Star Clusters in M31: V. Internal Dynamical Trends: Some Troublesome, Some Reassuring

TL;DR

This study provides the largest dataset of internal velocities and structural parameters for M31 globular clusters, revealing unexpected trends in mass-to-light ratios linked to metallicity and cluster mass, challenging existing models.

Contribution

It offers the largest dataset of M31 GCs with velocity and structural data, confirming unusual metallicity-related M/L ratio trends and proposing a non-standard initial mass function explanation.

Findings

Mass-to-light ratios decline with metallicity, contrary to models.

A shallower mass function explains observed M/L ratios.

Mass-to-light ratio correlates with cluster mass, consistent with dynamical evolution.

Abstract

We present internal velocity dispersions and precise radial velocities for 200 globular clusters (GCs) in M31 that are derived using new high-resolution spectra from MMT/Hectochelle. Of these, 163 also have King model structural parameters that allow us to estimate their mass-to-light ratios. This is, by far, the largest such dataset available for any galaxy, including the Milky Way. These data strongly confirm earlier suggestions that the optical and near-infrared mass-to-light ratios of M31 GCs decline with increasing metallicity. This behavior is the opposite of that predicted by stellar population models for a standard initial mass function. We show that this phenomenon does not appear to be caused by standard dynamical evolution. A shallower mass function for metal-rich GCs (with dN/dM ~ M^-0.8 to M^-1.3 below one solar mass) can explain the bulk of extant observations. We also observe a consistent, monotonic correlation between mass-to-light ratio and cluster mass. This correlation, in contrast to the correlation with metallicity, is well-explained by the accepted model of dynamical evolution of GCs through mass segregation and the preferential loss of low-mass stars, and these data are among the best available to constrain this process.