UC Berkeley Student Finds Three 'Backsplash' Dwarf Galaxies Near M101
Three isolated dwarf galaxies that stopped making stars may have been flung out by the Pinwheel Galaxy, testing a prediction of cold dark matter.
Julian Shapiro, an undergraduate at UC Berkeley who began the project in high school, has found three isolated dwarf galaxies that have stopped forming stars. His single-author paper is in The Astrophysical Journal, and UC Berkeley reported the discovery on October 6, 2026. The galaxies are now cataloged as Shapiro Dwarf Galaxies I, II and III.
What makes them odd is the combination. Each sits far from other galaxies, yet each is “dead”, with no new stars forming. Small galaxies usually keep making stars unless something removes their gas, so quiet, isolated, quenched dwarfs are rare. Shapiro proposes they are backsplash galaxies: dwarfs that plunged through a massive host, most likely the Pinwheel Galaxy (Messier 101), lost their gas on the way through, and were flung back out. Shapiro Dwarf Galaxy II might instead be a faint satellite of the spiral galaxy NGC 5585.
The standard cosmological model, LambdaCDM, is built on cold dark matter, and it predicts that backsplash galaxies should be common. Nearby examples have been hard to find, so resolved candidates in a calm neighborhood give a direct test of that prediction. The backsplash history is a proposed explanation, not yet a measured orbit, and one of the three may turn out to be something else. For the bigger picture, see our guide to galaxies and guide to dark matter.
What to watch next: precise distances and motions for the three dwarfs would show whether their paths really trace back through M101.
Primary source
This summary is based on UC Berkeley News and the peer-reviewed paper. Read the original for full details, data and caveats.
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