Ten billion years ago, the young Milky Way survived a titanic merger with a neighboring galaxy and eventually consumed the whole thing. Now remnants of this fossilized galaxy are still floating in the core of our galaxy – and astronomers have discovered that nearly a third of the current population of the Milky Way came from this dismantled rival.
Galaxy Evolution isn't pretty at all. Big, impressive, beautiful galaxies like our own Milky Way hide a history of violence. Galaxies grow by hitting and consuming something small enough to get in their way, including a merger or two with an equally sized neighbor.
But even though our galaxy now looks calm and silent, the bones of its enemies were scattered across the galaxy. It's not obvious – at first glance, a star is like any other star. But through massive stellar surveys like APOGEE (the Apache Point Observatory's Galactic Evolution Experiment, part of the Sloan Digital Sky Survey) we can piece the clues together.
APOGEE has mapped almost half a million stars so far, but not in visible light – in infrared. This has given him an unprecedented power to look deep into the core of our galaxy, a place where dust and gas are too thick to let visible light through. And by searching deep within our core, astronomers discovered a fossilized remnant of a galaxy that collided with ours nearly ten billion years ago.
The evidence of the merger is in the form of outstanding properties. If you record the properties of stars such as their chemical composition and speed, you can get a feel for what a “normal” Milky Way star looks, looks and smells like. And if some stand out, you know they must have come from elsewhere.
Astronomers have used this technique to find fusion debris, but all of these stars have lived on the galactic outskirts. But now, APOGEE has found evidence of a merger based on observations from the core that suggests the collision event must have occurred a long time ago.
An artistic impression of what the Milky Way could look like from above. The colored rings show the rough extent of the fossil galaxy known as Herakles. The yellow point shows the position of the sun. Credit: Danny Horta-Darrington (Liverpool John Moores University), NASA / JPL-Caltech and SDSS
The rogue stars discovered with APOGEE belonged to an ancient galaxy astronomers named Hercules. It was a big deal: it is likely that Hercules contributed a hundred billion stars (and maybe more) to our current galactic population.
It's a surprise too. It is believed that spiral galaxies like the Milky Way shouldn't collide as massively. However, with further investigation planned, astronomers are sure to find more remains and fossils across the galaxy, revealing the true story of our past.