Dynamical friction in Newtonian and Milgromian dynamics

Abstract We examine the orbital evolution of a compact massive object orbiting a galaxy in both Newtonian and Modified Newtonian Dynamics (MOND) frameworks, considering two scenarios: one where the object starts inside the galaxy’s baryonic matter distribution and another where it begins far outside the galaxy. Assuming the systems discussed here are in the MOND regime, in the first scenario, the dynamical friction (DF) timescale is shorter in MOND than in Newtonian models because of stronger friction from enhanced gravitational effects of baryonic matter. We also find that increasing the galaxy’s half-mass radius weakens DF in Newtonian models due to a lower dark-matter density at larger distances. In the second scenario, when the object starts far outside the galaxy, the lack of baryonic matter reduces DF in MOND. In contrast, dark matter in Newtonian models continues to have a significant effect. These results suggest that orbital decay during galaxy mergers may occur more slowly in MOND than in ΛCDM. We conclude that comparisons of DF in MOND and Newtonian models require caution, as DF is stronger in the Newtonian regime at large distances but more effective in MOND once the object reaches the baryonic matter distribution. The initial placement of a massive object within a galaxy affects the DF efficiency in MOND, with a stronger DF near the center and a weaker DF at larger distances. We also identify a universal scaling relation for DF timescales, which impacts galaxy mergers and the orbital behavior of massive objects.