A massive galaxy that appeared less than two billion years after the Big Bang does not rotate on itself. This specimen, named XMM-VID1-2075, contradicts all astronomers' expectations.
According to classical models, galaxies acquire natural rotation as they form, under the influence of gravity and the gas streams that pour into them. Yet this distant galaxy seems globally motionless, without any overall movement.
Even before the detailed study by the James Webb telescope, the galaxy XMM-VID1-2075 had already intrigued researchers. Thanks to the Keck Observatory in Hawaii, they discovered that it was already very massive, containing several times the mass of our own Milky Way. Moreover, it had stopped forming new stars, a state generally associated with much older galaxies.
The James Webb telescope allowed them to go further by measuring the internal motions of the galaxy. Among three galaxies from the same era studied, one rotated clearly, another showed irregular motions, and XMM-VID1-2075 displayed random displacements, without any well-defined global rotation. This profile matches certain very massive local galaxies, but its presence so early in the universe is surprising.
To explain this lack of rotation, astronomers envision a major collision scenario with another galaxy rotating in the opposite direction. Such a collision could have canceled out the overall rotational motion. Webb's images also show an excess of light on one side, which could indicate the presence of an interacting companion object.
Researchers now plan to search for other similar galaxies in the young universe. By comparing their findings with computer simulations, they hope to better understand how galaxies evolved after the Big Bang.
What is a 'slow rotator'?
In astronomical jargon, a 'slow rotator' galaxy is one whose stars do not rotate in an orderly fashion around a central axis. Unlike spiral galaxies such as the Milky Way, where stars have a well-defined circular motion, slow rotators exhibit random trajectories. This behavior is often the result of multiple galaxy mergers that have scrambled the initial rotation.
In general, these galaxies are massive and old. Their discovery in the young universe, less than 2 billion years after the Big Bang, challenges the accepted timescales for these processes. Astronomers must now determine whether this galaxy is an exception or a sign that galactic mergers were more frequent early in cosmic history.
Observations from the James Webb telescope are crucial for identifying these objects, because their faint light and small size made them invisible to previous telescopes. By mapping internal motions, Webb allows scientists to distinguish slow rotators from galaxies with classic rotation.