Mass-to-light ratios for M31 globular clusters: age-dating and a surprising metallicity trend

We have obtained velocity dispersions from Keck high-resolution integrated spectroscopy of 10M31 globular clusters (GCs), including three candidate intermediate-age GCs. We show that these candidates have the same V-band mass-to-light (M/LV ) ratios as the other GCs, implying that they are likely to be old.We also find a trend of derived velocity dispersion withwavelength, 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 re-analysis of allM31 GC M/LV values. In a combined sample of 27 GCs, we show that the metal-rich GCs have lower M/LV than the metal-poor GCs, in conflict with predictions from stellar populationmodels. Fragmentary data for other galaxies support this observation. TheM31 GC fundamental plane is extremely tight, and we follow up an earlier suggestion by Djorgovski to showthat the fundamental plane can be used to estimate accurate distances (potentially 10% or better).