The Milky Way galaxy has had a violent history. It did not grow though simple and calm accretion, but rather through the aggressive mergers of multiple smaller galaxies. Recently, a team of astronomers found the remnants of perhaps the oldest merger, right in our stellar neighborhood.
The Milky Way’s stars have a wide variety of ages, sizes, and abundances of metals (elements heavier than hydrogen or helium). Stars that are born together tend to share many properties, because they all come from the same primordial gas cloud. But even though stars may be born together, they quickly disperse as they wander along their own orbits. So astronomers turn to subtle differences like metal abundances to help them pick out distinct populations. And it’s through this technique that astronomers can find populations of stars that weren’t born in our galaxy at all, but instead hopped on the ride during a violent merger event.
In a paper recently appearing on the arXiv preprint server, a team of astronomers studied a curious population of 20 stars currently orbiting in the vicinity of the Sun. Unlike the Sun, however, these stars have almost no metals. This means that the stars are incredibly ancient. Since heavy elements come from fusion inside of stars, it takes many cycles of stellar life and death to build up metals in newer generations.
While these stars don’t have a lot of metals they do have some, all in roughly the same amounts, indicating that they share a common origin.
However, half the stars are orbiting in one direction around the Milky Way, while the other half have opposite orbits.
Detective work
Puzzle pieces in place, researchers got to work making sense of it all. For one, these stars are far older than the typical star in the disk of the Milky Way, indicating that they likely weren’t born in our galaxy. Simulations of the evolution and distribution of elements inside galaxies suggest that the most likely birthplace of these stars would have been a small dwarf galaxy, weighing no more than roughly 2 percent of the current mass of the Milky Way.
The universe has many creative ways to produce heavy elements: giant stars exploding in a supernova at the ends of their lives, merging neutron stars, and white dwarves accumulating enough material to explode in their own kind of supernova. The particular mix of heavy elements that the astronomers found in the population of stars suggest that they have been enriched by exploding giant stars and merging neutron stars, but not white dwarf explosions. This means that the stars’ parent galaxy had a very brief and extreme life, with a lot of star formation (and star death) without enough time for white dwarves to go supernova.
The astronomers dubbed the parent galaxy of these interloper stars Loki, after the trickster god of Norse mythology. While it’s difficult to pinpoint the exact time when Loki and the Milky Way merged, researchers can turn to one other clue: the fact that these stars don’t all share the same direction in their orbits.
To explain that strange behavior, Loki must have merged with the Milky Way extremely early in its history, well before our own galaxy developed its own disk of stars that all mostly share a common orbital direction. When the galaxies merged, Loki’s stars scattered in all directions.
Considering that the Milky Way began coalescing around 10-11 billion years ago, this would make Loki potentially one of its first merger encounters, and this strange population of 20 stars one of the oldest remnant populations left in the galaxy.
It’s through this kind of detective work that astronomers are piecing together not just the story of Loki, but all of the mergers that lead to our present-day galaxy.
