Astronomers explain the origin of isolated ultra-diffuse galaxies

A recent study examined the origin of ultra-diffuse dwarf galaxies (UDGs), a rare type of galaxies containing a small number of stars that are spread out over large regions in space, and have an extremely low surface brightness, making them highly difficult to detect. 

Using sophisticated computer simulations, scientists detected a few “quenched” UDGs (ultra-diffuse galaxies that have stopped producing stars) in low-density environments in the universe. 

“What we have detected is at odds with theories of galaxy formation since quenched dwarfs are required to be in clusters or group environments in order to get their gas removed and stop forming stars,” said co-author Laura Sales, an associate professor of Physics and Astronomy at the University of California, Riverside. 

“But the quenched UDGs we detected are isolated. We were able to identify a few of these quenched UDGs in the field and trace their evolution backward in time to show they originated in backsplash orbits.” 

According to Professor Sales, a backsplash galaxy looks like an isolated galaxy in the present, but in the past was a satellite of a more massive system. “Isolated galaxies and satellite galaxies have different properties because the physics of their evolution is quite different,” she explained. “These backsplash galaxies are intriguing because they share properties with the population of satellites in the system to which they once belonged, but today they are observed to be isolated from the system.”  

Scientists believe that in the past, quenched UDGs coalesced within halos of dark matter with unusually high angular momentum. Similar to a cotton candy machine, such an extreme environment might have spun out dwarf galaxies that were unusually stretched out. Although they evolved within galaxy clusters, at a certain moment in the past, interactions within the cluster most probably ejected the UDGs into the void, giving them wide, boomerang-like trajectories (the so-called “backsplash orbits”). During this process, the UDGs gas was stripped away, leaving them unable to produce stars (“quenched”).

“These orbits are almost like those of comets in our solar system,” said Professor Sales. “Some go out and orbit back around, and others may come in once and then never again. For quenched UDGs, because their orbits are so elliptical, they haven’t had time to come back, even over the entire age of the universe. They are still out there in the field.”

The research is published in the journal Nature Astronomy.

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By Andrei Ionescu, Earth.com Staff Writer