Astronomers have detected what they believe to be one of the earliest instances of “galactic cannibalism” — when one galaxy consumes one of its smaller neighbors — in an ultrafaint dwarf galaxy called Tucana II. The findings stem 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 dozens of dwarf galaxies surrounding the Milky Way. They are thought to be artifacts left over from the first galaxies in the universe — and Tucana II is among the most primitive of them. 
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 away from its center but remain in the small galaxy’s gravitational pull. 
The configuration of stars provides the first evidence that the galaxy contains an extended dark matter halo — a region of matter three to five times larger than scientists originally believed — in order to keep a gravitational hold 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, in order to bound these stars that are so far away,” one of the authors of the study, MIT graduate student Anirudh Chiti, said in a statement. “This means that other relic first galaxies probably have these kinds of extended halos too.”
Every galaxy is believed to be held together by a halo of dark matter, a type of hypothetical matter thought to make up over 85% of the universe, MIT News explains. But the new findings represent the first time one has been detected in an ultrafaint dwarf galaxy. 
“Without dark matter, galaxies would just fly apart,” Chiti said. “[Dark matter] is a crucial ingredient in making a galaxy and holding it together.”
The vicinity of the Tucana II ultra-faint dwarf galaxy, as imaged with the SkyMapper Telescope.
Anirudh Chiti, MIT
Scientists also found that these far-flung stars are older than the stars at Tucana II’s core — the first evidence of such an imbalance in this type of galaxy. Their discovery points to the possibility that the galaxy 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, that then spilled all its stars into the outskirts.”
Using a telescope’s imaging filter, astronomers are able to study the metal content of a galaxy’s stars to determine just how primitive it is. They had previously found stars at Tucana II’s core with such low metal content that the galaxy was identified as the most chemically primitive of the known ultrafaint dwarf galaxies.
New research found the outer stars were three times more metal-poor than the ones at the center, making them even more primitive. 
“This probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought,” Frebel said. “We have thought that the first galaxies were the tiniest, wimpiest galaxies. But they actually may have been several times larger than we thought, and not so tiny after all.”
An early galactic merger is one likely explanation for the imbalance. Galactic cannibalism occurs “constantly” across today’s universe, 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,” Frebel said. “And it turns out this ancient galaxy may have its own cannibalistic history.”
The team hopes to use their approach to discover even older, more distant stars in other ultrafaint dwarf galaxies.