The WAGGS project -- III. Discrepant mass-to-light ratios of Galactic
  globular clusters at high metallicity
  Observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs)
disagree with theoretical predictions. This discrepancy is of fundamental
importance since stellar population models provide the stellar masses that
underpin most of extragalactic astronomy, near and far. We have derived radial
velocities for 1,622 stars located in the centres of 59 Milky Way GCs - twelve
of which have no previous kinematic information - using integral-field unit
data from the WAGGS project. Using N-body models, we then determine dynamical
masses and M/L ratios for the studied clusters. Our sample includes NGC 6528
and NGC 6553, which extend the metallicity range of GCs with measured M/L up to
[Fe/H] ~ -0.1 dex. We find that metal-rich clusters have M/L more than two
times lower than what is predicted by simple stellar population models. This
confirms that the discrepant M/L-[Fe/H] relation remains a serious concern. We
explore how our findings relate to previous observations, and the potential
causes for the divergence, which we conclude is most likely due to dynamical
effects.
