Astronomers have detected what they believe is one of the first cases of “galactic cannibalism” – when a galaxy consumes one of its smallest neighbors: in an ultra-weak dwarf galaxy called Tucana II. The discoveries come from the discovery of an ancient halo of dark matter, located in a galaxy 163,000 light-years from Earth.
Toucan II is just one of dozens of dwarf galaxies surrounding the Milky Way. They are believed to be artifacts left over from the first galaxies in the universe, and Toucan II is one of the most primitive.
In a new study, published Monday in the journal Nature Astronomy, astrophysicists report the detection of nine unknown stars on the edge of Toucan II, using the SkyMapper telescope in Australia and the Magellan telescopes in Chile. The stars are surprisingly far from their center, but remain in the gravitational pull of the small galaxy.
The configuration of the stars provides the first evidence that the galaxy contains an extended halo of dark matter, a region of matter three to five times larger than scientists originally believed, in order to maintain gravitational control of its distant stars. The findings suggest that the the first galaxies in the universe they were much more massive than previously thought.
“Tucana II has a lot more mass than we thought, in order to tie these stars that are so far away,” one of the study’s authors, MIR graduate student Anirudh Chiti, said in a statement. “That means probably other early relic galaxies will also have this type of extended halos.”
All galaxies are believed to be joined by a halo of dark matter, a type of hypothetical matter believed to exceed 85% of the universe, MIT News reports. But the new findings represent the first time one has been detected in an ultra-weak dwarf galaxy.
“Without dark matter, galaxies would just separate,” Chiti said. “[Dark matter] it’s a crucial ingredient in making a galaxy and holding it together. “
Anirudh Chiti, MIT
Scientists also found that these distant stars are older than the stars in the core of Toucan II, the first evidence of an imbalance in this type of galaxy. Their discovery points to the possibility that the galaxy may have been the product of one of the first mergers between two galaxies in the universe, which scientists refer to as “galactic cannibalism.”
“We may be seeing the first signature of galactic cannibalism,” said MIT professor Anna Frebel. “A galaxy may have eaten one of its slightly smaller, more primitive neighbors, which then spilled all its stars on the outskirts.”
Using the image filter of a telescope, astronomers can study the metallic content of the stars in a galaxy to determine how primitive it is. They had previously found stars in the core of Tucana II with such a low metal content that the galaxy was identified as the most chemically primitive of the known ultra-thin dwarf galaxies.
New research found that outer stars were three times poorer in metal than those in the center, making them even more primitive.
“This probably also means that the first galaxies formed in much larger dark matter halos than previously thought,” Frebel said. “We thought the first galaxies were the smallest and smallest. But they can actually be several times larger than we thought and, after all, not so small.”
A probable galactic fusion is one of the probable explanations for the imbalance. According to MIT News, galactic cannibalism occurs “constantly” in the current universe, but mergers in the primitive universe are not as safe.
“Toucan II will finally be eaten by the Milky Way, without mercy,” Frebel said. “And it turns out that this ancient galaxy may have its own cannibal history.”
The team hopes to use its focus to discover even older, more distant stars in other ultra-thin dwarf galaxies.