Explaining the mass-to-light ratios of globular clusters

Context. The majority of observed mass-to-light ratios of globular clusters are too low to be explained by “canonical” cluster models, in which dynamical effects are not accounted for. Moreover, these models do not reproduce a recently reported trend of increasing $M/L$ with cluster mass, but instead predict mass-to-light ratios that are independent of cluster mass for a fixed age and metallicity. Aims. This study aims to explain the $M/L$ of globular clusters in four galaxies by including stellar evolution, stellar remnants, and the preferential loss of low-mass stars due to energy equipartition. Methods. Analytical cluster models are applied that account for stellar evolution and dynamical cluster dissolution to samples of globular clusters in Cen A, the Milky Way, M 31 and the LMC. The models include stellar remnants and cover metallicities in the range $Z = 0.0004{-}0.05$. Results. Both the low observed mass-to-light ratios and the trend of increasing $M/L$ with cluster mass can be reproduced by including the preferential loss of low-mass stars, assuming reasonable values for the dissolution timescale. This leads to a mass-dependent $M/L$ evolution and increases the explained percentage of the observations from 39% to 92%. Conclusions. This study shows that the hitherto unexplained discrepancy between observations and models of the mass-to-light ratios of globular clusters can be explained by dynamical effects, provided that the globular clusters exhibiting low $M/L$ have dissolution timescales within the ranges assumed in this Letter. Furthermore, it substantiates that $M/L$ cannot be assumed to be constant with mass at fixed age and metallicity.