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

The Milky Way is surrounded by dozens of dwarf galaxies believed to be remnants of the very first galaxies in the universe. One of the most primitive of these galactic fossils is Tucana II – an ultra-faint dwarf galaxy 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 that is surprisingly far from the center, but nonetheless caught in the gravity of the small galaxy. This is the first evidence that Tucana II is home to a sprawling halo of dark matter – a region of gravity-bound matter that the researchers calculated was three to five times as massive as scientists estimated. This discovery of distant stars in an ancient dwarf galaxy implies that the very first galaxies in the universe were probably also vast and more massive than previously thought.

“Tucana II has a lot more mass than we thought to bind these stars so far away,” said MIT graduate student Anirudh Chiti. “This means that other remnants of early galaxies are likely to have these elongated halos as well.”

The researchers also found that the stars at the edge of Tucana II are more primitive than the stars at the core of the Milky Way. This is the first evidence of such a stellar imbalance in an ultra-dim dwarf galaxy.

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

“We may be seeing the first signature of galactic cannibalism,” said Anna Frebel, the Silverman Family Career Development Associate Professor of Physics at MIT. “One galaxy may have eaten one of its slightly smaller, more primitive neighbors, who then spilled all of its stars to the suburbs.”

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

Not-so-floppy galaxies

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

Chiti and Frebel wondered whether the old galaxy might harbor other, even older stars, that might shed light on the formation of the universe’s first galaxies. To test this idea, they obtained observations from Tucana II using the SkyMapper Telescope, a ground-based optical telescope in Australia that offers far-reaching views of the southern sky.

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

“Ani’s analysis shows a kinematic connection that these distant stars move in lockstep with the inner stars, like bath water running down the drain,” added Frebel.

The results suggest that Tucana II must have a sprawling dark matter halo three to five times as massive as previously thought to keep gravity on these distant stars. Dark matter is hypothetical matter believed to make up more than 85 percent of the universe. Each galaxy is believed to be held together by a local concentration, or halo, of dark matter.

“Without dark matter, galaxies would fly apart,” Chiti. says. “[Dark matter] is a critical ingredient in creating a galaxy and keeping it together. “

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

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

“A cannibalistic history”

Chiti and Frebel followed up their initial results with observations of Tucana II taken by the Magellan telescopes in Chile. With Magellan, the team focused on the galaxy’s metal-poor stars to infer their relative metallicity, and found that the outer stars were three times more metal-poor and therefore more primitive than those in the center.

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

A likely explanation for the imbalance could be an early galactic amalgamation, where one small galaxy – possibly one of the first generation of galaxies to emerge in the universe – swallowed up another nearby galaxy. This galactic cannibalism is constantly occurring across the universe today, but it was unclear whether early galaxies merged in a similar way.

“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 plans to use their approach to observe other ultra-faint dwarf galaxies around the Milky Way, hoping to discover even older, more distant stars.

“There are probably many more systems, maybe all of them, with these stars blinking at their edge,” Frebel says.