Our Galaxy Was Walloped by a Neighbour in Its Not-So-Distant Past, New Analysis Suggests

By Ryan F. Mandelbaum on at

Space is a chaotic, ever-changing place. But that’s not limited to exploding stars and colliding black holes. Even our own Milky Way galaxy could have recently received a massive jolt from which it is still recovering.

Since the Gaia mission’s massive data drop last week, scientists around the world have been hard at work crunching the information, releasing over 30 new research papers in just two days following the announcement. One especially interesting paper describes the possibility that some strong gravitational force sent ripples through our galaxy, like a stone into a pond, between 300 million and 900 million years ago.

“We have here provided the clearest evidence that our own Galaxy disk has suffered from perturbations, bringing it to an out-of-equilibrium state, which may well be due to the interaction with an external satellite galaxy,” write the authors, led by Teresa Antoja from the Universitat de Barcelona in Spain, in the paper published last week on the arXiv preprint server.

The European Space Agency released its Gaia data last Wednesday, and with it came new information about 1.7 billion stars in the Milky Way. That included 1.3 billion stars’ apparent motion in the sky and actual velocities for over 7 million stars. The researchers behind the newest study used this information to study the galaxy in phase space—fancy talk for how its stars’ velocities vary with their position.

The stars’ motion revealed that they could still be recovering from a gravitational wallop a few hundred million years ago—relatively recently in the universe’s terms. This might have been due to a close approach from the Sagittarius Dwarf Galaxy, a collection of stars orbiting the Milky Way around 70,000 light years away.

“This work shows that the stellar disk of the Milky Way is a dynamically active place, where spiral arms and the Galactic bar leave their marks on the orbits of stars like ripples in a pond—a pond that has perhaps recently felt the splash of a small stone in the form of a merging dwarf galaxy,” astronomer Stacy McGaugh from Case Western Reserve University told Gizmodo. “It is a living and breathing beast that is sensitive to be poked and prodded and even tickled a little bit.”

It’s important to note that, as these results appear on the arXiv server, they haven’t yet been officially reviewed by other scientists. Antoja declined Gizmodo’s request for a comment due to embargo policies from the journal the research has since been submitted to.

But this finding is just one of many results so far gleaned from the new data, demonstrating how much left there is to learn about our galaxy. Here’s a map of the Milky Way rotating, for example:

And Leah Crane at New Scientist reported that the Gaia data release has created further confusion around a long-standing mystery. You might remember that scientists get two different numbers for the expansion of the Universe based on the two different ways they measure it. Gaia observations of regularly brightening stars called Cepheid variables further validated the discrepancy between the two numbers—thus adding evidence that we might be actually measuring two different rates of cosmic expansion, rather than, say, one of the measurement methods being off.

Astronomers will continue to pull more interesting data from the Gaia mission as they perform more analyses. And while it may sound cliché, it seems like the right time to say it: The Milky Way is so much weirder than we thought. [arXiv]