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A small and ancient dwarf galaxy named Tucana II orbiting the Milky Way holds a great secret. According to a new study of the stars around the object, gravitationally linked to it at great distances, its dark matter halo is much more massive than previously thought.
In fact, it is absolutely huge. Although Tucana II’s stellar mass is only about 3,000 times the mass of the Sun, its dark matter halo is 10 million times the mass of the Sun. That’s about three to five times as massive as previous estimates.
This suggests that the first galaxies in the Universe could have been much more massive than we thought.
“Tucana II has a lot more mass than we thought, in order to bind these stars that are so far away,” said MIT astrophysicist Anirudh Chiti. “This means that other early relic galaxies probably also have these kind of extended halos.”
The Milky Way has a whole swarm of dwarf galaxies. These are small, weak clusters of stars that are very poor in metal, revealing that they are very old, as metals took a long time to form in the hearts of stars and to spread throughout 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 is also among the oldest, with almost no metal to be found. Chiti and her team were investigating these stars, hoping to find an even older star population.
They took observations using the Australian National University’s SkyMapper Telescope and ran the results through a Chiti algorithm designed to detect stars low in metals. In addition to the stars at the heart of Tucana II, the algorithm detected nine new stars, at fairly large 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. These stars far from the core of the dwarf galaxy were orbiting it, gravitationally linked.
Yet the previously estimated properties of the galaxy did not include enough mass to produce the type of gravitational force that would keep these distant stars bound. Which meant that there was a mass there that we could not see or detect directly. Which in turn meant dark matter.
We don’t know what dark matter is, but there is an invisible mass in the Universe responsible for creating all the extra gravity, making galaxies spin faster, and bending space-time – and there’s has much more than normal matter. It’s dark matter, and we think it’s the glue that binds galaxies together.
“Without dark matter, galaxies would just fly to pieces,” Chiti said. “[Dark matter] is a crucial 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 estimate of the dark matter mass of Tucana II, eventually arriving in the range of 10 million solar masses. This is the first proof that ultra-thin dwarf galaxies can have so much dark matter, and it raises many puzzles.
“It probably also means that the first galaxies formed in halos of dark matter much larger than previously thought,” said astrophysicist Anna Frebel from MIT. “We thought the early galaxies were the smallest and wimest galaxies. But they may have been several times larger than we thought, and not that small after all.”
So where the hell did he get all this dark matter from? A clue to this could be in the stars of the galaxy. When the team studied data from Magellan’s telescopes in Chile, they found that not all stars had the same metallicity.
In fact, they were quite sharply divided between two populations. The stars on the outskirts of Tucana II had three times the metallicity of the stars in the center, suggesting two distinct 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 that such a chemical difference between stars has been observed in an ancient galaxy, but it is possible that the reasons are similar: once upon a time, Tucana II was not one, but two galaxies that have merged, combining their dark matter halos.
“We are perhaps witnessing the first signing of galactic cannibalism,” Frebel said. “A galaxy may have eaten one of its slightly smaller, more primitive neighbors, which then knocked over all of its stars in the periphery.”
Either way, research shows that the extended reach of these tiny satellite galaxies can now be observed and characterized, meaning others like Tucana II could be identified. There are even two candidates – the ultra-thin dwarf galaxies Segue 1 and Bootes I each have a star an extended distance from the galactic core.
The team plans to use their techniques to find and study more stars and galaxies of this type.
“There are probably a lot more systems, maybe all of them, that have these blinking stars around their periphery,” Frebel said.
The research was published in Nature astronomy.
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