Formation of Faint and Ultra-Faint dwarf spheroidal galaxies in an extreme collapsing scenario.

The dwarf spheroidal galaxies are the faintest dwarf galaxies around the Milky Way , the oldest and most dark matter-dominated systems known, offering a unique chance to explore the formation of the first galaxies and the behavior of dark matter on small scales. The formation of these objects is an on-going debate, specially in the isolated scenarios. To analyze this scenario, we consider the dissolving cluster model adapted to faint and ultra-faint dwarf spheroidals, where initially the stars of the galaxy are distributed in a fractal pattern within the center of the dark matter halo, building the faint luminous component observed. We tested a collapsing (non virial equilibrium) formation scenario modelling Ursa Major II by performing numerical simulations using AMUSE, with the stars following a fractal distribution within a dark matter halo corresponding to a Plummer distribution and observed the evolution during 1 Gyr. We obtain an object of non spherical shape, which size depends completely on the initial fractal radius and will become stable only when this radius is larger than the dark matter halo scale-length.