A group of stars buried inside our own galaxy may be the leftover remains of a smaller galaxy that the Milky Way swallowed billions of years ago, according to new research.
The findings, published in Monthly Notices of the Royal Astronomical Society, include the team’s nickname for the absorbed galaxy Loki, after the Norse trickster god, which they believe was consumed during the Milky Way’s earliest growth period.
The case for further research
The Milky Way is what it is, in part, because of how much it consumed as it grew. Cosmologists have known for a long time that big galaxies tend to grow by absorbing smaller ones. The new paper lays out evidence that one of those smaller galaxies can still be identified from its remnants.
Loki, which fits this description perfectly, is thought to have been a dwarf galaxy. By the standards of mature galaxies, it was small. By the standards of anything else, it was a respectable galaxy in its own right that would have still contained billions of stars even if it were a fraction of the size of the Milky Way’s current few hundred billion.
To trace the remnants of Loki, the researchers focused on a group of 20 metal-poor stars sitting on the galactic plane, the flat band where most of the Milky Way’s stars are. These metal-poor stars are old, having formed in the early universe, retaining the chemical signature of whatever was around them at the time, effectively making them excellent fossil records of the galaxy’s early history.
As the researchers put it: “These building blocks merged at early epochs, dispersing their stellar, gaseous, and dark matter content into the forming proto-galaxy.”
Signs of a violent youth
The 20 stars in the sample looked different from the metal-poor stars around them. Their compositions didn’t match, nor did their orbits. The stars in question carried chemical traces of violent events from the early universe, including supernovas and the collisions of neutron stars, both of which fling heavy elements out into space.
What was missing proved just as revealing as what was present. The stars didn’t show any signs of contamination from white dwarfs, which are what’s left when a sun-sized star runs out of fuel and casts off its outer layers. They take billions of years to form. If the original galaxy these stars came from had been around long enough to produce white dwarfs, the chemistry of those white dwarfs would have left a mark. In this case, it didn’t.
To the team, this suggests that whatever galaxy these stars came from didn’t last very long before it was absorbed. It was short-lived enough that white dwarfs never had a chance to develop in it. Such a brief existence fits the profile of a small dwarf galaxy that ran into something much larger early in its existence.
The team has been careful about how strong a claim they’re making. The sample is admittedly small; nonetheless, twenty stars are enough to flag a pattern.
However, it is not enough to confirm that Loki existed as a separate galaxy or to map out its original structure. Larger datasets will be needed before any of that becomes possible, the researchers noted.
Sources: Oxford Academic, NYP, Phys.org