Dark-matter core structure and dynamical-friction efficiency in DFUWS 68

Determine why DFUWS 68 may have a non-cored dark-matter profile unlike other ultra-diffuse galaxies, or how dynamical friction could operate efficiently enough to move its massive globular-cluster candidate toward the centre if the galaxy resides in a cored dark-matter halo.

Background

The brightest point source in DFUWS 68, designated 68e, lies close to the galaxy’s photometric and kinematic centre and has a stellar mass exceeding 106 solar masses. Its old, metal-poor stellar population places it in the globular-cluster-infall region of nuclear-star-cluster diagnostic diagrams, leading to the interpretation that it may be a massive globular cluster or a merger of several globular clusters that is sinking inward through dynamical friction.

Dynamical friction is expected to become inefficient in cored dark-matter haloes because of core stalling. Since many ultra-diffuse galaxies are thought to possess dark-matter cores, the nature of DFUWS 68’s halo and the apparent or potential inward migration of source 68e present an unresolved problem. The available spectra do not have sufficient resolution or signal-to-noise to measure the velocity dispersion and dynamical mass needed to distinguish between these possibilities.

References

This raises an interesting question as to the nature of DFUWS 68's dark matter halo. In general, it is expected that dynamical friction is much less efficient in cored dark matter halos due to a process known as `core stalling' \citep{Read2006, BinneyAndTremaine}. Many UDGs have been shown to have cores (see e.g., \citealp{Gannon2020, Gannon2022, Gannon2026} and \citealt{Levitskiy2025}) so it leaves an open question as to why DFUWS 68's dark matter profile is perhaps not cored and thus different to other UDGs, or how dynamical friction has managed to be so efficient in a cored halo.

— The Stellar Populations of Two Quiescent Low Surface Brightness Dwarf Galaxies in Low Density Environments  (2609.00693 - Forbes et al., 1 Sep 2026) in Section 6.2, “Possible Off-Centre Nuclear Star Cluster” (the paragraph beginning “This raises an interesting question”)

A major uncertainty, however, is whether the present-day radii of the Euclid GCs are representative of their birth radii, which remain unconstrained. In the specific case of the Fornax dSph, previous studies have shown that assuming larger initial galactocentric radii can substantially increase the orbital decay timescales and alleviate, or even remove, the GC timing problem \citep{Cole12,Boldrini19,Meadows20}. Whether a similar scenario applies to the GC populations observed by Euclid remains unknown, as their birth-radius distribution is currently unconstrained.

— Constraints on the fuzzy dark matter mass using globular clusters in dwarf galaxies from Euclid ERO data  (2609.11328 - Besnard et al., 10 Sep 2026) in Section 3.1, “Fornax dwarf galaxy globular clusters”