Scientists spot signs of 'fossil galaxy' lurking in the heart of the Milky Way

Say hello to Heracles, a previously hidden galaxy that likely smooshed into the Milky Way billions of years ago.

This image shows the Milky Way seen from Earth. The reddish rings highlight where the stars of Heracles linger within our galaxy.
Danny Horta-Darrington (Liverpool John Moores University), ESA/Gaia, and the SDSS

Our very own Milky Way galaxy had a dramatic childhood. Astronomers have unveiled a new chapter in its memoir with the discovery of a likely "fossil galaxy" hidden near its heart.

The proposed fossil galaxy is named Heracles for the Greek hero. It probably tangled with the Milky Way around 10 billion years ago, back when our galaxy was a baby.

"Stars originally belonging to Heracles account for roughly one third of the mass of the entire Milky Way halo today -- meaning that this newly-discovered ancient collision must have been a major event in the history of our galaxy," the Sloan Digital Sky Survey (SDSS) said in a statement Thursday. The SDSS was involved in the research.

This illustration shows how Heracles fits into the heart of the Milky Way if it were seen from above. The yellow dot points out our sun.

Danny Horta-Darrington (Liverpool John Moores University), NASA/JPL-Caltech, and the SDSS

A research team led by Liverpool John Moores University (LJMU) graduate student Danny Horta published a paper on Heracles this week in the Monthly Notices of the Royal Astronomical Society journal. 

To find Heracles, the team spotted stars that didn't match the Milky Way's. "These stars are so different that they could only have come from another galaxy. By studying them in detail, we could trace out the precise location and history of this fossil galaxy," said Horta.

We've seen evidence of dramatic galactic mergers in the Milky Way's deep past. Recent studies have looked into a time when our galaxy gobbled up a dwarf galaxy called Gaia-Enceladus

Heracles has been particularly elusive, since signs of its existence are obscured by interstellar dust clouds. The research team used the SDSS Apache Point Observatory Galactic Evolution Experiment (APOGEE) to peer through this mess using near-infrared light. "To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars," said study co-author and LJMU astrophysicist Ricardo Schiavon.

The Milky Way may not be done with its galaxy-colliding ways. A Milky Way-Andromeda galaxy crash is lurking billions of years into the future. It's tough being a galaxy. Sometimes you're the Milky Way, eating them up. Sometimes you're Heracles, the one getting swallowed.