A tiny, ancient dwarf galaxy named Tucana II orbiting the Milky Way has been harbouring a big secret. According to a new study of stars around the object, gravitationally bound to it at great distances, its dark matter halo is a lot more massive than we thought.
In fact, it's absolutely huge. Although the stellar mass of Tucana II is around just 3,000 times the mass of the Sun, its dark matter halo clocks in at 10 million times the mass of the Sun. That's around three to five times more massive than 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, in order to bound these stars that are so far away," said astrophysicist Anirudh Chiti of MIT. "This means that other relic first galaxies probably have these kinds of extended halos too."
The Milky Way has a whole swarm of attendant dwarf galaxies. These are small, faint clusters of stars that are very low in metal, revealing that they are very old, since metals took some time to form in the hearts of stars and propagate through the Universe.
Tucana II, located about 163,000 light-years from Earth, is among the smallest. Based on the metallicity of its star population, it's also among the oldest, with almost no metals to be found. Chiti and his team were investigating these stars, hoping to find a population of even older stars.
They took observations using the Australian National University's SkyMapper telescope and ran the results through an algorithm Chiti designed to pick out metal-poor stars. In addition to the stars within the heart of Tucana II, the algorithm detected nine new stars, at quite extended distances.
Data collected by the Gaia satellite – an ambitious project to map the Milky Way in three dimensions, including the motions of the stars – confirmed it. Those stars far from the core of the dwarf galaxy were in orbit around it, gravitationally bound.
Yet the previously estimated properties of the galaxy did not include enough mass to produce the kind of gravitational strength that would keep those distant stars bound. Which meant that there was some mass there that we could not see, or detect directly. Which meant, in turn, dark matter.
We don't know what dark matter is, but there is some unseen mass out there in the Universe responsible for creating all the extra gravity, making galaxies spin faster, and bending spacetime – and there is a lot more of it than normal matter. That's dark matter, and we believe that it's the glue that binds galaxies.
"Without dark matter, galaxies would just fly apart," Chiti said. "[Dark matter] is a crucial ingredient in making a galaxy and holding it together."
Based on the positions and motions of the stars, the team was able to update the estimate for the dark matter mass of Tucana II, ultimately arriving in the range of 10 million solar masses. This is the first evidence that ultrafaint dwarf galaxies can have that much dark matter, and it raises a lot of puzzles.
"This probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought," said astrophysicist Anna Frebel of MIT. "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."
So, where the heck did it get all that dark matter from? A clue to that could be in the galaxy's stars. When the team studied data from the Magellan Telescopes in Chile, they found that not all the stars had the same metallicity.
In fact, they were pretty starkly divided between two populations. The stars on the outskirts of Tucana II were three times lower in metallicity than the stars in the centre, suggesting two separate stellar 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 such a chemical difference between stars has been seen in an ancient galaxy, but it's possible that the reasons why are similar: once upon a time, Tucana II was not one, but two galaxies that merged, combining their dark matter haloes.
"We may be seeing the first signature of galactic cannibalism," Frebel said. "One galaxy may have eaten one of its slightly smaller, more primitive neighbours, that then spilled all its stars into the outskirts."
However it happened, the research demonstrates that the extended reach of these tiny satellite galaxies can now be observed and characterised, which means others like Tucana II could be identified. There are even two candidates – ultrafaint dwarf galaxies Segue 1 and Bootes I each have one star at an extended distance from the galactic core.
The team plans to use their techniques to find more such stars, and more such galaxies, and study them.
"There are likely many more systems, perhaps all of them, that have these stars blinking in their outskirts," Frebel said.
The research has been published in Nature Astronomy.