Not all galaxies get a fair shot at growing up—and the crowded ones may be the most stunted of all. But here’s where it gets controversial: could living in a “cosmic city” actually hold galaxies back from reaching their full potential?
Astronomers are finding that a galaxy’s environment is not just background scenery—it actively shapes how that galaxy lives, grows, and eventually ages. A major new data release from the Deep Extragalactic Visible Legacy Survey (DEVILS) pulls back the curtain on how surroundings influence galactic evolution, especially for galaxies that existed billions of years ago and have been changing ever since.
What DEVILS actually is
DEVILS is a long-term observational project led by researchers at the International Centre for Radio Astronomy Research (ICRAR) and the University of Western Australia. It has just released its first major batch of data, packed with detailed measurements for thousands of galaxies from a region known as D10, which overlaps with the well-studied COSMOS field.
This first data release, described in a paper in the Monthly Notices of the Royal Astronomical Society and led by Associate Professor Luke Davies, represents about a decade of observing and analyzing the distant universe. The catalogue includes information on galaxy shapes (morphology), distances (redshifts), brightness and colors (photometry), spectra, as well as how galaxies sit in groups, clusters, and dark matter haloes. In simpler terms, it doesn’t just look at the galaxies themselves—it maps their neighborhoods too.
Why this survey is different
Many previous surveys have studied how galaxies change over cosmic time, but they often focused on the big-picture trends rather than the fine-grained details of local environments. DEVILS, by contrast, zooms in on galaxies that lived up to about five billion years in the past and compares them to galaxies in the nearby universe. This lets researchers see how properties such as star formation, mass, and structure evolve from then until now.
Lead author Luke Davies has described the approach using a landscape analogy: earlier work often treated the universe like a map of continents and oceans—useful, but coarse. DEVILS drills down to the “mountains, hills, valleys, and plateaus,” meaning the small-scale structures and local conditions around individual galaxies. That extra resolution is crucial when trying to understand how subtle environmental differences can change a galaxy’s life story.
A visual look at the data
One of the key images from the DEVILS data release showcases just how rich this dataset is. On the left, you see the distribution of different kinds of galaxies and their associated dark matter haloes located roughly 3.5 to 4 billion light-years away. Isolated galaxies are marked one way, galaxies in groups another, and each halo is drawn with a circle whose color and size relate to its mass and physical extent.
The middle panel zooms in on a single group, showing the individual galaxies labeled by their morphological type—think spirals, ellipticals, and other familiar shapes. Next to this, bars indicate how the galaxies in that group spread out in stellar mass and star formation rate, making it easy to see how varied even a single group can be. On the right, an image from the Subaru Hyper Suprime-Cam reveals the same group in exquisite detail, with member galaxies highlighted so you can pick out who belongs to the “cosmic family” and who is just passing by.
The big finding: crowded galaxies grow slower
Here’s the headline result that might surprise people: galaxies in crowded regions tend to grow more slowly than galaxies living more or less alone. That is, if a galaxy is part of a dense group, cluster, or a close pair, it generally forms new stars at a reduced rate compared to truly isolated systems.
Researchers often talk about two broad classes of galaxies. On one side are blue, gas-rich galaxies actively forming stars—these are the lively, “young” systems, full of cold gas that can collapse into new stars. On the other side are red, gas-poor, quiescent galaxies, which have used up or lost most of their star-forming material and are no longer building many new stars. The current understanding is that most galaxies start off as smaller, blue, star-forming systems and gradually transition into red, quiescent ones over cosmic time.
As the universe ages, the fraction of quiescent galaxies increases, meaning more galaxies have effectively slowed or shut down their star formation. DEVILS helps show that this transition is strongly influenced by where a galaxy lives. And this is the part most people miss: environment isn’t just a minor tweak—it can be a main driver in how quickly a galaxy’s star-forming “fuel tank” gets emptied.
What happens in dense environments
In regions packed with galaxies—such as clusters, groups, or tight pairs—the cold gas that galaxies need in order to form new stars often doesn’t survive unscathed. It can be heated up, disturbed, or stripped away entirely. Once that cold gas is gone or disrupted, star formation slows dramatically or stops altogether, a process astronomers call “quenching.”
Several physical mechanisms can cause this. Ram-pressure stripping, for instance, occurs when a galaxy plows through the hot, thin gas that fills a cluster; the pressure effectively sweeps out its own gas, like wind stripping leaves off a tree. Tidal interactions, where galaxies tug on each other gravitationally, can rearrange or eject gas and stars. In some cases, these interactions can trigger brief bursts of star formation; in others, they can rob a galaxy of the very material it needs to keep growing.
A graphic picture of slowed growth
One of the key graphics associated with this work illustrates how the rate of star formation changes with environment. It shows that in busier regions, where galaxies are packed more closely together, the typical star formation rate drops. In other words, the more crowded the cosmic “neighborhood,” the harder it becomes for galaxies to keep building new stars and increasing their mass.
Luke Davies summarizes this neatly by likening dense regions to the “bustling city centres of the cosmos.” Galaxies there not only grow more slowly but also develop structures that differ significantly from their more isolated cousins. That might mean more spheroidal shapes, thicker disks, or other architectural differences tied to their environmental history.
Galaxies as people: a helpful analogy
To make this idea even more intuitive, Davies compares galaxies to human beings. Just as a person’s upbringing and surroundings help shape their personality, opportunities, and lifestyle, a galaxy’s environment shapes its evolution. Someone who has always lived in a busy city might have very different habits and experiences than someone who grew up in a remote, quiet community.
Likewise, a galaxy embedded in a dense cluster undergoes frequent interactions, encounters strong gravitational forces, and experiences gas stripping processes that an isolated galaxy never faces. Over billions of years, those environmental differences add up, leaving clear imprints on how the galaxy looks, how massive it becomes, and how actively it forms stars.
Star formation, mass, and density
Another figure from the DEVILS work digs into how star formation and stellar mass behave across different types of galaxies and environments. It separates galaxies by morphological class (for example, spirals versus ellipticals) and examines how their star formation rate changes as the local density of galaxies increases.
In these plots, galaxy surface density describes how many galaxies occupy a region of space—higher density means more cosmic “neighbors.” The left-hand panels show how the median star formation rate shifts with this density, while the right-hand panels track how stellar mass changes. The results show a strong pattern: star formation rates decline noticeably as local density rises, whereas stellar mass tends to increase more gently with density.
Put simply, as you move into more crowded territory, galaxies are typically more massive but are forming new stars less vigorously. That suggests galaxies in dense structures may have built up mass earlier and then had their star formation choked off over time. To help interpret these trends, the team also compares their DEVILS results to data from another major project, COSMOS 2020, reinforcing that these environmental effects show up consistently across different surveys.
DEVILS as a foundation for the future
The DEVILS dataset is not an endpoint—it is a foundation. Other astronomers around the world will be able to use these catalogues to test their own ideas about galaxy evolution, environment, and cosmology. Just as DEVILS builds on previous surveys, its data will now feed into new studies looking at everything from dark matter haloes to the life cycles of specific galaxy types.
Davies and his collaborators already have their eyes on the next step. They plan to extend this work using WAVES, the Wide Area VISTA Extragalactic Survey. WAVES will collect data on an even larger number of galaxies and environments, broadening the statistical base and filling in more pieces of the cosmic puzzle.
Looking ahead with WAVES
According to the team, DEVILS provides a detailed snapshot of galaxy evolution across a specific slice of cosmic time and space, while WAVES will scale that perspective up dramatically. By studying many more galaxies over wider areas, astronomers hope to refine their understanding of how different environments, over many epochs, conspire to build the universe we see today.
In practical terms, WAVES should help answer questions such as: How universal are the trends seen in DEVILS? Are the same environmental effects at play in all types of cosmic structures, or are there exceptions? And could there be rare environments where galaxies actually thrive rather than struggle when surrounded by neighbors?
A gentle twist: is “crowded = bad” too simple?
Here’s a subtle but potentially controversial angle: while the overall trend is that dense environments suppress star formation, interactions in these regions can sometimes ignite intense bursts of star formation before quenching sets in. So is it fair to say crowded environments only “hurt” galaxy growth, or do they partly help galaxies build up mass quickly before shutting them down?
Another nuance is that galaxies may arrive in dense environments already partially evolved, meaning their history outside the cluster or group also matters. That raises an important question for future work: how much of a galaxy’s fate is written by its birthplace, and how much by where it ends up?
Your turn: what do you think?
So, what’s your take on this cosmic social experiment? If galaxies in quiet, isolated regions can keep calmly forming stars for longer, while those in crowded clusters grow fast and then burn out, which environment would you call more “successful” for a galaxy’s long-term evolution?
Do you agree that environment should be seen as one of the main drivers of a galaxy’s fate, almost like a cosmic version of nurture versus nature? Or do you think internal factors—like the galaxy’s own mass or black hole activity—matter more in the end? Share where you stand in the comments: does the idea that “crowded galaxies struggle to grow” make sense to you, or do you see a different interpretation of the same evidence?
