Spinning rugby balls—the rotation of the most massive galaxies

By targeting the most massive galaxies in our universe, astronomers have studied how their stars move. The results are surprising: while half of them spin around their short axis as expected, the other half turn around their long axis. Such kinematics are most likely the result of a special type of galaxy merger, involving already massive, similar-mass galaxies. This would imply that the growth of the most massive and the largest galaxies is governed by these rare events.

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Credit: MUSE/D. Krajnovic

The upper panels show maps of the measured mean stellar velocity of two example galaxies: Blue colour means that stars in this part of the galaxy move towards us and red colours away from us. The type of rotation for the galaxy on the left is typical for galaxies. The majority of galaxies rotate like this. The rotation around the long axes of the galaxy on the right is unusual, and applies only to a small fraction of galaxies. This fraction increases as galaxies become more massive.

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The rotation curve of a system can tell us quite a bit about how the mass is distributed.

For galaxies, there is a surprising amount of mass out from the center revealed by galactic rotation curves.