Abstract
Context. Dynamical friction can be used to distinguish Newtonian gravity and modified Newtonian dynamics (MOND) because it works differently in these frameworks. This concept, however, has yet to be explored very much with MOND. Previous simulations showed weaker dynamical friction during major mergers for MOND than for Newtonian gravity with dark matter. Analytic arguments suggest the opposite for minor mergers. In this work, we verify the analytic predictions for MOND by high-resolution N-body simulations of globular clusters (GCs) moving in isolated ultra-diffuse galaxies (UDGs). Aims. We test the MOND analog of the Chandrasekhar formula for the dynamical friction proposed by Sánchez-Salcedo on a single GC. We also explore whether MOND allows GC systems of isolated UDGs to survive without sinking into nuclear star clusters. Methods. The simulations are run using the adaptive-mesh-refinement code Phantom of Ramses. The mass resolution is 20 M⊙ and the spatial resolution 50 pc. The GCs are modeled as point masses. Results. Simulations including a single GC reveal that, as long as the apocenter of the GC is over about 0.5 effective radii, the Sánchez-Salcedo formula works excellently, with an effective Coulomb logarithm increasing with orbital circularity. Once the GC reaches the central kiloparsec, its sinking virtually stops, likely because of the core stalling mechanism. In simulations with multiple GCs, many of them sink toward the center, but the core stalling effect seems to prevent them from forming a nuclear star cluster. The GC system ends up with a lower velocity dispersion than the stars of the galaxy. By scaling the simulations, we extend these results to most UDG parameters, as long as these UDGs are not external-field dominated. We verify analytically that approximating the GCs by point masses has little effect if the GCs have the usual properties, but for massive GCs such as those observed in the NGC 1052-DF2 galaxy, further simulations with resolved GCs are desirable.
Highlights
Modified Newtonian dynamics, or Milgromian dynamics (MOND; Milgrom 1983a,b) was introduced four decades ago as a possible explanation for the missing mass or gravity in galaxies not relying on an elusive dark matter component in these stellar systems.Under modified Newtonian dynamics (MOND) theory, rather than adding dark matter, the laws of gravity or inertia have to be adjusted in the regime of small acceleration a a0, where a0 is on the order of 10−10 m s−2 and would constitute a new natural constant
We explore whether MOND allows globular clusters (GCs) systems of isolated ultra-diffuse galaxies (UDGs) to survive without sinking into nuclear star clusters
We first sought to test for the first time the validity of the proposed MOND analog of the Chandrasekhar formula using high-resolution, self-consistent simulations
Summary
Modified Newtonian dynamics, or Milgromian dynamics (MOND; Milgrom 1983a,b) was introduced four decades ago as a possible explanation for the missing mass or gravity in galaxies not relying on an elusive dark matter component in these stellar systems (see Famaey & McGaugh 2012, for a review). In the context of MOND, irrespective of their formation scenario, such galaxies in the field are very well suited to the study of dynamical friction because they are deep in the MOND regime all the way down to their center. Analytic arguments suggested that GCs of galaxies in the deep MOND regime experience strong dynamical friction and sink quickly to the centers of the galaxies (Ciotti & Binney 2004; Nipoti et al 2008). Many UDGs contain many GCs (Lim et al 2018, 2020), which might even be exceptionally massive (van Dokkum et al 2018a) and experiencing exceptionally strong dynamical friction This brings up the question of whether GCs could have survived orbiting in (relatively isolated) UDGs since their formation 10 Gyr ago in MOND, or if MOND predicts that such GCs should have sunk to the centers of UDGs and formed nuclear star clusters.
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