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Current Events: Milky Way’s Warp Caused by Galactic Collision

By Stacy Palen

According to this article published by the European Space Agency, the extremely precise mapping being done by the Gaia satellite shows that the warp in the Milky Way Galaxy is moving far faster than expected. This implies that it is caused by interactions with another galaxy rather than by magnetic fields or the dark matter halo.

Below are some questions to ask your students based on this article.

1) What is unprecedented about the data that Gaia is taking now?

Answer: Gaia is taking more precise position and velocity data on a very large number of objects.

2) What do astronomers mean when they talk about a “warp” in the disk of the Milky Way?

Answer: They mean that one side of the disk is twisted up, while the other side is twisted down.

3) Where else in your study of astronomy have you run into the term precession? How does that use of the term compare to the one used here?

Answer: Precession was also used to talk about how Earth’s pole points in different directions over a 26,000-year period. It’s similar to this usage because if you imagine an axis perpendicular to the warped disk, that axis would likewise point in different directions over time. The time scale here, however, is much longer.

4) What is it about the warp that makes it possible for astronomers to rule out all causes but one?

Answer: The speed of the warp matters. It moves too fast for anything but an intergalactic collision to be causing it.

5) Consider what you have learned about galaxy formation and interactions. If you were an astronomer, what might you look for in other spiral galaxies in order to confirm this result?

Answer: If warps are caused by interactions, then every time I see one, I should also see small companion galaxies that are very close—with orbits close enough to pass through the disk of the larger spiral galaxy.

6) The Sagittarius dwarf galaxy may be the cause of the Milky Way’s warped disk. What is the ultimate fate of the Sagittarius dwarf galaxy?

Answer: The Sagittarius dwarf galaxy will ultimately be absorbed by the Milky Way.

7) Speculate: after the Sagittarius dwarf galaxy meets its ultimate fate, what do you expect to happen to the warp in the Milky Way’s disk?

Answer: The warp should slowly disappear, once there is no longer a galaxy tugging the disk around.


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