Left by Galactic ‘Intruder,’ Astronomers Say
The gigantic structure of gas in the galaxy group known as Stephan's Quintet may have had a violent origin, according to a new study.
Scientists have discovered an unexplained structure made of hydrogen gas that stretches for nearly two million light years near a famous group of interacting galaxies, according to a new study.
The strange gas trail may have been formed by an “intruder” galaxy that collided with this galactic group, known as Stephan’s Quintet, about a billion years ago, but it will take more observations to understand the mystery of its formation.
Stephan’s Quintet is a spectacular group of five galaxies located about 300 million light years from Earth with a rich observational history. Named after Édouard Stephan, the astronomer who discovered it in 1877, the quintet was the first compact galaxy group ever spotted, and has since been imaged by countless observatories, including NASA’s James Webb Space Telescope, which is the most powerful observatory ever launched.
Now, scientists led by Cong Xu, a researcher at the National Astronomical Observatories in Beijing, have studied Stephan’s Quintet with the biggest single-dish telescope on Earth: China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST). FAST was able to peer 100 times deeper into the galaxy group than past observations, revealing the never-before-seen gas structure that appears to be associated with the quintet, according to a study published on Wednesday in Nature.
Stephan’s Quintet “is unique among compact groups of galaxies,” said Xu and his colleagues in the study. “Observations have previously shown that interactions between multiple members, including a high-speed intruder galaxy currently colliding into the intragroup medium, have probably generated tidal debris.”
“The details and timing of the interactions and collisions remain poorly understood because of their multiple nature,” the team continued. However, the researchers added that diffuse structures made of atomic hydrogen gas can reveal clues about the dynamic history of galaxy groups.
“Atomic hydrogen is the least bound component of galaxies and is therefore the easiest (and hence first) to be stripped off and spread around during interactions,” the team said. “Thus, the distribution of the very diffuse atomic hydrogen and its velocity field can provide new information about the earliest interactions.”
In other words, the billion-year backstory of Stephan’s Quintet is written in these diffuse gas structures that are starting to come into focus, thanks to next-generation observatories such as FAST. While some diffuse gas structures have been seen in the quintet before, the new study is the first to report the enormous curved trail of hydrogen near the galaxy group. The structure extends for nearly two million light years in FAST’s observations and it may be even larger, as it literally runs out of the frame.
Xu and his colleagues speculate that the feature could be the fallout of ancient interactions that occurred when fast-moving galaxies collided with Stephan’s Quintet. One possible culprit is a galaxy called NGC 7320a, which is traveling at a breakneck pace of 6,702 kilometers per second.
“A hypothetical scenario for the formation of the diffuse feature is that NGC 7320a…passed through Stephan’s Quintet approximately 1.5 billion years ago…and pulled out from one of the core member galaxies of Stephan’s Quintet a tidal tail, which developed into the diffuse feature we see now,” the team said in the study.
“Another possibility is that…the diffuse feature could be the product of a high-speed head-on collision between another old intruder and one of the core members of Stephan’s Quintet,” the researchers added, noting that a galaxy called Anon 4 could be this ancient intruder. “In this scenario, the collision triggers an expanding density wave that pushes gas in an extended [atomic hydrogen] disk of the target galaxy outwards to form a very large ring, of which the diffuse feature is the high-density part.”
However, these proposed origin stories for the structure raise puzzling new questions. Current models suggest that gas structures should dissipate within 500 million years due to the harsh ultraviolet radiation that exists in the intergalactic medium. With that in mind, it’s unclear how this particular feature has survived on a billion-year timescale, assuming that it was formed by ancient interactions within Stephan’s Quintet.
“Our observations require a rethinking of properties of gas in outer parts of galaxy groups and demand complex modeling of different phases of the intragroup medium in simulations of group formation,” the team said. “These questions can only be answered by more sophisticated models that are built upon the existing simulations for the formation and evolution of Stephan’s Quintet.”
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