A small ancient dwarf galaxy called 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 over long distances, its halo of dark matter is much more massive than I thought.
In fact, it is absolutely huge. Although the stellar mass of Tucana II is about 3,000 times larger than the Sun, its halo of dark matter is 10 million times larger than the mass of the Sun. This is about three to five times higher than previous estimates.
This suggests that the first galaxies in the Universe could have been much more massive than we knew.
“Tucana II has a lot more mass than I thought, to bind these stars that are so far away,” said astrophysicist Anirudh Chiti of MIT. “This means that other relic galaxies probably have these types of extended halos.”
The Milky Way has a whole swarm of accompanying dwarf galaxies. These are groups of small, faint stars with a very low metal content, revealing that they are very old, because metals took some time to form in the hearts of stars and spread 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 is also among the oldest, with almost no metal to be found. Chiti and his team investigated these stars, hoping to find an even older star population.
They took observations using the Australian National University’s SkyMapper telescope and disseminated the results using a Chiti algorithm designed to pick metal-poor stars. In addition to the stars in the heart of Tucana II, the algorithm detected nine new stars at fairly long distances.
The data collected by the Gaia satellite – an ambitious project for mapping 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 in gravitation orbit around it.
However, the previously estimated properties of the galaxy did not include enough mass to produce the type of gravitational force that would hold those distant stars together. Which meant there was a 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 some unseen mass there in the Universe, responsible for creating all the extra gravity, making galaxies spin faster and bending space-time – and there is much more than normal matter. This is dark matter and we believe it is the adhesive that binds the galaxies.
“Without dark matter, galaxies would just fly,” Chiti said. “[Dark matter] it is a crucial ingredient in creating and maintaining a galaxy. “
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 10 million solar masses. This is the first proof that ultrafaint dwarf galaxies can have so much dark matter and pick up a lot of puzzles.
“This also means that the first galaxies probably formed in much larger dark matter halos than previously thought,” said astrophysicist Anna Frebel of MIT. “I thought the first galaxies were the smallest and most beautiful galaxies. But in fact, they may have been a few times bigger than I thought, and after all, not so small.”
So where the hell did he get all the dark matter? A clue to this could be in the stars of the galaxy. When the team studied data from Chile’s Magellan Telescopes, they found that not all stars had the same metallicity.
In fact, they were quite clearly 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 came from elsewhere, such as a collision with another galaxy.
This is the first time such a chemical difference has been observed between stars in an ancient galaxy, but the motives may be similar: once, Tucana II was not one, but two galaxies that merged, combining their matter. dark. nimburi.
“We may see the first signature of galactic cannibalism,” Frebel said. “It is possible that a galaxy ate one of its slightly smaller, more primitive neighbors, which then shed all its stars on the periphery.”
However, research shows that the extensive coverage of these small satellite galaxies can now be observed and characterized, which means that other people such as Tucana II could be identified. There are even two candidates – the ultrafaint dwarf galaxies Segue 1 and Bootes I each have a star at an extended distance from the galactic core.
The team intends to use their techniques to find more such stars and more such galaxies and study them.
“There are probably many other systems, maybe all of them, that have these stars flashing on their periphery,” Frebel said.
The research was published in Nature Astronomy.