A small, old dwarf system called Tucana II that orbits the Milky Way contains a great secret. According to a new study of stars orbiting the object at great distances, the halo of dark matter is much more massive than we thought.
In fact, it’s absolutely great. Although the star mass of Tucana II is about 3000 times the mass of the sun, the halo of dark matter is 10 million times the mass of the sun. This is about three to five times as massive 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 that are so far away,” said astrophysicist Anirudh Chiti of MIT. “This means that other galaxies from the remnants probably also have such elongated halos.”
The Milky Way has a whole swarm of dwarf systems. These are small, dim clusters of stars that are very low in metal, revealing that they are very old, as metals took some time to form in the hearts of stars and propagate through the universe.
Tucana II, about 163,000 light-years from Earth, is one of the smallest. Based on the metallicity of its star population, it is also one of the oldest with almost no metals. Chiti and his team examined these stars, hoping to find a population of even older stars.
They take observations using the SkyMapper telescope of the Australian National University and execute the results through an algorithm designed by Chiti to pick out metal-poor stars. In addition to the stars at the heart of Tucana II, the algorithm has detected nine new stars at very long 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 this. Those stars far from the core of the dwarf system were in orbit around it, gravitationally bound.
Yet the previously estimated properties of the galaxy did not contain enough mass to produce the kind of gravity that would keep those distant stars bound. This meant that there was a mass that we could not see or detect directly. Which in turn meant dark matter.
We do not know what dark matter is, but there is an unseen mass in the universe that is responsible for creating all the extra gravity, spinning galaxies faster and bending space-time – and there is much more than normal matter. It is dark matter, and we believe it is the glue that binds galaxies.
“Without dark matter, galaxies would just fly apart,” Chiti said. “[Dark matter] is an important ingredient in making a galaxy and holding it together. ‘
Based on the positions and movements of the stars, the team was able to update the estimate for the dark matter mass of Tucana II, eventually reaching about 10 million solar masses. This is the first evidence that ultra-dull dwarf systems can have so much dark matter, and this raises many mysteries.
“It probably also means that the earliest galaxies formed in much larger dark matter halos than previously thought,” said astrophysicist Anna Frebel of MIT. “We thought the first galaxies were the smallest, most volatile galaxies. But it was perhaps a few times larger than we thought, and yet not so small.”
So, where did it get all the dark matter from? A clue to that may be in the stars of the galaxy. When the team studied data from the Magellan telescopes in Chile, they found that not all stars have the same metal.
In fact, they were fairly sharply divided between two populations. The stars on the outskirts of Tucana II were three times less metallic than the stars in the center, indicating that there were two different star populations. In the Milky Way, this can happen when a population arrives at stars from elsewhere, such as a collision with another galaxy.
This is the first time such a chemical difference has been seen between stars in an ancient galaxy, but it is possible that the reasons for this are similar: once, Tucana II was not one, but two galaxies that merged and their dark matter combined. halo.
“We may see the first sign of galactic cannibalism,” Frebel said. “One galaxy may have eaten one of its slightly smaller, more primitive neighbors, which then spilled all its stars into the suburbs.”
Either way, the research shows that the extensive range of these small satellite systems can now be observed and characterized, meaning that others like Tucana II can be identified. There are even two candidates – ultra-five dwarf systems Segue 1 and Bootes I each have one star at a long 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 probably a lot more systems, maybe all, that are cutting these stars in their suburbs,” Frebel said.
The research was published in Natural Astronomy.