A small, ancient dwarf galaxy called Tucana II orbiting the Milky Way hides a great secret. According to a new study of stars around the object, bound to them by gravity at great distances, the dark matter halo is much more massive than we thought.
In fact, it is absolutely huge. Although the stellar mass of Tucana II is about 3,000 times the mass of the sun, the dark matter halo is 10 million times more massive than the sun. That is about three to five times as heavy as previous estimates.
This suggests that the earliest galaxies in the Universe could have been much more massive than we knew.
“Tucana II has a lot more mass than we thought to bind these stars so far away,” said astrophysicist Anirudh Chiti of MIT. “This means that other remnants of early galaxies are likely to have these elongated halos as well.”
The Milky Way has a swarm of companion dwarf galaxies. These are small, faint star clusters with a very low metal content, showing that they are very old, as it took some time to form metals in the hearts of stars and propagate through the universe.
About 163,000 light-years from Earth, Tucana II is one of the smallest. Based on the metallicity of its star population, it is also one of the oldest, with almost no metals to be found. Chiti and his team investigated these stars, hoping to find a population of even older stars.
They took observations using the Australian National University’s SkyMapper telescope and passed the results through a Chiti algorithm designed to select metal-poor stars. In addition to the stars at the heart of Tucana II, the algorithm detected nine new stars at fairly great distances.
Data collected by the Gaia satellite – an ambitious project to map the Milky Way in three dimensions, including the movements of the stars – confirmed it. Those stars far from the core of the dwarf galaxy were orbiting around them, gravity-bound.
But the galaxy’s previously estimated properties don’t include enough mass to produce the kind of gravity that would keep those distant stars bound. Which meant there was a mass out there that we couldn’t see or detect directly. That, in turn, meant dark matter.
We don’t know what dark matter is, but there is an invisible mass in the Universe that is responsible for creating all of the extra gravity, making galaxies spin faster and bend spacetime – and there’s a lot more of it than normal matter. That’s dark matter, and we believe it’s the glue that binds galaxies.
“Without dark matter, galaxies would just fly apart,” Chiti said. “[Dark matter] is a critical ingredient in creating a galaxy and keeping it together. “
Based on the positions and motions of the stars, the team was able to update the Tucana II dark matter mass estimate, which ended up in the range of 10 million solar masses. This is the first evidence that ultra-faint dwarf galaxies can contain so much dark matter, and it raises many puzzles.
“This probably also means that the earliest galaxies formed in much larger halos of dark matter than previously thought,” said MIT astrophysicist Anna 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.”
So, where the hell did it get all that dark matter from? An indication of this could be in the stars of the Milky Way. When the team studied data from the Magellan telescopes in Chile, they found that not all stars had the same metallicity.
In fact, they were split pretty grimly across two populations. The stars at the edge of Tucana II were three times less metallic than the stars at the center, indicating two separate star populations. In the Milky Way, this can happen if a population of stars has arrived from elsewhere, such as a collision with another galaxy.
This is the first time that such a chemical difference between stars has been observed in an ancient galaxy, but it is possible that the reasons for this are similar: Once upon a time, Tucana II was not one, but two galaxies merging and their dark matter halos combined.
“We may be seeing the first signature of galactic cannibalism,” said Frebel. “One galaxy may have eaten one of its slightly smaller, more primitive neighbors, who then spilled all of its stars to the suburbs.”
However it happened, the research shows that the extended range of these small satellite galaxies can now be observed and characterized, meaning others such as Tucana II could be identified. There are even two candidates: ultra-faint dwarf galaxies Segue 1 and Bootes I each have one star at a greater distance from the galactic core.
The team plans to use their techniques to find and study more such stars and more such galaxies.
“There are probably many more systems, maybe all of them, with these stars blinking at their edge,” Frebel said.
The research is published in Nature astronomy.