The cosmos, with its vast expanse and unfathomable mysteries, is always revealing new layers of complexity, constantly challenging our understanding of the universe’s origins and evolution. Recent observations by an international team of astronomers, led by experts from the University of Geneva (UNIGE), have added yet another layer to the puzzle of cosmic history. Using the power of the James Webb Space Telescope (JWST), they have uncovered startling evidence that galaxies ceased forming stars far earlier than previously predicted—just 700 million years after the Big Bang. This discovery is reshaping our understanding of galaxy formation, challenging long-held assumptions, and prompting a reconsideration of theoretical models that have guided astronomers for decades.
The Early Universe: A Time of Rapid Galaxy Growth
For years, the scientific community believed that the earliest galaxies, those formed in the first billion years following the Big Bang, would primarily be actively forming stars—vibrant, young galaxies constantly creating new stars from the surrounding gas. These galaxies, often referred to as “star-forming” or “active,” are expected to be the norm in the young universe. However, new observations are shattering this expectation.
The James Webb Space Telescope has revolutionized our ability to observe distant objects in the universe, especially in the infrared spectrum. This allows scientists to peer deeper into the early cosmos than ever before, revealing galaxies in stages of evolution that were previously hidden from view. The most recent breakthrough, led by the UNIGE team, presents evidence that not only were galaxies forming stars during the universe’s infancy, but some of them stopped star formation much earlier than expected, giving rise to what are called “red and dead” galaxies.
What Are “Red and Dead” Galaxies?
To understand the significance of this discovery, we need to first define what a “red and dead” galaxy is. In the local universe, there exists a class of galaxies that have stopped forming stars, and their stars have aged to the point where they no longer emit the blue light characteristic of young, hot stars. Instead, these galaxies appear red, as they are dominated by older, cooler stars that emit light primarily in the red part of the spectrum. These galaxies are often referred to as “quenched” or “quiescent.”
Typically, “red and dead” galaxies are massive, elliptical-shaped systems that have formed over billions of years. They are thought to have undergone a prolonged period of rapid star formation, only to later “shut down” through a process known as quenching. Quenching occurs when a galaxy’s gas supply is exhausted or when external factors—such as powerful galactic winds or supermassive black hole activity—suppress further star formation.
The formation of such galaxies, particularly those that are massive and “red and dead,” is thought to take a long time. The standard model suggests that it should take billions of years for galaxies to build up a large enough stellar mass and undergo the quenching process. Therefore, the discovery of galaxies that have already quenched in the early universe, as early as 700 million years after the Big Bang, presents a major challenge to this model.
The Discovery of the “Dead” Galaxy: A New Record
The team at UNIGE, using data from the JWST’s NIRSpec (Near Infrared Spectrograph) instrument, observed a galaxy that was already “dead”—a massive quiescent galaxy—at a record-breaking redshift of 7.29. This galaxy, named RUBIES-UDS-QG-z7, formed stars at an astonishing rate, accumulating more than 10 billion solar masses of stellar material within the first 600 million years after the Big Bang. Yet, despite this rapid star formation, it stopped forming stars very early, ceasing to produce new stars much sooner than expected.
The implications of this discovery are profound. The galaxy’s early cessation of star formation implies that the processes responsible for quenching galaxy growth are much more efficient—and much earlier—than previously thought. In fact, RUBIES-UDS-QG-z7 suggests that massive quiescent galaxies (MQGs) are far more abundant in the early universe than theoretical models have predicted. The discovery pushes back the timeline for the cessation of star formation, indicating that some galaxies may have evolved into their “red and dead” states much earlier than models allow for.
What Does This Mean for Galaxy Formation?
The revelation that galaxies can stop forming stars within just 700 million years after the Big Bang raises significant questions about the mechanisms of galaxy evolution. Current theoretical models of galaxy formation are based on the idea that galaxies grow by accumulating gas, which is then turned into stars. Over time, galaxies continue to grow in size and mass as star formation continues, until the process of quenching occurs.
However, RUBIES-UDS-QG-z7 and other similar galaxies discovered in the same study suggest that the quenching process may be far more rapid than previously thought. Models must now account for the possibility that the cessation of star formation can occur earlier and more efficiently than current theories allow.
The findings also suggest that the factors responsible for quenching—such as the feedback from supermassive black holes, the outflows of gas driven by stellar winds, or other galactic interactions—may be more powerful or have a more immediate impact on star formation than previously realized.
The Surprising Size and Density of Early Galaxies
Another surprising aspect of the RUBIES-UDS-QG-z7 discovery is the galaxy’s physical size. Despite its impressive stellar mass, this “dead” galaxy is relatively small, measuring only about 650 light-years across. This small size suggests that the galaxy had a high stellar mass density, comparable to the densest regions of more familiar “red and dead” galaxies observed at later stages of cosmic evolution, when galaxies are already well-established. This suggests that the centers of some of the oldest and most massive galaxies in the universe may have been formed much earlier than anticipated.
The discovery of a small, dense, quiescent galaxy at such an early stage in the universe’s history raises intriguing possibilities for the future evolution of galaxies. It hints that some galaxies may already be in the process of evolving into the massive elliptical galaxies that populate the cores of galaxy clusters today.
Implications for Cosmic Evolution Models
The detection of such a distant and massive quiescent galaxy has profound implications for our understanding of cosmic evolution. For one, it challenges the current models of galaxy formation and evolution, which predict that such “red and dead” galaxies should take much longer to form. The existence of massive quiescent galaxies as early as 700 million years after the Big Bang suggests that key elements of these models—such as the effects of stellar winds, black hole feedback, and gas accretion—need to be revisited and refined.
In particular, astronomers may need to reconsider the role of supermassive black holes in regulating star formation. The rapid formation and quenching of galaxies in the early universe may be linked to the powerful feedback processes from these black holes, which could be more efficient in suppressing star formation than previously assumed.
Moreover, the study of these early galaxies can help answer some of the most fundamental questions about the universe’s formation. What mechanisms triggered the rapid cessation of star formation in these early galaxies? How did galaxies evolve from actively star-forming systems to the “red and dead” galaxies we see today? These questions remain at the forefront of astronomical research, and discoveries like RUBIES-UDS-QG-z7 will undoubtedly continue to shape our understanding of the universe’s origins.
Looking Ahead: The Future of Cosmic Exploration
The discovery of early “dead” galaxies such as RUBIES-UDS-QG-z7 is just one of many groundbreaking findings expected from the James Webb Space Telescope. As astronomers continue to observe the distant universe with unprecedented clarity, they are uncovering a more complex and nuanced picture of cosmic evolution than ever before. These discoveries are not only reshaping our understanding of galaxies and their formation but also offering profound insights into the fundamental processes that govern the universe.
As the scientific community absorbs the implications of this discovery, astronomers will no doubt revise their models and continue to refine our understanding of how galaxies form, evolve, and eventually stop forming stars. The early universe, once thought to be dominated by young, star-forming galaxies, is now revealed to be far more diverse and complex, with galaxies evolving much more rapidly and undergoing dramatic changes at earlier stages than previously thought.
In the coming years, as more data from JWST and other telescopes pours in, astronomers will continue to explore these intriguing “red and dead” galaxies, unlocking more secrets of the cosmos and redefining our place within it. The universe, as always, is full of surprises—and the story of galaxy formation is far from over.
Reference: Andrea Weibel et al, RUBIES Reveals a Massive Quiescent Galaxy at z = 7.3, The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/adab7a