Kinematics of a globular cluster with an extended profile: NGC5694

TL;DR

This study analyzes the kinematics of the remote globular cluster NGC5694 using medium-resolution spectra, revealing a flat velocity dispersion profile inconsistent with simple isotropic models.

Contribution

It provides detailed kinematic measurements of NGC5694 and demonstrates that both isotropic and anisotropic models can fit the observed profiles, highlighting complexities in cluster dynamics.

Findings

Weak rotation detected in the cluster.

Velocity dispersion remains flat beyond 2 arcmin.

Standard isotropic King models do not fully explain the data.

Abstract

We present a study of the kinematics of the remote globular cluster NGC5694 based on GIRAFFE@VLT medium resolution spectra. A sample of 165 individual stars selected to lie on the Red Giant Branch in the cluster Color Magnitude Diagram was considered. Using radial velocity and metallicity from Calcium triplet, we were able to select 83 bona-fide cluster members. The addition of six previously known members leads to a total sample of 89 cluster giants with typical uncertainties <1.0 km/s in their radial velocity estimates. The sample covers a wide range of projected distances from the cluster center, from ~0.2 arcmin to 6.5 arcmin = 23 half-light radii (r_h). We find only very weak rotation, as typical of metal-poor globular clusters. The velocity dispersion gently declines from a central value of sigma=6.1 km/s to sigma = 2.5 km/s at ~2 arcmin = 7.1= r_h, then it remainins flat out to the next (and last) measured point of the dispersion profile, at ~4 arcmin = 14.0 r_h, at odds with the predictions of isotropic King models. We show that both isotropic single-mass non-collisional models and multi-mass anisotropic models can reproduce the observed surface brightness and velocity dispersion profiles.