A small, ancient dwarf galaxy called Tucana II that orbits the Milky Way has been keeping a big secret. According to a new study of stars around the object, gravitationally bound to it at great distances, its halo of dark matter is much more massive than we thought.
In fact, it is absolutely huge. Although the stellar mass of Toucan II is about 3,000 times the mass of the Sun, its halo of dark matter is around 10 million times the mass of the Sun. It is about three to five times more massive than previous estimates.
This suggests that the first galaxies in the Universe may have been much more massive than we knew.
“Toucan II has a lot more mass than we thought, in order to bind these stars that are so far apart,” said MIT astrophysicist Anirudh Chiti. “This means that other early galaxies in the relics probably also have this type of extended halos.”
The Milky Way has a whole swarm of auxiliary dwarf galaxies. These are small, weak clusters of stars, very low in metal, which reveal that they are very old, as the metals took a while to form in the heart of the stars and propagate through the Universe.
Toucan II, located about 163,000 light-years from Earth, is among the smallest. Based on the metallicity of its star population, it is also one of the oldest, almost without metals. Chiti and his team were researching these stars, hoping to find an even older population of stars.
They made observations using the Australian National University’s SkyMapper telescope and published the results using a Chiti algorithm designed to select metal-poor stars. In addition to the stars in the heart of Tucana II, the algorithm detected nine new stars, at fairly long distances.
Data collected by the Gaia satellite, an ambitious project to map the Milky Way in three dimensions, including the movements of stars, confirmed this. Those stars far from the core of the dwarf galaxy were orbiting around them, gravitationally bound.
However, the previously estimated properties of the galaxy did not include too much to produce the type of gravitational force that would hold those distant stars together. Which meant there was some mass there that we couldn’t see or detect directly. Which in turn meant dark matter.
We do not know what dark matter is, but there is an invisible mass in the Universe responsible for creating all the extra gravity, making galaxies rotate faster and double space-time, and there is much more than that. normal matter. This is dark matter and we believe it is the glue that binds the galaxies together.
“Without dark matter, galaxies would just separate,” Chiti said. “[Dark matter] it’s a crucial 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 of the dark matter mass of Tucana II, eventually reaching the range of 10 million solar masses. This is the first evidence that ultra-bright dwarf galaxies can have so much dark matter, and it raises many riddles.
“This probably also means that the first galaxies formed in much larger dark matter halos than previously thought,” said MIT astrophysicist Anna Frebel. “We thought the first galaxies were the smallest and smallest. But they can actually be several times larger than we thought and, after all, not so small.”
So where the hell did he get all that dark matter? A clue in this regard could be in the stars of the galaxy. When the team studied data from Magellanic telescopes in Chile, they found that not all stars had the same metallicity.
In fact, they were quite divided between two populations. The stars on the outskirts of Tucana II had three times less metallicity than the stars in the center, suggesting two separate stellar populations. In the Milky Way, this can happen if a population of stars has arrived from other places, such as a collision with another galaxy.
This is the first time a chemical difference between stars has been seen in an ancient galaxy, but it is possible that the reasons are similar: once, Tucana II was not one, but two galaxies that fused combining their dark matter. . nimbo.
“We may be seeing the first signature of galactic cannibalism,” Frebel said. “A galaxy may have eaten one of its most primitive, slightly smaller neighbors, which then spilled all its stars on the outskirts.”
However, research shows that the range of these tiny satellite galaxies can now be observed and characterized, meaning that others such as Toucan II could be identified. There are even two candidates: the ultrafaint dwarf galaxies Segue 1 and Bootes I have a star each at an extended distance from the galactic core.
The team plans to use their techniques to find more stars and more galaxies of this type and study them.
“There are probably a lot more systems, maybe all of them, that have these stars flashing on their outskirts,” Frebel said.
The research has been published in Nature Astronomy.