Carlos Solis
Gaby
Astronomers have found galaxies that shut off star formation within the universe’s first billion years after the Big Bang. Consequently, researchers have taken a step closer to understanding how early galaxies evolve.
What Caused These Galaxies to Go Dormant?
Using sensitive spectroscopic data from the James Webb Space Telescope (JWST), an international team of astronomers stumbled upon 14 dormant galaxies with a wide range of masses.
One major factor that can halt star formation is a supermassive black hole at a galaxy’s center. These giants emit intense radiation that heats and expels the cold gas needed to form new stars. Larger neighboring galaxies can also siphon off or heat that cold gas, causing star formation to stop. As a result, a galaxy can remain dormant indefinitely or slowly fade away.
Another cause is stellar feedback: gas in a galaxy can be heated and driven out by processes such as supernova explosions, strong stellar winds, or radiation pressure from stars. That can push a galaxy into a temporary quiet phase, as reported by Live Science.
“Typically, this is a temporary phase lasting about 25 million years,” said Alba Covelo Paz, a PhD student at the University of Geneva and the lead author of the study. Over millions of years, expelled gas can fall back and warm gas can cool again. Once enough cold gas is present, star formation can restart in the galaxy.
A Surprise from JWST
Until now, astronomers had only identified four dormant galaxies from the first billion years of the universe. Of those, three had masses under a billion solar masses, while one exceeded 10 billion solar masses. Given that, researchers did not expect to find many quiet galaxies so early.
The first discovery of such a galaxy was reported in 2024. Paz says that initial finding was a real shock. Astronomers had observed that galaxy with Hubble before, but they couldn’t tell it was dormant until JWST began operations.
Unlike Hubble, JWST can not only detect the light from these galaxies but also provide the spectroscopy needed to determine whether new stars are forming.
Paz and her colleagues searched for galaxies caught between bursts of star formation and periods of inactivity. They examined the light from about 1,600 galaxies, looking for spectral signs that new stars were not forming and for signatures of middle-aged or older stellar populations. Ultimately, they identified 14 galaxies with masses ranging from 40 million to 30 billion solar masses that had ceased forming stars.
“All of them stopped forming stars 10 to 25 million years before we observed them,” said Paz. That timing implies these galaxies experienced intermittent star formation rather than a steady, continuous process. They had been quiet for 10 to 25 million years, a relatively short dormancy that suggests stellar feedback may have been responsible and that star formation might resume.
“However, we cannot confirm this for sure because we do not know how long they will remain dormant, and if they stay in this state for another 50 million years, it would mean that the reason for their fading is different,” explained Paz.
The “Sleeping Beauties” Project
Dormant galaxies are extremely rare, so much about them remains mysterious. Astronomers hope upcoming observations will shed more light on these quiet star factories.
Paz says a future JWST program called “Sleeping Beauties” will be dedicated to finding dormant galaxies in the early universe. The program will help scientists better estimate how long a galaxy stays in a quiet phase and improve understanding of how galaxies form stars.
Photo: NASA
