Made-to-measure modelling of globular clusters

Abstract

We present the first application of the made-to-measure method for modelling dynamical systems to globular clusters. Through the made-to-measure algorithm, the masses of individual particles within a model cluster are adjusted while the system evolves forward in time via a gravitational N-body code until the model cluster is able to reproduce select properties of an observed cluster. The method is first applied to observations of mock isotropic and anisotropic clusters while fitting against the cluster's 3D or projected density profile, density weighted mean-squared velocity profile, or its density profile with individual mean-squared velocity profiles assuming 10% uncertainty in each of these observables. We find that a cluster's 3D density profile can easily be reproduced by the made-to-measure method, with minor discrepancies in the outer regions if fitting against a cluster's projected surface density or projected kinematic properties. If an observed cluster is anisotropic, only fitting against the cluster's density profile and individual mean-squared velocity profiles will fully recover the full degree of anisotropy. Partial anisotropy can be recovered as long as two kinematic properties are included in the fit. We further apply the method to observations of the Galactic globular cluster M4 and generate a complete 6D representation of the cluster that reproduces observations of its surface density profile, mean-squared proper motion velocity profile, and mean-squared line of sight velocity profile. The M2M method predicts M4 is primarily isotropic with a mass of 9.2 ± 0.4 × 104 M⊙ and a half-mass radius of 3.7 ± 0.1 pc.