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Dark matter halo found around ancient ‘cannibal’ galaxy

Space is a violent place, strewn with galaxies that have cannibalized each other to survive. Some of these survivors are galactic fossils, revealing glimpses of the past because they formed in the early universe.

One of them, known as Tucana II, has a much more intriguing story to tell than scientists previously believed, according to a new study.

Our own Milky Way galaxy is ringed by smaller galaxies, called dwarf galaxies, that are also considered to be galactic fossils.

One such galactic fossil is Tucana II, a very primitive and ancient ultrafaint dwarf galaxy located 163,000 light-years from Earth. Scientists know it’s a primitive galaxy because it contains stars with low metal content.

Metals and other heavy elements are created when stars explode. Stars that formed in the early days of the universe were lower in metals than the stars that came after them.

“Stars manufacture elements throughout their lifetimes, which they spread into the surrounding gas when they explode as powerful supernovae,” said study coauthor Joshua Simon, an astronomer at the Carnegie Institution for Science in Washington, DC, in a statement.

“These raw materials are then incorporated into new stars, making each successive stellar generation more chemically complex than its predecessors. As a result, we know that stars containing very small amounts of most elements are incredibly old.”

Astrophysicists used the ground-based SkyMapper telescope in Australia to observe Tucana II in the southern sky, which is visible from Australia. The researchers found stars at the edge of the primitive galaxy that are very distant from the galaxy’s center, yet still ensnared by the gravitational pull of the galaxy.

Study author Anirudh Chiti, a graduate student at the Massachusetts Institute of Technology, developed an algorithm to locate the stars with the lowest metal content, which revealed the stars on the outskirts of the galaxy.

“Ani’s analysis shows a kinematic connection, that these far-out stars move in lockstep with the inner stars, like bathwater going down the drain,” said study coauthor Anna Frebel, the Silverman family career development associate professor of physics at MIT, in a statement.

This connection suggests that the dwarf galaxy has an extended dark matter halo, or an area of material bound to the galaxy’s gravity, which is essentially holding on to these stars.

Massive halo

Dark matter is invisible matter that is believed to make up more than 85% of the universe, providing it with structure. Astronomers for decades have searched for physical evidence of this unseen matter, which provides structure in galaxies that are called halos. Without the structure of dark matter, galaxies would fall apart; it’s essentially the glue that holds them together.

In Tucana II, this dark matter halo is between three to five times more massive than scientists thought. It’s the first evidence that an ultrafaint dwarf galaxy could possess such an extended halo like this.

“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.”

The study published Monday in the journal Nature Astronomy.

The stars on the edge of Tucana II are also much more primitive than the stars located near the galaxy’s center. Researchers believe this means that the galaxy may be the byproduct of one of the first galactic mergers in the universe between two young galaxies, and that one of them was just a little less primitive than the other one.

“We may be seeing the first signature of galactic cannibalism,” Frebel said. “One galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts.”

The researchers also observed Tucana II using the Magellan telescopes in Chile to study the metal-poor stars on the outskirts. These stars were even more depleted in metals than those at the galaxy’s center, the scientists found.

“This is the first time we’ve seen something that looks like a chemical difference between the inner and outer stars in an ancient galaxy,” Chiti said.

This discovery of an imbalance in the galaxy’s stars further suggests cannibalism, where a small galaxy gobbled up a neighboring galaxy.

These violent events happen often throughout the universe.

“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 researchers plan to observe and study other galaxies like Tucana II that are located around the Milky Way because they could find even older and more distant stars hanging out in these galaxies.

“There are likely many more systems, perhaps all of them, that have these stars blinking in their outskirts,” Frebel said.

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