RESOLVING THE LEDGER
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The dark matter content of ultra-diffuse galaxies (UDGs) is the subject of considerable debate 1 , 2 , 3 , 4 , 5 . Stellar streams, which form when a host galaxy tidally strips stars from an orbiting stellar system, provide a powerful technique to constrain the dark matter content of external galaxies 6 . The stripped stars form long, thin leading and trailing tidal arms that persist for billions of years. Stellar streams from globular clusters (GCs) are particularly sensitive probes of dark matter halos and substructure 7 , 8 , 9 , 10 . GC streams are expected to exist in a variety of host galaxy types 11 , 12 but, so far, they have only been observed in the Milky Way (MW). Here we present evidence for the first, to our knowledge, extragalactic GC stellar stream, identified in deep Hubble Space Telescope (HST) imaging of the UDG UGC 9050-Dw1. The stream’s morphology, colour and apparent association with a compact source support the GC progenitor interpretation observationally and we reproduce the observed surface brightness with simulated GC stellar populations. We use generative stream modelling, which fits dynamical models directly to the stream morphology, to constrain the mass of the progenitor and present the first stream-based halo constraint for an UDG. The stream models point to a GC origin and suggest a massive dark matter host halo. By extending the reach of GC stream analysis to external galaxies, this work opens a new chapter in dark matter science.
Recent capabilities to detect low-surface-brightness objects have led to the discovery of numerous UDGs 13 , 14 : galaxies with dwarf-like stellar masses but MW-like extents 15 . The methods available for inferring their detailed mass and density distribution remain limited, as their characteristic low surface brightness means that measuring velocity dispersions or rotation curves is challenging 16 .
Figure 1 shows observations of the UDG UGC 9050-Dw1 by the HST and the Canada–France–Hawaii Telescope (CFHT), in which we highlight a feature resembling one arm of a stellar stream extending from a GC candidate. The thin stream candidate and its presumed parent GC have a projected distance of about 2.5 kpc to the centre of UGC 9050-Dw1, which is probably associated with the low-surface-brightness spiral galaxy UGC 9050 at a distance of 35.2 ± 2.5 Mpc (refs. 17 , 18 ). UGC 9050-Dw1 was first identified in CFHT images as part of a diffuse dwarf galaxy search 19 . The HST image was presented in a detailed analysis of UGC 9050-Dw1’s GC population, which found that most of the GCs probably formed simultaneously during a past dwarf merger event 17 . The stream-like feature, which we name Oyashio (after a cold Pacific ocean current, pronounced [ ], inspired by the nomenclature introduced in ref. 20 ), is identified independently in the HST and CFHT data, ruling out imaging or data processing artefacts as its origin. All available data are shown in Extended Data Fig. 1 .
A comparison between the background-subtracted image counts, f , of the stream candidate and the standard deviation of the background, σ , gives a signal prominence of \(({f}_{{\rm{stream}}}-\,{\overline{f}}_{{\rm{background}}})/{\sigma }_{{\rm{background}}}=7.34\) in the combined HST image. We measure the width of the stream candidate to be w ± σ = 72.3 ± 8.9 pc, assuming a Gaussian profile (see Methods and Extended Data Fig. 2 ). The amplitude, A , of this fit gives a signal-to-noise ratio of \(A/\sqrt{{\sigma }_{A}}=5.2\) for the HST image and similar fits to the CFHT images give ratios ranging between 2.3 and 3.9 in all g-band, r-band and i-band images, whereas the feature is not detectable in the u and z bands.
GC streams in the MW have widths ranging from a few tens to a few hundred pc (ref. 21 ). The width of Oyashio is much smaller than any known stream from a dwarf galaxy progenitor (for example, the Orphan–Chenab MW stream, which has a low-mass dwarf progenitor but a width >200 pc (ref. 22 )), pointing towards a GC origin. The length of the Oyashio stream arm is 2 kpc based on a by-eye estimate.
The stellar population of a GC progenitor and its stream are of the same age and metallicity and therefore follow the same track (isochrone) in a colour-magnitude diagram. In the MW, these tracks can be analysed at the level of individually resolved stars. At the distance of UGC 9050-Dw1, we must consider the integrated-light equivalent: comparing the overall colour of the stream and progenitor. Many of the GC candidates in UGC 9050-Dw1 have similar colours, probably because of simultaneous formation 17 , so we also expect this particular GC candidate and its stream to align with the colours of the other GCs in the host. In Fig. 2 , we show that the Oyashio stream and progenitor cluster candidates have overlapping colours, as expected if they share the same stellar population, and both fall within the GC candidate selection box used in ref. 17 . The GC candidate has a colour of F555W-F814W = 1.1 ± 0.1 and the stream-like feature has F555W-F814W = 1.0 ± 0.2. This is slightly bluer than the average GC in the MW (refs. 23 , 24 ), which aligns with the findings in ref. 17 that the clusters in UGC 9050-Dw1 are consistent with a more recent origin.
To test whether a GC stream with this morphology at this location is dynamically plausible, we apply the X-Stream sampler 6 , which translates stream imaging into constraints on stream progenitors and host dark matter halos. See Methods and Extended Data Table 1 for details on the model parameters. In Fig. 3a , we show the likelihood surfaces and sampler constraints on the progenitor mass, halo mass and on the inner density slope of the halo, γ , from the sampler applied to control points generated within a mask of width w ±2 σ = 143.7 pc. The orange contours show the 68% and 95% credible regions for runs with a dark matter halo concentration of c = 5 and the grey contours show the same regions for the c = 2 runs.
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