Astronomers have discovered a rare dwarf galaxy, named SDSS J011754.86+095819.0 (dE01+09), that is drifting in isolation. This tiny galaxy was likely ejected from its original group billions of years ago, providing new evidence that a galaxy's current location may not reveal its full history. The discovery challenges previous assumptions about how isolated galaxies form and evolve.
Key Takeaways
- Astronomers identified a rare 'runaway' dwarf galaxy, dE01+09, located far from any galaxy group.
- The galaxy likely experienced a violent ejection from the NGC 524 group about 3.5 billion years ago.
- Star formation in dE01+09 ceased approximately 8.3 billion years ago, consistent with being 'quenched' within a dense environment.
- This discovery suggests that isolated galaxies can be survivors of dramatic gravitational interactions.
- The findings were published in the journal Astronomy & Astrophysics.
A Cosmic Castaway: The Discovery of dE01+09
The newly identified galaxy, dE01+09, is small, faint, and no longer forms stars. Such galaxies are typically found within dense galaxy clusters. In these crowded environments, strong gravitational forces and harsh conditions often strip galaxies of their gas, effectively stopping new star formation. However, dE01+09 is located more than 3.9 million light-years away from its closest likely host, the NGC 524 group in the constellation Pisces. This distance places it well beyond the group's gravitational influence.
"Why is this single refugee galaxy sitting there?" asked Sanjaya Paudel, a research professor at Yonsei University in South Korea. Paudel led the discovery team. "For such an ejection to occur, it has to have had a very special, peculiar orbit."
Interesting Fact
The galaxy dE01+09 is so isolated that researchers confirmed no massive galaxies exist within about 2.3 million light-years of its current position. This level of isolation for a dwarf galaxy with its characteristics is highly unusual.
Identifying the Isolated Galaxy
Paudel's team used a machine learning model to find dE01+09. This model was trained on 5,000 previously cataloged early-type dwarf galaxies. The team then scanned imaging data from the Sloan Digital Sky Survey and the DESI Legacy Imaging Survey. Out of 751 potential candidates, dE01+09 stood out due to its isolation and the clear absence of recent star formation.
The radial velocity of dE01+09, which is its speed relative to Earth, closely matches that of the five galaxies in the NGC 524 group. This similarity supports the idea that dE01+09 was once part of that group. Deeper observations in the future could potentially reveal faint tidal tails or other features that would visually confirm this past connection.
Tracing a Violent Past
Spectroscopic analysis of dE01+09 revealed crucial details about its history. The galaxy stopped forming stars approximately 8.3 billion years ago. This timing suggests it was 'quenched' while still within a dense galaxy environment, such as the NGC 524 group. The process of quenching removes the gas necessary for new stars to form.
Background Information: Galaxy Quenching
Galaxy quenching is a process where a galaxy stops forming new stars. This often happens in dense environments like galaxy clusters. Intense gravitational interactions can strip away a galaxy's gas supply, or powerful active galactic nuclei can heat the gas, preventing it from cooling and collapsing to form stars.
Paudel's team estimates that dE01+09 entered the NGC 524 group several billion years ago as a young, actively star-forming galaxy. Around 8.3 billion years ago, powerful gravitational forces within the group stripped away its gas. This event effectively shut down star formation in dE01+09. After this quenching event, the galaxy likely continued to orbit within the group for billions of years.
The Ejection Event
The research suggests that dE01+09 was likely expelled from the NGC 524 group about 3.5 billion years ago. This ejection probably occurred after a close gravitational encounter involving multiple galaxies. Such interactions can act like a cosmic slingshot, accelerating a galaxy to escape velocity and flinging it out of its group.
"We can't say exactly when, because we don't have a full history," Paudel noted. "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. However, their interactions can also lead to other outcomes. Sometimes smaller galaxies are captured, while others simply pass by. In rare cases, like dE01+09, they are violently ejected. The outcome of these interactions largely depends on the specific orbital paths of the galaxies involved.
Significance of the Discovery
Such dramatic ejections are more commonly observed in massive galaxy clusters. Finding one in a smaller group environment, like the NGC 524 group, makes dE01+09 an especially rare and important case. This discovery provides a unique opportunity to study the processes that can lead to galactic isolation.
Key Data Point
The galaxy dE01+09 is located over 3.9 million light-years from the NGC 524 group, a distance far beyond the typical gravitational influence of such a group.
Paudel and his team are now actively searching for more of these 'runaway' dwarf galaxies. Finding additional examples could help astronomers understand how often such ejections occur across the universe. It could also shed new light on the complex life cycles and evolutionary paths of the universe's smallest galaxies. These findings highlight that a galaxy's current tranquil state might hide a turbulent and violent past.
The results of this study were published on September 11 in the journal Astronomy & Astrophysics.