The Discovery of Dormant Galaxies in the Early Universe
The recent discovery of dormant galaxies, existing in the first billion years after the Big Bang, is reshaping our understanding of how galaxies evolve. This groundbreaking finding comes from the advanced observations conducted by the James Webb Space Telescope (JWST), which has revealed galaxies that had stopped forming stars at an unexpectedly early stage in the universe’s history. These galaxies are considered to be in a brief “quiet phase,” but they provide valuable insights into galaxy evolution and the potential for future star formation.
A study published on the preprint server arXiv details the work of an international team of astronomers led by Alba Covelo Paz, a doctoral student at the University of Geneva. Their research challenges traditional assumptions about the early universe and could change how we view the growth and development of galaxies, particularly their star-forming processes.
A Surprising Finding
When astronomers first directed the JWST toward the distant universe, they expected to find galaxies teeming with star formation—a common feature of the universe’s youth. Instead, they encountered something unexpected: galaxies that had completely halted their star formation. These celestial bodies, referred to as “dormant galaxies,” were not following the typical pattern of continuous star formation seen in later epochs.
These galaxies have been nicknamed “Sleeping Beauty” galaxies, suggesting they are in a state of hibernation, waiting for a trigger to awaken. This discovery was surprising because previous models suggested that early galaxies should have been actively forming stars due to the abundance of gas and other materials. However, by analyzing the near-infrared light emitted by these galaxies, the JWST was able to detect that star formation had ceased.
Timing of Dormancy
What makes this discovery even more intriguing is the timing of the dormancy. These galaxies stopped forming stars relatively early in the universe’s life, within the first billion years after the Big Bang—much earlier than previously thought. According to the study, the cessation of star formation could be temporary, with the possibility that these galaxies may one day resume their stellar activities when conditions allow.
Factors Behind Star Formation Pauses
The research team led by Alba Covelo Paz sought to understand why some early galaxies would suddenly stop forming stars. Several factors could explain the dormant state of these galaxies. One of the most significant is the activity of supermassive black holes at their centers. These black holes can emit intense radiation that heats up surrounding gas, preventing it from cooling and condensing into new stars.
In addition to supermassive black holes, interactions between neighboring galaxies could also play a role. When galaxies collide or come close to each other, gravitational forces can strip away essential cold gas, halting star formation. Stellar feedback is another contributing factor, where gas is heated and expelled by violent stellar processes such as supernovas or intense stellar winds. Once the gas is ejected, the galaxy enters a temporary phase where it can no longer form stars.
“This is usually a temporary phase, which usually lasts about 25 million years,” said Alba Covelo Paz. This insight is crucial in understanding the evolutionary patterns of galaxies. While this phase of dormancy is brief in cosmic terms, it may leave a lasting imprint on the galaxy’s future star formation history.
A Wide Range of Dormant Galaxies
Before the advent of the JWST, astronomers had only discovered a handful of dormant galaxies in the early universe. However, with the new data from the telescope, astronomers have now identified 14 dormant galaxies, some of which have masses ranging from 40 million to 30 billion solar masses. This wide range of masses challenges previous assumptions that dormant galaxies in the early universe were either small or excessively large.
The team used the JWST’s powerful spectroscopic capabilities to examine light from over 1,600 galaxies, searching for clues about star formation. “We now found 14 sources supporting this burstiness process, and we found that all of them have halted star formation between 10 [million] and 25 million years before we observed them,” explained Paz. This discovery is groundbreaking because it suggests that star formation does not follow a continuous path in all galaxies. Instead, some galaxies experience bursts of star formation followed by long periods of dormancy.
The Burstiness of Star Formation
The new findings reveal a pattern in early galaxy evolution that could change how scientists think about star formation. The concept of “burstiness” refers to the intermittent nature of star formation, where galaxies experience short bursts of activity, followed by long periods of quiescence. According to Paz, the dormant phase is “usually a temporary phase, which lasts about 25 million years.” This period of dormancy appears to be part of a larger cyclical pattern in the star formation process, where the galaxy pauses and then resumes star formation depending on various internal and external factors.
However, there is still uncertainty regarding the long-term behavior of these dormant galaxies. “We cannot confirm it for sure because we don’t know how long they will remain dormant, and if they happen to stay dormant for another 50 million years, this would mean the cause of their quenching is different,” Paz remarked. This raises important questions about whether some galaxies may indeed remain dormant indefinitely, suggesting a possible end to their stellar evolution.