Our home galaxy has a secret buried inside. A new study published in the Monthly Notices of the Royal Astronomical Society suggests that the Milky Way swallowed an ancient dwarf galaxy billions of years ago, and its stellar remains are still embedded within ours.
Researchers have named this lost galaxy Loki, after the Norse trickster god, and the name is quite fitting because it remained hidden in plain sight for a very long time.
How did astronomers find Loki?
The discovery came down to star chemistry. The first stars that formed after the Big Bang were made almost entirely of hydrogen and helium.
Over billions of years, later generations of stars fused those elements into heavier ones. Stars with very little of those heavier elements are therefore considered ancient, and astronomers call them metal-poor.
Researchers studied a group of 20 metal-poor stars sitting inside the Milky Way’s galactic disk, which is the flat, spinning region where most of our galaxy’s stars live. That’s what made them stand out. Metal-poor stars typically turn up in the galaxy’s outer halo, not the disk. Their presence suggests that they came from somewhere else entirely.
The chemical evidence backed that up. The team found chemical traces of supernovas and neutron star mergers, but no evidence of white dwarf explosions. White dwarfs take billions of years to form, so their absence suggests Loki burned out long before it could produce them.
The orbits of Loki’s stars reveal a secret about our galaxy’s past
Here’s where it gets stranger. Of the 20 stars, 11 travel in the same direction as the Milky Way, and nine go in the opposite direction. This suggests an extremely early merger, back when our galaxy’s own orbits were still chaotic and unsettled. Researchers believe all 20 stars came from one system, not two.
In fact, Loki isn’t the only galaxy the Milky Way consumed. A separate 2020 study identified an ancient galaxy called Kraken that merged with our galaxy around 11 billion years ago. The Milky Way, it turns out, has always had a big appetite.