The wobbling Milky Way

The origin of the Milky Way’s warped shape has been debated since its discovery in 1957. Data from the Gaia satellite suggest that the warp is moving – and moving very fast. Our result favours an interaction with a satellite as a possible explanation for the warp’s origin.
The wobbling Milky Way

Like many disk galaxies, our Milky Way is not flat, but warped in its outer parts. Such a feature, known as ‘the Galactic warp’, indicates that our Galaxy might be subject to external forces acting on it. Warps can therefore potentially reveal fundamental clues on the formation history of galaxies.

Our Galaxy offers a unique case study for warps, as we can study it on a star-by-star basis. However, which mechanism is actually at work for our own Galaxy has been debated for decades. Since their discovery, warps in disk galaxies have stirred the imagination of theorists, who proposed numerous ideas to explain their formation and persistence. In this respect, the measurements of stellar motions can potentially offer key information, as they trace the underlying forces of the Galaxy.

Our understanding of our own Galaxy changed drastically after data from the Gaia satellite became available, leading to exciting results in different fields of astronomy. With the advent of the second data release (April 2018), we had access for the first time to high-precision astrometric measurements of billions of stars. We therefore thought that it was the perfect moment to study the kinematic signature of the Galactic warp, that is, the signal that the warp induces in the stellar motions.

When we looked at the data, we found that Gaia observations couldn’t be reproduced by a static model – that is, a warp figure frozen in time. We therefore realized that the warp had to move somehow. After some tests, we developed a warp model whose orientation changes over time and found that the warp speed was surprisingly high. Based on the obtained velocity, the warp would complete one rotation around the Galactic center in 600 to 700 million years – how did this value compare with the predictions from dynamical models available in the literature?

Galaxies, including our Milky Way, are expected to be surrounded by a large amount of unseen matter (called 'dark matter halo'). If its shape is non-spherical and the disk of the galaxy is not aligned with it, the galaxy will be warped, and the warp is expected to move ('precess') - even if the galaxy is isolated. However, according to this model, the velocity of the warp would be very slow - much slower than what we observe in real data. For this reason, something more powerful would be needed to explain our results. 

Like a stone thrown into water, another smaller galaxy might be perturbing the Milky Way’s disk, warping its surface and causing it to move as fast as we measure. This is the most convincing interpretation that we found to explain our results, and suggests that forces from interacting satellite galaxies are playing a significant and ongoing role in shaping the outer disc of the Milky Way. 

We now have a new question: is the Sagittarius dwarf galaxy the culprit?

Lint to the ESA press release here.

Image credits: Stefan Payne-Wardenaar/ESA.

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