The discovery of a handful of stars at the edge of a fossil galaxy has shed new light on dark matter and provided clues to a possible early case of galactic cannibalism, the researchers say.
Tucana II is an ultra-faint dwarf galaxy about 163,000 light-years from Earth and is believed to be a remnant of the formation of the first galaxies in the universe. It was already known to contain ancient stars, including some with very low metal content, indicating that they formed shortly after the Big Bang.
Now, researchers say they have discovered a handful of stars far from the center of Tucana II, revealing that the galaxy is larger than previously thought and offering new clues about its formation.
Writing in the diary Nature astronomy, researchers from the UK, the US and Australia report how they identified stars using images captured by the 1.3m SkyMapper telescope at the Australian National University, along with data from the Gaia satellite, which tracks stars on the Pathway. Milky.
In all, the team was able to find and study nine new stars, nearly doubling the number of known stars in Tucana II.
Crucially, the newly discovered stars were about 3,000 light years from the center of the galaxy, compared to about 725 light years from previously known stars, and were found to have even lower metal content, suggesting that they are older. .
The team says that one possibility is that the outer stars may have originated elsewhere, suggesting that Tucana II may have resulted from the merger of two early galaxies, a process believed to have occurred in other, younger galaxies, and that it has been suggested by simulations to have occurred in ancient dwarf galaxies like Tucana II.
“Our finding provides tentative evidence for such behavior in a primitive relic galaxy, indicating that its formation may also have been shaped by the same processes,” the team write.
The team adds that it is surprising to find stars so far from the center of a galaxy with so few stars, and examining their orbits ruled out the possibility that their position was downward and other objects were pulling them away from the galaxy.
Instead, the researchers say the most likely explanation is that the stars are held in place by the gravitational pull of the galaxy itself.
“In order to hold onto those stars and not be destroyed by the Milky Way by the tides, Tucana II needs to have a lot of mass,” said Dr. Denis Erkal, author of the study from the University of Surrey.
However, with so few stars in the galaxy, that means Tucana II would have to contain roughly four times as much dark matter than previously thought.
The team says that means the first galaxies may have been much larger affairs than previously expected, and now it would be interesting to take a closer look at other ancient dwarf galaxies to see if they also have stars far from their centers.
Erkal added: “This is the first time that we have been able to detect stars this far from one dwarf and we will need to do more observations of other dwarfs to see if this is true in general or if it is special for Tucana II.”
Professor Justin Read, director of physics at the University of Surrey, who was not involved in the work, said the study confirmed a long-standing theory that these tiny dwarf galaxies should be surrounded by a vast, invisible “halo” of matter. dark. .
“It suggests that our current ideas about what dark matter is are on the right track,” he said. “While we still don’t know what dark matter is made of, observations like this bring us closer to an answer.”
Hans-Walter Rix, Director of the Max Planck Institute for Astronomy in Heidelberg, said: “This is a really good article, with excellent scientific craftsmanship that convincingly shows that there are stars in this small galaxy farther away than previously known. “.
But he cautioned that the proposed galactic cannibalism scenario was currently just a theory. “The path to the implications proposed by the results for galactic cannibalism in the early universe seems plausible and intriguing, but not unique or very compelling,” he said.
George is Digismak’s reported cum editor with 13 years of experience in Journalism