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

Abstract

AbstractWe have usedHubble Space Telescopeand ground-based photometry to determine totalV-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,GaiaDR2 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.