The M31 Globular Cluster Luminosity Function

Abstract

We combine our compilation of photometry of M31 globular clusters and probable cluster candidates with new near-infrared photometry for 30 objects. Using these data we determine the globular cluster luminosity function (GCLF) in multiple filters for the M31 halo clusters. We find a GCLF peak and dispersion V⁰₀=16.84+/-0.11, σ_t=0.93+/-0.13 (Gaussian σ=1.20+/-0.14), consistent with previous results. The halo GCLF peak colors (e.g., B⁰₀-V⁰₀) are consistent with the average cluster colors. We also measure V-band GCLF parameters for several other subsamples of the M31 globular cluster population. The inner third of the clusters have a GCLF peak significantly brighter than that of the outer clusters (ΔV⁰~0.5 mag). Dividing the sample by both galactocentric distance and metallicity, we find that the GCLF also varies with metallicity, as the metal-poor clusters are on average 0.36 mag fainter than the metal-rich clusters. Our modeling of the catalog selection effects suggests that they are not the cause of the measured GCLF differences, but a more complete, less contaminated M31 cluster catalog is required for confirmation. Our results imply that dynamical destruction is not the only factor causing variation in the M31 GCLF: metallicity, age, and cluster initial mass function may also be important. Based partially on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.