^{\prime}$ from the center of cluster and steadily decreasing outward. We statistically confirm the small amplitude of the cluster rotation, as suggested in the past by several authors. This new analysis represents a significant improvement with respect to previous results in literature and provides a fundamental observational input for the modeling of the cluster dynamics.