The Milky Way May Have Already Collided With Another Galaxy

Astronomers predict that our Milky Way galaxy is on a collision course with Andromeda, and we’ve got just a few billion years to brace for impact. However, the Milky Way might have a history of smashing up other galaxies. A new analysis suggests that our galaxy may have collided with a recently discovered dwarf galaxy called Antlia 2.

Scientists discovered Antlia 2 in late 2018 in orbit of the Milky Way. It’s an unusual object due to its extremely low density. While it’s about the same size as the Large Magellanic Cloud satellite galaxy, it’s about 10,000 times more diffuse. According to the team from the Rochester Institute of Technology, the current state of Antlia 2 and perplexing ripples in the Milky Way’s hydrogen gas disk (discovered about ten years ago) could be explained by a collision between the two galaxies. Of course, Antlia 2 came out of it much worse off than the Milky Way.

Using data collected by the European Space Agency’s Gaia satellite, Sukanya Chakrabarti and her team calculated the past trajectory of Antlia 2. Based on the models the team generated, Antlia 2 may have collided with the Milky Way several hundred million years ago.

We often think of galaxies as dense, unified objects with wall-to-wall stars. However, galaxies are mostly empty space. When they “collide,” it’s unlikely that any two stars will actually run into each other. Instead, the gravitational interaction may fling stars out into deep space or cause them to migrate from one galaxy to the other. Clouds of dust and gas may also merge, causing an uptick in star formation.

Antlia 2 is similar to the Large Magellanic Cloud (above), but thousands of times fainter.

So, the Milky Way is still largely the same as it was before running into Antlia 2. However, the smaller galaxy was wrecked by the gravity of its larger neighbor. That accounts for the very diffuse current state. The team also used its models to rule out another alleged candidate for the cause of those ripples in the Milky Way: the Sagittarius dwarf galaxy. The model projects no likely collisions between that galaxy and the Milky Way in the past. This work could solve the ripple mystery.

Scientists hope that studying Antlia 2 and its orbit will reveal some clues to the nature of dark matter, a separate mystery scientists are still a long way from unraveling.

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