Astronomers have discovered more than dozens of “dormant” galaxies that paused star formation within the first billion years of the Big Bang.
Discoveries made with James Webb Space Telescope (JWST) data can illuminate the fascinating stages of early galaxy life and provide more clues about galaxy evolution.
There are several reasons why galaxies stop forming new stars. One is the presence of an extra-large black hole in the center. These giants emit intense radiation that heats up and depletes cold gas, the most important component of star formation. Furthermore, larger adjacent galaxies can remove this cold gas or heat it and stop at star formation. As a result, these galaxies may remain dormant indefinitely or “quenched.”
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Another reason galaxies become inactive is the feedback of stars. At that time, galaxy gases are warmed and exiled for stellar processes such as supernova, intense star winds, or pressures associated with starlight. Therefore, the galaxy goes through a temporary “quiet” period.
“This is usually a temporary stage, usually lasting around 25 million years,” Alba Covelo Paz, a doctoral student at the University of Geneva and lead author of a new study explaining the findings, told Live Science in an email. For millions of years, the extruded gas is dropped and the warm gas is cooled again. Once again there is enough cold gas, the galaxy can begin to form new stars.
Dormant phases are commonly observed in nearby galaxies, but astronomers have only discovered four dormant galaxies in the first billion years of the universe. Of these, three were below the 1 billion solar mass, and one had over 10 billion solar masses. The limited observations and scattering properties of dormant galaxies were not sufficient to capture clear pictures of early star formation.
Related: “Previously Unthinkable”: James Webb’s Telescope Breakes Its own Record Again, Discovers the Most Known Galaxy in the Universe
However, using JWST’s sensitive spectroscopic data, an international team of astronomers discovered 14 dormant galaxies with extensive masses in the early universe, indicating that dormant galaxies are not limited to low or very high mass ranges.
The findings were uploaded to the Preprint database ARXIV on June 27th and have not yet been peer-reviewed.
Take a breather
Researchers were not expecting to see dormant galaxies in early universes. As these galaxies are young, they should form a lot of new stars vigorously, astronomers thought. However, in a 2024 paper, the researchers described the first discovery of dormant galaxies in early universes.
“The first discovery of a dormant galaxy in early universe was such a shock, as the galaxy had previously been observed in Hubble, but we were unable to know it was dormant until JWST,” Pas said.
That’s because unlike the Hubble Space Telescope, both JWST’s NirSpec instruments can see the light from these galaxies redshifted to near-infrared wavelengths, and can also provide spectral details about it.
Astronomers wanted to know why early galaxies stopped forming stars and whether this was common among the wide-ranging star masses. One hypothesis was that before the galaxy began again, it had a quiet era with star-forming explosions. Path and her team searched for the galaxy between the bursts of star formation. They used Galaxy data published in the Dawn JWST archive.
They looked into the light of around 1,600 galaxies, looking for signs of no new stars being formed. They also focused on the clear signatures of middle-aged or old stars in the galaxy light. The team discovered 14 galaxies ranging from around 40 million to 30 billion solar masses that have paused star formation.
“We’ve now found 14 sources supporting this explosion process, and we found that all of them have stopped star formation during the 10. [million] And Paz explained 25 million years ago, as we observe. In other words, we found that these 14 galaxies follow the stop-and-go fashion of star formation rather than continually forming stars.
This relatively short snooze hint could quiet the feedback of stars such as supernova and stellar winds and eventually reopen the stellar factory, Paz said.
But there is still uncertainty, she added. “We can’t confirm for sure because we don’t know how dormant they will remain dormant, and if they happen to remain dormant for 50 million years, this means that they’re causing quenching differently,” Paz explained.
This scenario suggests that the galaxy is dead. Nevertheless, the current characteristics of these galaxies support a cycle of fit and initiation.
Doubtous galaxies are very rare, so much about them remains mystical. However, astronomers hope that future observations will help shed light on these dozing-out star factories. The upcoming JWST program, called “Sleeping Beauties,” will focus on discovering dormant galaxies in early universes, Paz said. The program helps astronomers estimate how long the galaxy remains in this quiet stage and understand the ruptured star formation process.
“We still have a lot of unknowns, but we are one step closer to unraveling this process,” Paz said.
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