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

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 $σ/ m_χ\approx$ 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.

Figures (6)

• Figure 1: Initial NFW density profile Fischer_2025 used for our simulations (black) and progenitor halos from a cosmological zoom-in simulation (blue). The latter are 60 halos from Yang_2023 within a mass range of $[5.6 \times 10^8, 5.6 \times 10^{9}] \, \mathrm{M_\odot}$, a subset of the halos shown by Zhang_2025. The gray band indicates a density range between being a factor of five times lower and two times higher relative to the black curve. This range covers roughly the scatter in the cosmological zoom-in simulations and is used later in the paper.
• Figure 2: Average density as a function of radius. Left: collisonless run Fischer_2025. Middle: velocity-independent run Fischer_2025. Right: velocity-dependent run Fischer_2025. In the middle and right panels, the legend specifies the time relative to the collapse time ($\tau = t / t_*$), as specified by Eq. \ref{['eq:collapse_time']}. Whereas in the left panel, we show the results for the collisionless simulation for the same times as in the right panel. The dashed line for $\tau = 0$ corresponds to the initial conditions. Additionally, we display the data for various observed dwarf satellite galaxies as explained in the main text. To highlight some of them, we show them in colour and label them with their names. We note that the upper limit for Eridanus II is not a measure of the density within the physical half-light radius but the density core inferred from its star cluster Orkney_2022. Moreover, Ursa Major III might be a star cluster instead of a dwarf galaxy Rostami_Shirazi_2025Cerny_2025, but its nature remains ambiguous Adams_2026. In the left panel, we additionally display a gray band to give an idea of the variation between CDM satellites. It is the same as in Fig. \ref{['fig:progenitor']}, with the only difference that we are showing the average density here. The dark gray line is the analytic description of the initial NFW profile.
• Figure 3: Central density as a function of time. The velocity-dependent run (simulation Y by Fischer_2025) is used to show the average central density of the halo within 0.03 kpc as a function of time relative to the collapse time (Eq. \ref{['eq:collapse_time']}). In addition, the same UFD satellite galaxies as in Fig. \ref{['fig:density_profile']} are given. We show the results for the velocity-dependent cross-section only, as the results for the velocity-independent cross-section Fischer_2025 are qualitatively very similar.
• Figure 4: Gravitational bound mass as a function of time. The bound mass for the different DM models, as indicated in the legend, is shown. The mass was computed by considering particles that initially belonged to the subhalo only and ignoring the host system.
• Figure 5: Correlations of the MW UFDs with the pericentre distance of their orbit. Upper panel: The average density within the physical half-light radius is displayed as a function of the pericentre to host distance. Middle panel: The physical half-light radius of the UFDs is shown. Lower panel: Stage of gravothermal evolution as a function of the pericentre distance. We use the gravothermal evolution stage that we assigned to the UFDs in Fig. \ref{['fig:central_density']}.