GLIMPSE-CO1: the most massive intermediate-age stellar cluster in the Galaxy

The stellar cluster GLIMPSE-C01 is a dense stellar system located in the Galactic plane. Though often referred to in the literature as an old globular cluster traversing the Galactic disc, previous observations do not rule out that it is an intermediate-age (less than a few Gyr) disc-borne cluster. Here, we present high-resolution near-infrared spectroscopy of over 50 stars in the cluster. We find an average radial velocity is consistent with being part of the disc, and determine the cluster's dynamical mass to be (8 +/- 3) x 104 M-circle dot. Analysis of the cluster's M/L ratio, the location of the red clump and an extremely high stellar density, all suggest an age of 400-800 Myr for GLIMPSE-C01, much lower than that for a typical globular cluster. This evidence therefore leads us to conclude that GLIMPSE-C01 is a part of the disc population, and is the most massive Galactic intermediate-age cluster discovered to date.