A high school student has spotted three small, solitary galaxies that appear to have stopped making stars.
Julian Shapiro discovered them while exploring images of the sky accessible to astronomers. He was still in high school. Now a student at the University of Berkeley, he continued his research to understand why these three galaxies appear so inactive.
The three 'dead' galaxies, faintly visible at the center of these images
A galaxy does not make stars forever. To give birth to new ones, it needs gas. As long as a small galaxy retains enough of it, it can continue producing stars. But if it loses that reserve, star formation eventually stops. Astronomers then call it a “dead” galaxy, even though its old stars are still shining.
The three newcomers were spotted in the region of M101, the large galaxy nicknamed the Pinwheel. They are small, faint, and difficult to distinguish from the background sky. The observations show almost no signs of young stars. Their activity therefore appears to have slowed dramatically, or even stopped.
One mystery remains: how did they lose their gas? We know of dwarf galaxies that no longer form stars, but they often live near a giant galaxy. In that environment, the surrounding gas can strip away their own reserve. Yet the three objects detected now appear relatively isolated.
The researchers are therefore considering a turbulent history. These small galaxies may once have passed close to a large neighbor, which deprived them of some of their gas. They then moved away, eventually finding themselves in a much quieter region. Astronomers call these “backsplash” galaxies.
This scenario is plausible, but there is not yet any proof that the three galaxies followed such a path. Their distances remain poorly known, and one might ultimately turn out to be near another galaxy. There are also other ways to lose gas, even without directly encountering a giant galaxy.
Observations with the Hubble or James Webb telescopes could help locate these galaxies more precisely and determine the ages of their stars. This would make it possible to find traces of their past and verify whether they were truly driven out of their former surroundings.