2026A&A...706A.333V

Context. The ultra-faint dwarf spheroidal galaxies (UFDs) around the Milky Way are the faintest, oldest, and most dark-matter-dominated systems known, providing an unique opportunity to understand early galaxy formation and how dark matter (DM) behaves in the smallest halos. There is an ongoing debate over their formation, despite the several proposed models, they fail to explain UFDs that appear to have evolved in isolation, without external tidal influence.
Aims. To explore the isolated models. We aim to prove if an extreme collapsing scenario could reproduce the size, velocity dispersion, and spherical shape of a classical UFD and to show how it would differ from a virial equilibrium one.
Methods. We considered the dissolving-star-cluster model, adapted to UFDs. Here, the stars are initially distributed in a fractal pattern within the center of the DM halo. Which builds the faint luminous component. We modeled the UFD Ursa Major II, in an initially collapsed formation scenario by performing multiple numerical simulations, using the Astrophysical Multipurpose Software Environment (AMUSE), where the stars follow a fractal distribution within a Plummer DM halo and studying their evolution through 1 Gyr.
Results. The simulations produce a non-spherical object, where the half-mass radius (R₅₀) completely depends on the initial fractal radius. The stability of the object is only reached when the fractal radius is larger than the DM scale length. To reproduce UMa II's 3D r_1/2 ≈ 184 pc, we require an initial fractal radius of ≈ 500 pc. The velocity dispersions are higher than the observed σ_LOS ≈ 5.6 kms^–1, depending on both the fractal and Plummer radii. These results demonstrate that nonequilibrium fractal collapse can reproduce the characteristic size of an UFD without invoking tidal stripping. However, to match the predicted kinematics with observations it may require lowering the mass of the DM halo.