Absolute -band magnitudes and mass-to-light ratios of Galactic globular clusters

Abstract

Abstract We have used Hubble Space Telescope and ground-based photometry to determine total V-band magnitudes and mass-to-light ratios of more than 150 Galactic globular clusters. We do this by summing up the magnitudes of their individual member stars, using colour-magnitude information, Gaia DR2 proper motions, and radial velocities to distinguish cluster stars from background stars. Our new magnitudes confirm literature estimates for bright clusters with $V<8$ , but can deviate by up to two magnitudes from literature values for fainter clusters. They lead to absolute mass-to-light ratios that are confined to the narrow range $1.4<M/L_V<2.5$ , significantly smaller than what was found before. We also find a correlation between a cluster’s $M/L_V$ value and its age, in agreement with theoretical predictions. The $M/L_V$ ratios of globular clusters are also in good agreement with those predicted by stellar isochrones, arguing against a significant amount of dark matter inside globular clusters. We finally find that, in agreement with what has been seen in M 31, the magnitude distribution of outer halo globular clusters has a tail towards faint clusters that is absent in the inner parts of the Milky Way.