MONDian dynamical modelling of NGC 288 with β ≠ 0

Abstract

NGC 288 is a diffuse Galactic globular cluster, it is remarkable in that its low density results in internal accelerations being below the critical MOND $a_{0}$ acceleration throughout. This makes it an ideal testing ground for MONDian gravity, as the details of the largely unknown transition function between the Newtonian and modified regimes become unimportant. Further, exact analytical solutions exist for isothermal spherical equilibrium structures in MOND, allowing for arbitrary values of the anisotropy parameter, $\beta$. In this paper we use observations of the velocity dispersion profile of NGC 288, which is in fact isothermal, as dynamical constraints on MONDian models for this cluster, where the remaining free parameters are adjusted to fit the observed surface brightness profile. We find the optimal fit requires $\beta =0$, an isotropic solution with a total mass of $3.5 \pm 1.1 \times 10^{4} M_{\odot}$.