Halo globular clusters observed with AAOmega: dark matter content, metallicity and tidal heating

TL;DR

This study analyzes spectroscopic data of ten Halo globular clusters to assess their dark matter content, tidal heating, metallicity, and dynamics, finding no need for dark matter or modified gravity to explain their kinematics.

Contribution

It provides a comprehensive analysis of multiple clusters, combining new observations with previous data to evaluate dark matter presence, tidal effects, and rotation, challenging some prior assumptions.

Findings

Velocity dispersion profiles explained by tidal heating.

All clusters have M/L_V < 5, no dark matter needed.

Distant clusters show no tidal heating signatures.

Abstract

Globular clusters have proven to be essential to our understanding of many important astrophysical phenomena. Here we analyse spectroscopic observations of ten Halo globular clusters to determine their dark matter content, their tidal heating by the Galactic disc and halo, describe their metallicities and the likelihood that Newtonian dynamics explain their kinematics. We analyse a large number of members in all clusters, allowing us to address all these issues together, and we have included NGC 288 and M30 to overlap with previous studies. We find that any flattening of the velocity dispersion profiles in the outer regions of our clusters can be explained by tidal heating. We also find that all our GCs have M/L_V < 5, therefore, we infer the observed dynamics do not require dark matter, or a modification of gravity. We suggest that the lack of tidal heating signatures in distant clusters indicates the Halo is not triaxial. The isothermal rotations of each cluster are measured, with M4 and NGC 288 exhibiting rotation at a level of 0.9 +/- 0.1 km/s and 0.25 +/- 0.15 km/s, respectively. We also indirectly measure the tidal radius of NGC 6752, determining a more realistic figure for this cluster than current literature values. Lastly, an unresolved and intriguing puzzle is uncovered with regard to the cooling of the outer regions of all ten clusters.