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

TL;DR

This study uses high-resolution SIDM simulations to explore the diverse dark matter density profiles of ultra-faint dwarf galaxies, suggesting many are in the gravothermal collapse phase, which explains their observed density variations.

Contribution

It demonstrates that SIDM models can account for the diversity and evolution stage of dark matter halos in MW ultra-faint dwarf galaxies through detailed simulations.

Findings

Most UFDs have high DM densities indicating they are in the gravothermal collapse phase.

Tidal stripping accelerates gravothermal evolution, explaining density diversity.

Large SIDM cross-sections are consistent with observed UFD densities and evolution stages.

Abstract

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 employ idealised high-resolution SIDM N-body simulations targeted to a MW-like system and compare 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 goes hand-in-hand with explaining 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.