Why do they stop growing?

Galaxies are astronomical objects composed of billions or even trillions of stars. Our Earth and solar system are just a tiny part of the Milky Way; it is one of countless galaxies in the universe. These immense systems are the fundamental building blocks of cosmic structure. During the universe’s 13.7 billion-year history, the first few billion years marked a golden age for galaxy formation, when galaxies actively turned their gas reservoirs into new stars.

However, recent discoveries reveal that some galaxies stopped growing as early as 1 billion years after the Big Bang—less than 10% of the universe’s current age. These “quiescent galaxies” have become a major astronomical mystery: Why did they stop growing so early?

Now, using data from the James Webb Space Telescope, Prof. Wu Po-Feng at National Taiwan University uncovered the secret behind these early galaxies’ rapid shutdown: Their gas reserves were rapidly blown away by powerful forces generated inside the galaxies. The findings are published in The Astrophysical Journal.

The researchers discovered a quiescent galaxy approximately 12 billion light-years away using the spectroscopic data taken by the James Webb Space Telescope. The light of this galaxy originated from a time when the universe was only 1.5 billion years old, around 10% of the current age of the universe.

The research revealed that the star formation rate plummeted dramatically in a short period and now the galaxy has almost stopped growing. Data further indicate that the galaxy’s gas is being expelled at speeds of more than 200 kilometers per second, depleting the raw material necessary for star formation.

The study also suggests the presence of a supermassive black hole at the galaxy’s center. The immense energy released by the black hole may drive the gas outflow, dispersing the fuel for star formation into space. This galaxy provides critical evidence for understanding the core mechanisms behind the rapid shutdown of star formation in the first billions of years of the universe and an important empirical foundation for developing theories of early cosmic evolution.

This galaxy is currently the most distant massive quiescent galaxy confirmed to exhibit such powerful gas outflows. “With the unparalleled sensitivity of the James Webb Space Telescope, we are not just glimpsing the early universe;” Prof. Wu said, “we begin to decode the physical processes behind its evolution.”

He plans to analyze more similar galaxies to determine whether gas outflows are a widespread phenomenon, whether the expelled gas might return to reignite star formation, or whether it will permanently escape, altering the galaxy’s ultimate fate. These studies will enhance our understanding of how galaxies transform over time, ultimately shaping the structures we recognize today.