A telescope floating in space is helping scientists solve a cosmic mystery: why did some ancient galaxies suddenly stop making new stars? The James Webb Space Telescope, or Webb for short, has revealed that these galaxies carry hidden scars from cosmic collisions billions of years ago.
An international team led by astronomers at the University of Nottingham in the United Kingdom used Webb to study galaxies that stopped forming stars around nine billion years ago. At that time, the universe was reaching its most active period — when many of the largest galaxies we see today were born. The team's findings appeared in the journal Monthly Notices of the Royal Astronomical Society.
"This was the epoch of peak activity in the universe, when many of the most massive galaxies we see today were formed," said Professor Omar Almaini, who led the research. "A long-standing problem has been to understand why these galaxies stop forming stars. With Webb, we can see detail that was completely hidden before, allowing us to search for clues to what drives this dramatic transformation."
The answer appears to be violence. Lead author Dr. David Maltby explained: "These galaxies look calm on the surface, but Webb allows us to see the subtle signs of past violence. The galaxies show clear signs of disturbance, telling us that something dramatic happened to them not long before their star formation shut down, most likely a merger with another galaxy."
When two gas-rich galaxies collide, they can produce very compact remnants. The team found that these recently "quenched" galaxies are exceptionally compact, supporting the merger theory. Their structures show subtle disturbances that add further evidence.
The research combined data from two major survey programs: the JWST PRIMER program, led by Professor James Dunlop at the University of Edinburgh, and the Ultra-Deep Survey, led by Almaini at Nottingham.
For centuries, humans have gazed at the night sky and wondered what lies beyond. Now, thanks to Webb's sharp infrared vision, we can finally read the stories written in ancient galaxies — even the ones written in the aftermath of cosmic violence.