Astronomers have for the first time discovered a stellar stream from a globular cluster outside the Milky Way inside ultra-diffuse galaxy UGC 9050-Dw1.
The discovery marks the first time scientists have demonstrated that such distant stellar streams can be used to map dark matter distribution in regions where the elusive mass cannot be observed directly.
The structure was detected in UGC 9050-Dw1, a dwarf galaxy characterized by an extremely low surface brightness.
According to the research team, whose findings were published in the journal Nature, observations of the galaxy revealed an elongated trail of stars escaping from a globular cluster under galactic gravitational forces.
Ultra-diffuse galaxies are faint stellar systems that can span sizes comparable to major galaxies like the Milky Way while containing only a fraction of their stars.
Globular clusters are dense, spherical collections of up to millions of stars bound together by gravity, commonly found orbiting galactic centers.
Stellar streams form as stars gradually break away from their parent cluster due to the gravitational pull of the host galaxy.
Because the motion of these escaping stars retains a record of the system's dynamical history, the shape and trajectory of a stream allow astronomers to reconstruct the underlying gravitational field.
Detecting these structures beyond the Milky Way has proven exceptionally difficult because stellar streams consist of faint, dispersed stars.
In the case of UGC 9050-Dw1, observations were further hampered by the native low brightness of the ultra-diffuse galaxy itself.
Mapping invisible mass
Using observational data of the stream in UGC 9050-Dw1, the study authors calculated new mass distribution estimates for the galaxy.
The calculations revealed a significant presence of dark matter, aligning with previous estimates for UGC 9050-Dw1 and broader theoretical models of ultra-diffuse galaxies.
The primary breakthrough of the study lies in the measurement technique itself.
A diagnostic method previously reserved for studying dark matter within the Milky Way was successfully tested in another galaxy for the very first time.
Dark matter does not emit, absorb, or reflect light in quantities sufficient for direct detection, leaving its presence to be inferred solely through its gravitational effects on visible cosmic structures.
Stellar streams provide a vital probe because their physical shapes act as a visible footprint left behind by invisible gravitational forces.
Future space observatory missions
Prior to this discovery, the application of stellar stream mapping was restricted entirely to observational data gathered inside the Milky Way.
Astronomers can now search for similar streams in other distant galaxies and compare their properties across different cosmic environments.
The technique is particularly valuable for ultra-diffuse galaxies, where a scarcity of bright stars makes standard mass distribution measurements challenging.
The researchers stated that the number of detected stellar streams is expected to grow substantially as new space observatories begin observations.
Data from the European Space Agency's Euclid space telescope and NASA's Nancy Grace Roman Space Telescope will allow scientists to apply this stream-mapping technique to a wider variety of galaxy types.
By expanding these measurements, astronomers aim to compare dark matter distribution across diverse galactic systems throughout the universe.
