Astronomers find origins of “galactic cannibalism” with discovery of ancient halo of dark matter

Astronomers have detected what they believe is one of the earliest examples of “galactic cannibalism” – when a galaxy consumes one of its smaller neighbors – in an ultra-thin 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.

Tucana II is just one of dozens of dwarf galaxies surrounding the Milky Way. They are believed to be artifacts left behind by 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 having detected nine previously unknown stars on the edge of Tucana II, using the SkyMapper telescope in Australia and the Magellan telescopes in Chile. The stars are terribly far from its 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 extensive dark matter halo – a region of matter three to five times larger than scientists originally believed – in order to maintain a gravitational grip on its stars distant. The results suggest that the first galaxies in the universe were much more massive than previously believed.

“Tucana II has a lot more mass than we thought, in order to bind these stars that are so far away,” one of the study’s authors, Anirudh Chiti, graduate student from MIT, said in a statement. “This means that other early relic galaxies probably also have these kind of extended halos.”

Each galaxy is believed to be held together by a halo of dark matter, a hypothetical type of matter that is believed to make up more than 85% of the universe, explains MIT News. But the new findings represent the first time that a person has been detected in an ultra-thin dwarf galaxy.

“Without dark matter, galaxies would fly to pieces,” Chiti said. “[Dark matter] is a crucial ingredient in creating a galaxy and keeping it together. “

Scientists have also discovered that these distant stars are older than the core stars of Tucana II – the first evidence of such an imbalance in this type of galaxy. Their discovery indicates the possibility that the galaxy may be the product of one of the first mergers between two galaxies in the universe, which scientists call “galactic cannibalism.”

“We are perhaps seeing the first signature of galactic cannibalism,” said Anna Frebel, professor at MIT. “A galaxy may have eaten one of its slightly smaller, more primitive neighbors, which then knocked over all of its stars in the periphery.”

Using a telescope’s imaging filter, astronomers are able to study the metal content of stars in a galaxy to determine how primitive it is. They had previously found stars in the heart of Tucana II with such a low metal content that the galaxy was identified as the most chemically primitive of the ultra-thin dwarf galaxies known.

New research has found that the outer stars are three times poorer in metals than those in the center, making them even more primitive.

“It probably also means that the first galaxies formed in halos of dark matter much larger than previously thought,” Frebel said. “We thought that the earliest galaxies were the tiniest and most wimpiest galaxies. But they may have been several times bigger than we thought, and not so tiny after all.

An early galactic merger is a likely explanation for the imbalance. Galactic cannibalism is happening “constantly” in today’s universe, according to MIT News, but mergers in the early universe are not so certain.

“Tucana 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 cannibalistic history.”

The team hopes to use their approach to discover even older, more distant stars in other ultra-thin dwarf galaxies.