In a distant region of the cosmos, astronomers have found a small, spectral galaxy that is most likely a cosmic castaway that was thrown from its original galactic group billions of years ago and left to fend for itself in near-total isolation.
The discovery offers unique observational support for the theory that some galaxies that are currently isolated might have survived violent ejections from dense galactic groups.
According to astronomers, these galaxies, which were formed by intense interactions in dense, turbulent environments billions of years ago, indicate that a galaxy’s current location may not fully reveal its past.
The recently discovered galaxy, known as SDSS J011754.86+095819.0, or dE01+09, seems to be one of these “runaway” systems.
Small, faint, and no longer forming stars galaxies like dE01+09 are usually found in dense galaxy clusters where harsh conditions and strong gravitational interactions deplete them of gas and prevent star formation.
However, the study, published in the journal Astronomy & Astrophysics, noted that dE01+09 is located more than 3.9 million light-years away from its most likely host, the NGC 524 group in the constellation Pisces, and is therefore well outside the group’s gravitational field.
In a recent interview with Space.com, Sanjaya Paudel, a research professor in the astronomy department at Yonsei University in South Korea, who led the discovery, said, “Why is this single refugee galaxy sitting there?”
For such an ejection to occur, “it has to have had a very special, peculiar orbit,” Paudel added.
Paudel’s team scanned imaging data from the Sloan Digital Sky Survey and the DESI Legacy Imaging Survey using a machine learning model trained on 5,000 previously cataloged early-type dwarf galaxies to find this cosmic subject.
The study points out that dE01+09 was unique among 751 candidates due to its isolation and lack of recent star formation. Additionally, the scientists verified that there are no massive galaxies within 2.3 million light-years of dE01+09.
“There is nothing nearby,” said Paudel, adding, “So in that sense, it’s pretty much isolated.”
The notion that dE01+09 was once a member of the NGC 524 group is supported by the fact that its radial velocity—the speed at which it moves in relation to Earth—closely resembles that of the five galaxies in that group.
According to Paudel, more thorough observations might show outward indications of that connection, like subtle tidal tails or other characteristics.
“Maybe in the future, with deeper observations, we might find something,” he said, “but not yet.”
The new study found that dE01+09 ceased star formation approximately 8.3 billion years ago, which is sufficient time for it to have been quenched within the group before being expelled.
According to Paudel’s team, dE01+09 was a young, star-forming galaxy when it joined the group several billion years ago. According to the study, strong forces quenched the gas it needed to form new stars about 8.3 billion years ago.
The galaxy then kept orbiting within the group until it was probably expelled after a close gravitational encounter with another galaxy some 3.5 billion years ago. According to the new study, this type of interaction propels galaxies outward at escape velocity, much like a cosmic slingshot.
“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.”
Usually, but not always, galaxies grow by merging with one another. Smaller galaxies are occasionally caught, while others just pass by or, in extremely rare instances like this one, are thrown completely out of their groups. Paudel stated, “It all depends on the orbit.”
The study points out that dE01+09 is a particularly unique and remarkable instance because such dramatic ejections are more frequently seen in massive clusters and infrequently seen in smaller group environments.
Paudel and his group are currently looking for more of these rogue dwarf galaxies. Additional examples could provide new insights into the intricate life cycles of the smallest galaxies in the universe and help astronomers better understand how frequently such ejections occur.
