Mass-to-Light Ratios for M31 Globular Clusters: Age-Dating and a Surprising Metallicity Trend

TL;DR

This study measures mass-to-light ratios of M31 globular clusters, finds a surprising inverse metallicity trend, and demonstrates the fundamental plane's utility for precise distance estimation.

Contribution

It provides new velocity dispersion measurements, confirms old ages for candidate intermediate-age clusters, and reveals an unexpected metallicity trend conflicting with stellar models.

Findings

Metal-rich GCs have lower M/L_V than metal-poor GCs.

The M31 GC fundamental plane is extremely tight.

Potential for 10% accurate distance estimates using the fundamental plane.

Abstract

We have obtained velocity dispersions from Keck high-resolution integrated spectroscopy of ten M31 globular clusters (GCs), including three candidate intermediate-age GCs. We show that these candidates have the same V-band mass-to-light (M/L_V) ratios as the other GCs, implying that they are likely to be old. We also find a trend of derived velocity dispersion with wavelength, but cannot distinguish between a systematic error and a physical effect. Our new measurements are combined with photometric and spectroscopic data from the literature in a reanalysis of all M31 GC M/L_V values. In a combined sample of 27 GCs, we show that the metal-rich GCs have *lower* M/L_V than the metal-poor GCs, in conflict with predictions from stellar population models. Fragmentary data for other galaxies support this observation. The M31 GC fundamental plane is extremely tight, and we follow up an earlier suggestion by Djorgovski to show that the fundamental plane can be used to estimate accurate distances (potentially 10% or better).