Astronomers discover rare ‘runaway’ dwarf galaxy hiding a violent past

Astronomers have discovered a tiny, ghostly galaxy in a remote corner of the universe, likely a cosmic castaway flung from its original galactic group billions of years ago and left to wander in near-total isolation.

The discovery provides rare observational evidence that some galaxies now found in isolation may actually be survivors of violent ejections from dense galactic groups. Shaped by dramatic interactions in crowded, turbulent environments billions of years ago, these galaxies suggest a galaxy’s present-day location may not reveal its full history, astronomers say.

The newfound galaxy, named SDSS J011754.86+095819.0, or dE01+09, appears to be one such “runaway” system. Galaxies like dE01+09, which are small, faint and no longer forming stars, are typically found in dense galaxy clusters where intense gravitational interactions and harsh conditions strip them of gas and shut down star formation. But dE01+09 lies more than 3.9 million light-years from its nearest likely host, the NGC 524 group in the constellation Pisces — well beyond the group’s gravitational reach, the study reports.

“Why is this single refugee galaxy sitting there?” Sanjaya Paudel, a research professor in the astronomy department at Yonsei University in South Korea, who led the discovery, told Space.com in a recent interview. For such an ejection to occur, “it has to have had a very special, peculiar orbit.”

To identify this cosmic subject, Paudel’s team used a machine learning model trained on 5,000 previously cataloged early-type dwarf galaxies to scan imaging data from the Sloan Digital Sky Survey and the DESI Legacy Imaging Survey. Out of 751 candidates, dE01+09 stood out for its isolation and lack of recent star formation, the study notes. The researchers also confirmed that no massive galaxies lie within about 2.3 million light-years of dE01+09.

“There is nothing nearby,” said Paudel. “So in that sense, it’s pretty much isolated.”

The speed at which dE01+09 moves relative to Earth, known to astronomers as radial velocity, closely matches that of the five galaxies in the NGC 524 group, reinforcing the idea that it once belonged there. Deeper observations could reveal visible signs of that connection, such as faint tidal tails or other features, Paudel explained. “Maybe in the future, with deeper observations, we might find something,” he said, “but not yet.”

Spectroscopic analysis revealed dE01+09 stopped forming stars around 8.3 billion years ago, long enough for it to have been quenched within the group before being ejected, according to the new study. Paudel’s team estimates dE01+09 entered the group several billion years ago as a young, star-forming galaxy. Approximately 8.3 billion years ago, powerful forces stripped away the gas it needed to make new stars, a process called quenching, the study notes.

Afterward, the galaxy continued orbiting within the group until about 3.5 billion years ago, when it was likely expelled after a close gravitational encounter between multiple galaxies. This kind of interaction acts like a cosmic slingshot, flinging galaxies outward at escape velocity, according to the new study.

“We can’t say exactly when, because we don’t have a full history,” said Paudel. “But we can say that at least 8.3 billion years ago, it was in the NGC 524 group, and then it was somehow kicked out.”

Galaxies typically grow by merging with one another, but not always. Sometimes smaller galaxies are captured, while others simply fly by — or, in rare cases like this, are slingshotted out of their groups altogether. “It all depends on the orbit,” said Paudel.

Such dramatic ejections are more commonly observed in massive clusters and are rarely seen in smaller group environments, making dE01+09 an especially unusual and noteworthy case, the study notes.

Now, Paudel and his team are searching for more of these runaway dwarf galaxies. Finding additional examples could help astronomers better understand how often such ejections occur and shed new light on the complex life cycles of the universe’s smallest galaxies.

The results of this study were detailed in a paper published Sept. 11 in the journal Astronomy & Astrophysics.