Astronomers detect an extensive halo of dark matter around the ancient dwarf galaxy

The Milky Way is surrounded by dozens of dwarf galaxies believed to be relics of the very first galaxies in the universe. Among the most primitive galactic fossils is Tucana II – an ultra-thin dwarf galaxy located about 50 kiloparsecs, or 163,000 light years, from Earth.

Now, MIT astrophysicists have detected stars on the edge of Tucana II, in a configuration surprisingly far from its center but nevertheless caught up by the gravitational pull of the small galaxy. This is the first evidence that Tucana II harbors an extended dark matter halo – a region of gravitational bonded matter that researchers have calculated to be three to five times more massive than scientists had estimated. This discovery of distant stars in an ancient dwarf galaxy implies that the very first galaxies in the universe were also likely large and 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,” says MIT graduate student Anirudh Chiti. “This means that other early relic galaxies probably also have these kind of extended halos.”

The researchers also determined that the stars on the outskirts of Tucana II are more primitive than the stars at the heart of the galaxy. This is the first evidence of such a stellar imbalance in an ultra-thin dwarf galaxy.

The unique configuration suggests that the ancient galaxy may have been the product of one of the first mergers in the universe, between two nascent galaxies – one slightly less primitive than the other.

“We are perhaps seeing the first signature of galactic cannibalism,” says Anna Frebel, associate professor of physics at Silverman Family Career Development 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.”

Frebel, Chiti and their colleagues published their results today in Nature astronomy.

Not so weak galaxies

Tucana II is one of the most primitive dwarf galaxies known, based on the metal content of its stars. Low-metal stars probably formed very early on, when the universe was not yet producing heavy elements. In the case of Tucana II, astronomers previously identified a handful of stars around the core of the galaxy with such a low metal content that the galaxy was considered the most chemically primitive of the ultra-thin dwarf galaxies known.

Chiti and Frebel wondered if the ancient galaxy could host other, even older stars that could illuminate the formation of the first galaxies in the universe. To test this idea, they obtained observations of Tucana II using the SkyMapper Telescope, an optical telescope based on the ground in Australia that offers a wide view of the southern sky.

The team used an imaging filter on the telescope to spot primitive, metal-poor stars beyond the galaxy’s core. The team executed an algorithm, developed by Chiti, through the filtered data to efficiently select stars with low metal content, including previously identified stars in the center and nine new stars much further away from the galactic core.

“Ani’s analysis shows a kinematic conection, that these distant stars move along with the inner stars, like bathwater going down a sewer,” Frebel adds.

The results suggest that Tucana II must have an extended dark matter halo that is three to five times larger than previously thought, for it to maintain a gravitational grip on these distant stars. Dark matter is a hypothetical type of matter that would make up over 85% of the universe. Each galaxy is believed to be held together by a local concentration, or halo, of dark matter.

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

The team’s results are the first evidence that an ultra-thin dwarf galaxy can harbor an extensive dark matter halo.

“It probably also means that the first galaxies formed in halos of dark matter much larger than previously thought,” says Frebel. “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.

“A cannibalistic story”

Chiti and Frebel continued their first results with observations of Tucana II taken by Magellan telescopes in Chile. With Magellan, the team focused on the metal-poor stars of the galaxy to infer their relative metallities, and found that the outer stars were three times as low in metals, and therefore more primitive, than those in the center. .

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

A likely explanation for the imbalance may be an early galactic merger, in which a small galaxy – possibly among the first generation of galaxies to form in the universe – swallowed another nearby galaxy. This galactic cannibalism occurs constantly throughout the universe today, but it was not clear if the early galaxies similarly merged.

“Tucana II will finally be eaten by the Milky Way, without mercy,” says Frebel. “And it turns out that this ancient galaxy may have its own cannibalistic history.”

The team plans to use their approach to observe other ultra-thin dwarf galaxies around the Milky Way, in hopes of discovering even older and more distant stars.

“There are probably a lot more systems, maybe all of them, that have these blinking stars in their periphery,” Frebel says.