The Milky Way Devoured Another Galaxy 10 Billion Years Ago

New data from the European Space Agency explains why some stars are going the wrong way, galactically speaking.

Milky Way
The Milky Way. | Getty Images
The Milky Way. | Getty Images

The Milky Way has a dark secret.

According to new data beamed in from European research spacecraft, our Milky Way galaxy encountered and devoured another nearby galaxy around 10 billion years ago.

More precisely, the Milky Way collided with the second galaxy, absorbing many of its stars and spiraling out a chaotic tangle of stellar matter — birthing new stars, altering the orbits of others, and sending some in the opposite direction of the Milky Way's own rotation.

That last bit provided the first clue to astronomers, who have long observed that some stars in our home galaxy are essentially going the wrong way, moving against the trailing arms of the galaxy. Other stars have been observed to spin in strange clusters, bolstering the theory that the Milky Way is actually the result of a number of previous mergers and meals.

New research published in the journal Nature confirms that a good number of these rogue stars have a common origin — a deceased galaxy dubbed Gaia-Enceladus. It also suggests that our galaxy was largely formed by a few big collisions, rather than a lot of smaller ones, which helps answer a question that has occupied astronomers for years.

Milky Way
On the left, a face-on view shows the spiral structure of the Galactic Disc, where the majority of stars are located, interspersed with a diffuse mixture of gas and cosmic dust. The disc measures about 100 000 light-years across, and the Sun sits about half way between its center and periphery. On the right, an edge-on view reveals the flattened shape of the disc. Observations point to a substructure: a thin disc some 700 light-years high embedded in a thick disc, about 3000 light-years high and populated with older stars. The edge-on view also shows the Galactic Bulge, located in the central portion of the Milky Way and hosting about 10 billion stars. | ESA

The discovery was made by an international team of researchers led by Amina Helmi, an astronomer with the University of Groningen in the Netherlands.

Helmi has spent most of her career studying the Milky Way for “fossil stars” that provide clues to the ultimate origin of our galaxy. She has pioneered a specific approach to the mystery by combining analysis of the stars’ placement, trajectory, and chemical compositions.

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The new research provides hard evidence for previous theories concerning the ancient galactic collisions. That evidence comes largely from data sent by the Gaia satellite mission, a major initiative from the European Space Agency.

The Gaia satellite, launched in 2013, recently beamed back a colossal chunk of new data detailing the position, brightness, and motion of more than 1.3 billion stars in the Milky Way. Helmi and her team have been crunching the numbers since April.

Milky Way
Artist's impression of debris of the Gaia-Enceladus galaxy. Gaia-Enceladus merged with our Galaxy during its early formation stages, 10 billion years ago, and its debris can now be found throughout the Galaxy. | ESA

In materials issued with the new research, Helmi said that the chemical signature of many wrong-way stars was demonstrably different than “native” Milky Way stars.

“And they are a fairly homogeneous group, which indicates they share a common origin,” she said. “The youngest stars from Gaia-Enceladus are actually younger than the native Milky Way stars in what is now the thick disk region. This means that the progenitor of this thick disk was already present when the fusion happened, and Gaia-Enceladus, because of its large size, shook it and puffed it up.”

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Size is a tricky concept when working on the galactic scale, but Helmi’s team estimate that Gaia-Enceladus, the dearly departed galaxy, was slightly more massive than the Small Magellanic Cloud (SMG), a dwarf galaxy with a total solar mass about one percent of the Milky Way. (By the way, the Milky Way is slowly eating the SMG, too.)

An interesting final note: When Helmi finally pieced together all the data from the Gaia info dump, she discovered that the numbers looked awfully familiar. They reminded Helmi of simulations performed by a former Ph.D. student more than ten years ago. The student’s simulations of the merging of a large disc-shaped galaxy with the young Milky Way produced results that were totally in line with the Gaia data.

“It was amazing to look at the new Gaia data,” Helmi said, “and realize that I had seen it before.”