Chance Glance at Old Hubble Image Reveals Rare Stellar Stream That May Reveal Dark Matter's Secrets

In late June 2023, David Hendel, a former astronomer now working in finance, was reading an astronomy paper on a train in New York City when an image caught his eye: a year-old Hubble Space Telescope picture of UGC 9050-Dw1, a faint dwarf galaxy about 115 million light-years away, with a thin ribbon of stars trailing off it. Hendel recognized the ribbon as a stellar stream, torn from a globular cluster by the galaxy's gravity.

It is an ultra-diffuse galaxy, so sparse in stars that most of its mass is thought to be dark matter, the invisible substance making up more than 80% of the matter in the universe but never directly detected. If confirmed, the stream would be the first such feature found outside the Milky Way, offering a cleaner setting to study dark matter's effects.

Finding no prior record of it, Hendel sent a screenshot to astrophysicist Sarah Pearson at Technical University of Denmark's DTU Space. The collaboration, led by Pearson and Julie Kiel Holm, a doctoral student at the Niels Bohr Institute at the University of Copenhagen, confirmed the find and named the stream Oyashio, Japanese for a cold Pacific current.

Astronomers have found roughly 100 stellar-stream candidates within the Milky Way, but one around another galaxy is extraordinarily rare, since they form deep inside their host galaxy, washed out by brighter starlight. Emily Cunningham of Boston University, not involved in the study, called it "a very exciting step forward," adding that the Vera Rubin Observatory and the Nancy Grace Roman Space Telescope should reveal many more.

The team recovered Oyashio independently in archival Hubble and Canada-France-Hawaii Telescope data, ruling out an imaging artifact, then ran thousands of simulations of globular cluster properties and dark matter distributions to match the stream's brightness and shape. The models suggest the galaxy holds far more dark matter than its sparse stars alone would imply.