Three-integral multicomponent dynamical models and simulations of the nuclear star cluster in NGC 4244

Abstract

Adaptive optics observations of the flattened nuclear star cluster in the nearby edge-on spiral galaxy NGC 4244 using the Gemini near-infrared integral field spectrograph (NIFS) have revealed clear rotation. Using these kinematics plus 2MASS photometry, we construct a series of axisymmetric two-component particle dynamical models with our improved version of NMAGIC, a flexible χ2-made-to-measure code. The models consist of a nuclear cluster disc embedded within a spheroidal particle population. We find a mass for the nuclear star cluster of M = 1.6+0.5 −0.2 × 107M� within ∼42.4 pc (2 arcsec). We also explore the presence of an intermediate-mass black hole and show that models with a black hole as massive as M• = 5.0 × 105M� are consistent with the available data. Regardless of whether a black hole is present or not, the nuclear cluster is vertically anisotropic (βz < 0), as was found with earlier anisotropic Jeans models. We then use the models as initial conditions for Nbody simulations. These simulations show that the nuclear star cluster is stable against nonaxisymmetric
perturbations. We also explore the effect of the nuclear cluster accreting star clusters at various inclinations. Accretion of a star cluster with mass 13 per cent that of the nuclear cluster is already enough to destroy the vertical anisotropy, regardless of orbital inclination.