Extragalactic streams & dark matter
In Holm, Pearson et al. (2026), Nature we present evidence for the first globular cluster stellar stream from beyond the Milky Way in Hubble Space Telescope data. We use photometry and generative stream modelling to constrain its origin, and provide constraints on the host galaxy’s dark matter potential. All characteristics of the stream point strongly to a disrupted globular cluster progenitor, and our dynamical bounds indicate that it orbits in a massive dark matter halo with a steeper density profile than typical for e.g. low-surface brightness dwarfs. This discovery serves as a precursor for upcoming stream detections with the Euclid and Roman Space Telescopes. By extending stream-based analyses to faint, low surface brightness systems, our work provides a new path towards testing competing models of dark matter with globular cluster stellar streams in galaxies beyond the Milky Way.
Nibauer & Pearson (2026) recently showed that the morphology of extragalactic streams alone can be used to stitch together radial profiles of dark matter halos. The paper introduces the code X-Stream, a new method that
uses images of stellar streams, the remnants of disrupted star clusters and galaxies, to map how dark matter is distributed. Using the GPU-powered code streamsculptor (Nibauer et al. 2025a), we generate thousands of simulated streams to compare with real observations and reveal the shape and density of dark matter halos. This approach can test ideas beyond the s
tandard cold dark matter model and explore how galaxies’ merger histories shape their halos. With upcoming missions such as Euclid, the Rubin Observatory, ARRAKIHS, and the Nancy Gra
ce Roman Space Telescope, X-Stream will help create the most detailed dark matter maps yet for thousands of galaxies across the universe. See Pearson's talk discussing this work at a
recent DESC LSST Telecon.
We are about to enter an exciting new era, where data of thousands of streams orbiting other galaxies will become available over the course of the next five years facilitated by the European Space Agency’s space-based survey mission, Euclid, the ground-based Vera C. Rubin Observatory (Rubin), and the Nancy Grace Roman Space Telescope (Roman). Pearson recently showed that it is feasible to use extragalactic stellar streams to learn about the dark matter halos of host galaxies (Pearson et al. 2022b). She modeled the Dwarf3 stellar stream in Centaurus A's halo (see image) and used it to set a lower limit on the dark matter mass of the host galaxy. This provides preliminary evidence that dark matter can be mapped in external galaxies with stellar streams. She found that there are many viable stream models that fit the data well, provided that Cen A has a halo mass larger than M200 > 4.70 × 10^12 Msun.