A galaxy that should not exist — at least not yet — has turned up in James Webb Space Telescope observations, and it is making theorists uncomfortable in a productive way. Designated XMM-VID1-2075, it formed less than two billion years after the Big Bang, yet it shows none of the rotation that young galaxies are supposed to have.
The wrong kind of motion for its age
Most galaxies rotate, their stars and gas circling on ordered orbits — angular momentum inherited from the infalling gas that built them. XMM-VID1-2075, which contains several times more stars than the Milky Way, instead has stars moving at random, with no detectable spin. That "slow rotator" character is normally a hallmark of old, nearby galaxies that have been reshaped by billions of years of evolution and repeated mergers. Finding it in the early universe is the surprise.
What could cancel a galaxy's spin so fast
"This one in particular did not show any evidence of rotation, which was surprising and very interesting," said Ben Forrest of UC Davis, who led the study. His team used Webb to track stellar motions in the galaxy and two contemporaries; earlier W. M. Keck Observatory data had already established that XMM-VID1-2075 was massive and had stopped forming stars. One candidate explanation is dramatic: a head-on collision between two galaxies spinning in opposite directions, which could cancel their rotation in a single event rather than grinding it down over eons.
The work, published in Nature Astronomy, is a reminder that the early universe Webb is revealing keeps running ahead of the models built to describe it — massive galaxies, and now dynamically old ones, assembling far faster than expected.
