To Join via Zoom
The cold dark matter (DM) model is extremely successful at describing the large scale structure of the universe, but it faces several challenges at galactic scales. In particular, DM-only simulations predict steep, ‘cuspy' central density profiles for DM haloes while observations favor ‘cores' with a constant density towards the center.
The introduction of baryonic physics in simulations alleviates this discrepancy, notably as feedback-driven outflow episodes contribute to expanding the DM distribution. I will first present different theoretical models describing core formation in DM haloes, either from small stochastic density fluctuations induced by stellar feedback that dynamically heat up the halo, sudden bulk outflows that reorganise the halo mass distribution, or the combination of dynamical friction from incoming satellites with outflows. Such models may also provide a simple understanding of the formation of ultra-diffuse galaxies.
Instead of invoking baryonic feedback processes within the cold dark matter model to solve the galactic-scale challenges, Modified Newtonian Dynamics (MOND) changes the gravitational law below a characteristic acceleration scale. One of its consequence as a classical modification of gravity is that the strong equivalence principle -- which requires the dynamics of a small, free-falling, self-gravitating system not to depend on the external gravitational field in which it is embedded -- should be broken. I will present how ultra-diffuse galaxies in the Coma cluster can be used as a testing ground for the corresponding external field effect of MOND, since they both have singularly low internal gravitational accelerations and reside within a strong external field.