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

TL;DR

This study uses HST, ground-based data, and Gaia to accurately measure the V-band magnitudes and mass-to-light ratios of over 150 Galactic globular clusters, revealing new insights into their properties and dark matter content.

Contribution

It provides updated magnitudes and mass-to-light ratios for a large sample of globular clusters, confirming some previous estimates and revealing new correlations with age.

Findings

Mass-to-light ratios are narrowly confined between 1.4 and 2.5.

A correlation exists between M/L_V and cluster age.

Globular cluster M/L ratios align with stellar isochrone predictions, indicating little dark matter.

Abstract

We have used HST 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 color-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 M31, 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.