The dark fate of ultra-faint dwarfs: Gravothermal collapse in action

Ultra-faint dwarf (UFD) galaxies are a promising probe for dark matter (DM) physics as they are the most DM-dominated systems known. The Milky Way (MW) hosts many UFDs for which the properties of their DM distribution have been inferred from measurements of their stellar kinematics. If DM has self-interactions beyond gravity, the UFD halos may undergo a gravothermal evolution, giving rise to a population of galaxies with more diverse DM density profiles. We investigate DM densities of MW UFDs in self-interacting dark matter (SIDM) models, with an aim of determining the stage of gravothermal evolution for their halos. Therefore, we employed idealised high-resolution SIDM N-body simulations targeted to a MW-like system and compared the properties of simulated satellites to those of the observed UFDs. We find that the gravothermal evolution of SIDM halos produces diverse DM distributions, aligning with observations of the MW UFDs. Most of the UFDs have high DM densities, indicating that their halos have passed the period of maximum core expansion and entered the collapse phase, i.e. their central density may increase with time. The depth to which they have evolved into the gravothermal collapse may vary strongly across the satellites. This allows SIDM to account for the diversity in their DM densities. Moreover, the acceleration of the gravothermal evolution by tidal stripping can help to explain the diversity of the UFDs, as the ones with smaller pericentre distances require having evolved further into the gravothermal catastrophe. Large SIDM cross-sections of $\sigma / m_\chi \approx$ 80 cm$^2$ g$^{-1}$ at a velocity of $v \approx$ 20 km s$^{-1}$ are plausible, as the halo densities of MW UFDs are consistent with the gravothermal evolution predicted in SIDM, with most of them being in the collapse phase.