A 'super' star cluster grown old: The most massive star cluster in the Local Group

Abstract

We independently redetermine the reddening and age of the globular cluster (GC) 037-B327 in M31 by comparing independently obtained multicolour photometry with theoretical stellar population synthesis models. 037-B327 has long been known to have a very large reddening value, which we confirm to be E(B -V) = 1.360 ± 0.013, in good agreement with the previous results. We redetermine its most likely age at 12.4 ± 3.2 Gyr. 037-B327 is a prime example of an unusually bright early counterpart to the ubiquitous 'super' star clusters presently observed in most high-intensity star-forming regions in the local Universe. In order to have survived for a Hubble time, we conclude that its stellar initial mass function (IMF) cannot have been top-heavy. Using this constraint, and a variety of simple stellar population (SSP) models, we determine a photometric mass of MGC = (3.0±0.5) × 107 M⊙, somewhat depending on the SSP models used, the metallicity and age adopted and the IMF representation. This mass, and its relatively small uncertainties, makes this object the most massive star cluster of any age in the Local Group. Assuming that the photometric mass estimate thus derived is fairly close to its dynamical mass, we predict that this GC has a (onedimensional) velocity dispersion of the order of (72 ± 13) km s -1. As a surviving 'super' star cluster, this object is of prime importance for theories aimed at describing massive star cluster evolution. © 2006 RAS.