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Abstract

Many astrophysical and galaxy-scale cosmological problems require a well-determined gravitational potential. Globular clusters (GCs) surrounding galaxies can be used as dynamical tracers of the luminous and dark matter distribution at large (kpc) scales. This M.Sc. project investigates - by means of the Auriga galaxy simulations and in anticipation of high-resolution IFU data of external galaxies - whether a novel action-based approach could provide a constraint for an axisymmetric approximation of the gravitational potential. In an axisymmetric potential, actions (radial JR, vertical Jz and angular momentum Lz) are integrals of motion and can be used to characterize and label orbits. In the Milky Way (MW), the assumption that stars in cold streams are on similar orbits was found to be a useful first-order constraint of its gravitational potential. In external galaxies, no individual stars but only GCs can be resolved. One could expect GCs from the same dwarf galaxy (DG) merger event to move at the present time on similar orbits in the host galaxy, analogously to stellar streams in the MW, and should therefore have similar actions in the true (axisymmetric) potential. We investigate this idea in one galaxy of the cosmological N-body simulation suite Auriga (Grand et al., 2017). As a first step, we present an effective strategy to fit analytic, axisymmetric, time-dependent potential models with slowly varying parameters to the simulation that are good enough to estimate actions. Then, we select stellar particles born in dwarf galaxies as proxies for GCs and follow the evolution of their orbital actions during the process of merging with a more massive galaxy. These actions show a significant variation over time. As a result, at z = 0, the stellar particles accreted in the same merger event show a very extended distribution in action space. We find that minimizing this distribution, however, cannot constrain the true potential since actions and their evolution are affected by complex physical processes during mergers. In local observations, we confirm this result in the stars of Gaia-Enceladus, one of the few DG mergers of our MW that we know of. Their action distribution is smeared out extensively. Based on these results, we propose that modellers need to find and develop more realistic distribution functions for GCs of a single DG merger event in simulations before being able to constrain the gravitational potential of external galaxies using action-based dynamical modelling of GCs.