The Milky Way is surrounded by over 50 dwarf galaxies (maybe more), some unchanged since they first formed 13 billion years ago. They are an incredible laboratory to study a slice of the early universe up close, so discovering something new about them can have big implications. A paper in Nature Astronomy does exactly that.
Astronomers have announced that dwarf galaxy Tucana II appears to have a larger dark matter halo than previously estimated, suggesting a more complex history for this galaxy and helping us better understand how galaxies might have evolved.
According to the leading theory of the universe, five-sixths of all matter in the universe doesn’t interact with light – for this reason, it is called dark matter. Dark matter doesn’t seem to interact much at all, it doesn’t clump up. We can tell it’s there by looking at the gravitational interaction of galaxies.
Usually, to estimate the size of the dark matter halo that enshrouds a galaxy, one checks how stellar clusters are moving around the center of the galaxy. Tucana II is an ultrafaint dwarf galaxy, so establishing its size is not an easy task. Astronomers have discovered new stars further out from the center than previous observations had reported. This suggests that the galaxy has a lot more dark matter and it weighs three to five times more than previous estimations.
"Tucana II has a lot more mass than we thought, in order to bound these stars that are so far away," lead author Anirudh Chiti, a graduate researcher at MIT, said in a statement. "This means that other relic first galaxies probably have these kinds of extended halos too."
Chemical analysis from the stars in Tucana II tells us that this dwarf galaxy is among the most primitive we have ever studied. This can be worked out by the amount of elements heavier than helium that we see in stars. This is because as stars die, they release these heavier elements.
Usually, the more primitive stars tend to be found around the center of a galaxy. However, the observations that revealed the larger extent of this galaxy showed that the stars further out were the more primitive ones. This counter-intuitive finding provides a clue on how Tucana II evolved.
"We may be seeing the first signature of galactic cannibalism," added co-author Anna Frebel, an associate professor of Physics at MIT. "One galaxy may have eaten one of its slightly smaller, more primitive neighbors, that then spilled all its stars into the outskirts."
The observations suggest that the earliest galaxies in the universe might have sported large dark matter haloes, and that galaxy mergers started very early in cosmic history.