Astronomers have discovered what they believe is one of the earliest examples of ‘galactic cannibalism’ – when one galaxy used up one of its smaller neighbors – in an ultra-faint dwarf galaxy called Tucana II. The findings come from the discovery of an ancient dark matter halo located in a galaxy 163,000 light-years from Earth.
Tucana II is just one of a dozen dwarf galaxies around the Milky Way. They are thought to be artifacts left over from the first galaxies in the universe – and Tucana II is one of the most primitive of these.
In a new study published Monday in the journal Nature Astronomy, astrophysicists report detecting nine previously unknown stars at the edge of Tucana II using the SkyMapper telescope in Australia and the Magellan telescopes in Chile. The stars are shockingly far from the center, but remain in the gravitational pull of the small galaxy.
The configuration of stars provides the first evidence that the galaxy contains an elaborate halo of dark matter – an area of matter three to five times larger than scientists initially thought – to keep gravity on its distant stars. The findings suggest that the earliest galaxies in the universe were much more massive than previously believed.
“Tucana II has a lot more mass than we thought to bind these stars so far away,” one of the study’s authors, MIT graduate student Anirudh Chiti, said in a statement. “This means that other remnants of early galaxies are likely to have elongated halos like this as well.”
Each galaxy is believed to be held together by a halo of dark matter, a type of hypothetical matter believed to make up more than 85% of the universe, MIT News explains. But the new findings represent the first time one has been detected in an ultra-dim dwarf galaxy.
“Without dark matter, galaxies would fly apart,” Chiti said. “[Dark matter] is a critical ingredient in creating a galaxy and keeping it together. “
Anirudh Chiti, MIT
Scientists also found that these distant stars predate the stars in the core of Tucana II – the first evidence of such an imbalance in this type of galaxy. Their discovery points to the possibility that the Milky Way could be 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. “One galaxy may have eaten one of its slightly smaller, more primitive neighbors, who then spilled all of its stars to the suburbs.”
Using a telescope’s imaging filter, astronomers can study the metal content of the stars of a galaxy to determine how primitive it is. They had previously found stars in the core of Tucana II with such low metal content that the galaxy was identified as the most chemically primitive of the known ultra-faint dwarf galaxies.
New research found that the outer stars were three times more metallic than the stars in the center, making them even more primitive.
“This probably also means that the earliest galaxies formed in much larger halos of dark matter than previously thought,” Frebel said. “We thought the first galaxies were the smallest, faintest galaxies. But they were maybe several times bigger than we thought, and yet not that small.”
Early galactic fusion is a likely explanation for the imbalance. Galactic cannibalism is “constantly” in the universe today, according to MIT News, but mergers in the early universe are not so certain.
“Tucana II will eventually be eaten by the Milky Way, no mercy,” said Frebel. “And it turns out that this ancient galaxy has its own cannibalistic history.”
The team hopes to use their approach to discover even older, more distant stars in other ultra-dim dwarf galaxies.