The internal kinematics of NGC 2808 and its multiple populations

TL;DR

This study uses 17 years of HST data to analyze the internal kinematics of multiple stellar populations in NGC 2808, revealing differences in anisotropy and energy equipartition that support theoretical models of cluster evolution.

Contribution

It provides the first detailed kinematic comparison of first- and second-generation stars in NGC 2808 across a large radial extent, confirming theoretical predictions.

Findings

Second-generation stars are more radially anisotropic in outer regions.

Central regions show higher energy equipartition than outskirts.

Marginal evidence that the most extreme second-generation stars are more radially anisotropic.

Abstract

We use new HST observations coupled with archival data spanning a total temporal baseline of 17 years to study the internal kinematics of the multiple populations in the globular cluster NGC 2808 from its center out to ~8 half-light radii (r_h). We detect different kinematical behaviors between the first- and second-generation populations. This is especially evident towards the external regions of the cluster, where second-generation stars are increasingly more radially anisotropic. Our results are in agreement with theoretical simulations that predict that second-generation stars, initially more concentrated in the inner regions, gradually diffuse outward and develop a stronger radially anisotropic velocity distribution with respect to the first-generation stars. We find the central regions of the cluster to exhibit a higher degree of energy equipartition than the outskirts; our analysis reveals similar levels of energy equipartition in the radial and tangential components of the motion within about 4 r_h, while outside 4 r_h the data suggest that the equipartition level of the radial component of the velocity dispersion is slightly higher than that of the tangential component. Finally, we measured the dispersion of the angular momentum L_z for the three main subpopulations along the main sequence, which provides further evidence of the differences in the velocity anisotropy of 1G and 2G stars and shows marginal evidence for the most extreme second-generation subpopulation being slightly more radially anisotropic than the other second-generation subpopulation.