The earliest galaxy 'swallowed' by the cannibal Milky Way left a scar at its heart

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The Milky Way has something of a torrid and violent history, given that our galaxy has fueled its own growth by cannibalistically devouring smaller galaxies. Now, astronomers have discovered evidence of our galaxy's earliest bout of cannibalism in the form of "scarring" at the heart of the Milky Way left behind by a galactic morsel it devoured around 12 billion years ago.

Working as part of the ARMA (Cluster Ages to Reconstruct the Milky Way Assembly) project, the team behind this discovery used dense conglomerations of ancient stars called globular clusters to find traces of a merger that occurred around 1.8 billion years earlier than the previously earliest known event of this type.

Prior to this, the history of the Milky Way colliding and merging with both smaller and larger galaxies had only been reconstructed up to around 10 billion years ago, around 3.8 billion years after the Big Bang. The earliest bout of cosmic cannibalism involved the collision with and consumption of the dwarf galaxy Gaia-Enceladus about 10 billion years ago. That means the earliest history of the Milky Way was, until now, shrouded in mystery.

"This work tells us what happened to the Milky Way in its infancy," team member Chiara Zerbinati of the Department of Physics and Astronomy at the University of Bologna, Italy, said in a statement. "It sheds light on a particularly significant event that influenced the entire subsequent evolution of the galaxy. If one of humanity's great questions is 'where do we come from?', we offer at least a piece of the answer."

A helping hand from Hubble

The key development in the quest to delve further back into the history of the Milky Way was delivered by the Hubble Space Telescope.

"We started with the ambition of reconstructing the assembly history of the Milky Way, that is, all the galactic merger events that led to its current appearance," team leader Davide Massari of the Italian Institute for Astrophysics (INAF) said. "We used globular clusters as tracers: especially in the inner regions of the galaxy, where extinction is highest, they are the only objects for which we can obtain excellent measurements of both orbital motion and age."

Astronomers developed a technique using Hubble data that allowed them to discover the ages of globular clusters with a far greater precision than had ever been possible before. This revealed that the globular clusters at the heart of our galaxy can be categorized in three distinct ways based upon their age and their metallicity, a measure of elements heavier than hydrogen and helium, which astronomers refer to as metals.

One group of globular clusters could be linked directly to the Gaia-Enceladus merger; one belonged to the original Milky Way; but there also existed a third group distinct enough from the other two groups that it must have come from some hitherto undiscovered merger event.

The team was able to deduce some things about this doomed galaxy from this third group of globular clusters. They found the unfortunate galaxy had around the same mass as Gaia-Enceladus, about 500 million times the mass of the sun. For comparison, the Milky Way today has a mass of around 1.5 trillion solar masses.