This is the region of sky in the DEVILS field. DEVILS stands for Deep Extragalactic VIsible Legacy Survey, and it's a deep, multiwavelength survey of galaxies that existed up to 5 billion years ago. DEVILS examines the galactic surroundings and how these galaxies have evolved up to the present day.
Welcome, dear readers of FreeAstroScience. Have you ever stared at an image of thousands of galaxies and wondered: do they all grow the same way, or does “cosmic city life” change them? This article, written by FreeAstroScience only for you, tackles that question. We’ll follow a new survey, DEVILS, that watched galaxies about 4–8 billion years ago and asked why those in crowded zones seem to stall. Stick with us to the end and you’ll see how environment, physics, and good data team up to shape the Universe you live in.
What do astronomers mean by a “crowded” cosmic neighbourhood?
When we say a galaxy lives in a crowd, we’re talking about where it sits in the cosmic web:
- Field / isolated galaxy – lives mostly alone, no big neighbours close by.
- Group – a few to a few dozen galaxies bound together.
- Cluster – hundreds or thousands of galaxies in one massive dark-matter halo.
- Filaments – giant bridges of galaxies linking clusters across space.
The new DEVILS survey (Deep Extragalactic Visible Legacy Survey) maps these environments for thousands of galaxies, roughly 0–5 billion light-years away from us.
The popular summary from Universe Today puts it nicely: galaxies in “bustling city centres of the cosmos” grow more slowly than their more rural cousins.
To keep things straight, here’s a quick overview.
| Environment | Typical number of bright galaxies | Typical galaxy surface density | Nickname |
|---|---|---|---|
| Isolated / field | 1–2 | < 0.1 Mpc−2 | Cosmic countryside |
| Galaxy group | 5–50 | ≈ 0.1–1 Mpc−2 | Small town |
| Galaxy cluster | 100–1000+ | > 1 Mpc−2 | Big city centre |
These numbers are ballpark values, but they capture the story: some galaxies live with just a few neighbours, others in packed cosmic downtowns.
How does the DEVILS survey watch galaxies grow?
To understand how environment changes galaxies, you need three things at once:
- Deep imaging to see faint galaxies.
- Spectroscopy to get accurate distances (redshifts).
- High completeness so you don’t miss neighbours.
DEVILS does exactly that. It uses the Anglo-Australian Telescope’s AAOmega spectrograph to observe three sky regions totaling about 4.5 square degrees, down to a near-infrared magnitude of YAB = 21.2. The D10 (COSMOS) region alone delivers:
- Redshift range: 0 < z < 1.2, median z ≈ 0.53
- 5442 new high-quality spectroscopic redshifts
- 4824 of those are unique, faint galaxies with no prior secure redshift
- Spectroscopic completeness at Y ≈ 21 boosted from ~50% to ~90%
Here’s a compact summary.
| Property | Value | Why it matters |
|---|---|---|
| Sky area | ≈ 1.5 deg² (D10 field) | Large enough to sample many environments |
| Magnitude limit | YAB < 21.2 | Reaches “typical” mass galaxies to z ≈ 1 |
| Redshift range | 0 < z < 1.2 | Covers ~8 billion years of cosmic history |
| New spectroscopic redshifts | ≈ 5440 | Improves distance accuracy dramatically |
| Spectroscopic completeness | ~90% at Y ≈ 21 | Lets astronomers trust local density estimates |
Because DEVILS re-observes faint galaxies until a reliable redshift is achieved, it avoids one of the big traps in earlier surveys: missing the shy, hard-to-measure galaxies that often live in dense places.
That high completeness is the quiet hero of the story.
How do we turn “neighbours on the sky” into a real density number?
Two galaxies can look close together on the sky yet be millions of light-years apart along the line of sight. That’s why redshift is vital.
The DEVILS team defines local galaxy surface density using the distance to the N-th nearest neighbour (for example, the 3rd or 5th closest galaxy) within ±1000 km/s in radial velocity.
Mathematically, the projected density around a galaxy is
where:
- is the projected distance to the N-th neighbour,
- N is usually 1, 3, 5, or 7,
- The unit is galaxies per square megaparsec (Mpc⁻²).
They then bin galaxies by log10(ΣN) and compute the median star-formation rate (SFR) and median stellar mass in each bin.
In the key test sample, they pick galaxies with:
- Redshift: 0.2 < z < 0.5
- Stellar mass: 10 < log10(M / M☉) < 11
This keeps the mass range narrow and well measured, which means any strong trend left over with density is likely due to environment rather than simple mass differences.
What did DEVILS reveal about star formation in crowded environments?
Here’s the “aha” moment.
When the team plotted median SFR against local density, they found a very clear pattern:
- At low density (ΣN < 0.1 Mpc⁻², essentially isolated galaxies), the median SFR sits around 0.8 M☉/yr.
- Once density rises above about 0.2 Mpc⁻², the median SFR starts to fall.
- In the densest regions, the median SFR is lower by about 1 dex (a factor of 10).
So, going from the outskirts to the crowded city centres, typical galaxies form stars about ten times more slowly.
At the same time, the median stellar mass only increases slightly with density for this carefully selected mass range. That tiny mass change cannot explain a 10× drop in SFR. The conclusion is that environment itself is shutting star formation down.
The full DEVILS data release shows that this pattern appears consistently in the redshift range 0.2–0.5, for galaxies with 10–11 in log10(M/M☉). Suppression of star formation in overdense environments is described as ubiquitous in this regime.
So, city life really does slow galaxies down.
Does galaxy shape or mass change the story?
You might guess that only certain shapes of galaxies are sensitive to environment. Maybe fluffy spiral discs get battered, while compact ellipticals shrug off the crowd?
DEVILS tests that idea by splitting galaxies into morphological classes (pure discs, discs with bulges, ellipticals) and repeating the SFR–density analysis.
The result:
- All classes show lower median SFR in the densest environments.
- Ellipticals and “disc + classical bulge” galaxies show a smooth decline as density increases.
- Pure discs and discs with diffuse bulges keep high SFR until the very highest densities, then drop sharply.
Next, the team slices the sample by stellar mass, into low-mass and high-mass bins, then repeats the whole exercise.
Here’s what they find:
- In both mass bins, when you mix all morphologies together you still see clear SFR suppression with density.
- Once you fix both morphology and mass, the stellar mass itself no longer changes significantly with density – but SFR still falls.
- Lower-mass galaxies show stronger suppression than higher-mass galaxies at the same density.
That last point fits our intuition: lower-mass galaxies are more fragile. When they plunge through a cluster, their gas is easier to strip away or heat up.
To make this easier to scan, here’s a summary table.
| Galaxy type | Stellar mass range | Trend with increasing density | Who is most affected? |
|---|---|---|---|
| All morphologies combined | 10 < log10(M/M☉) < 11 | Median SFR drops by ~1 dex from low to high density | Clear suppression across the whole sample |
| Ellipticals | Both low and high mass bins | Gradual SFR decline as density rises; slight mass increase | More quiescent in dense regions, especially at higher mass |
| Disc + classical bulge | Both bins | SFR decreases steadily with density | Environment steadily “turns down” star formation |
| Pure discs | Both bins | High SFR until the densest bin, then sharp drop | Low-mass discs in clusters suffer the biggest hit |
| Disc + diffuse / pseudo-bulge | Both bins | Similar to pure discs: sharp suppression only in extremes | Vulnerable in the very highest densities |
So the message is clear: environment quenches star formation across the board, but lower-mass and disc-dominated galaxies feel the squeeze most strongly.
Why do crowded environments shut galaxies down?
Physics gives us several ways to starve a galaxy of the cold gas it needs to make new stars. The DEVILS paper reviews the main suspects:
- Ram-pressure stripping – As a galaxy races through hot cluster gas, that gas pushes on its own gas like a wind, blowing it out of the disc.
- Tidal interactions – Nearby galaxies and the cluster’s gravity tug on stars and gas, disturbing or removing them.
- Strangulation or starvation – Fresh gas from the surrounding halo stops reaching the galaxy, so star formation slowly winds down.
- Harassment – Many fast, close flybys stir and heat the gas, making it hard to cool and collapse into stars.
In a sparse environment, a galaxy can quietly accrete gas from its halo, rotate peacefully, and keep forming stars for billions of years.
In a cluster, every orbit around the centre is like commuting through rush-hour traffic: the galaxy gets buffeted, stripped, and cut off from supplies. No wonder the “city centre” systems turn red and dead.
The Universe Today article uses a human analogy: someone raised in a packed city may end up with a very different life from someone raised in a remote village. Galaxies behave the same way; upbringing matters.
Why does high-quality spectroscopy matter so much?
You might ask, “Can’t we just use good photometric redshifts instead of expensive spectroscopy?”
DEVILS actually tested this by comparing its spectroscopic density–SFR relation with one built from the COSMOS2020 photometric catalogue.
Here’s what they found:
- Both samples show some suppression of SFR in high-density regions.
- But photometric-redshift densities have larger scatter and often overestimate the level of suppression at high density.
- Photometric redshift errors are often larger than ±1000 km/s, so true neighbours can be missed or mis-counted.
Even when they loosened the velocity window to ±5000 km/s, the picture didn’t change much. That suggests:
- high-precision, highly complete spectroscopic surveys are essential if we want reliable environmental measures,
- and those measures underpin our understanding of how galaxies shut down star formation in dense regions.
This is where DEVILS shines and why it will act as a benchmark for upcoming projects like WAVES and other wide spectroscopic surveys.
What does this mean for the story of the Universe?
Across cosmic time, the average SFR density of the Universe has dropped dramatically. Today’s Universe is much quieter than it was 8–10 billion years ago.
The DEVILS results show that part of this slowdown comes from environmental quenching:
- Galaxies migrate into denser structures – groups, clusters, filaments.
- When they do, processes in those environments strip, heat, or starve their gas.
- Their star formation fades, and they join the red, quiescent population.
Because DEVILS covers a broad range of environments with high completeness, it reveals that this environmental quenching is not some rare, exotic effect; it’s a common feature for galaxies with 10–11 in log10(M/M☉) at 0.2 < z < 0.5.
That means the cosmic city-building of structure formation isn’t just a pretty backdrop. It actively changes how galaxies grow and when they stop.
So, what should we take away from all this?
Let’s gather the threads:
- Environment matters. Galaxies in dense regions form stars up to ten times more slowly than similar-mass galaxies in isolation.
- This effect is widespread. It shows up across several billion years of look-back time and across many galaxy shapes and masses.
- Lower-mass galaxies take the biggest hit. They’re more easily stripped and starved in clusters.
- Good data changes the story. High-completeness spectroscopic surveys like DEVILS give a cleaner, more accurate view than photometric redshift samples alone.
On a human level, there’s something strangely relatable here. Galaxies don’t grow in isolation; they respond to their surroundings, just as we do. City life, cosmic or terrestrial, shapes what you can become.
At FreeAstroScience, we care about this not just as an astronomy result, but as a reminder of why careful reasoning matters. If we ignore data and let speculation run the show, the sleep of reason breeds monsters—in science, in society, and even in how we imagine the Universe.
So keep your curiosity awake. Come back to FreeAstroScience.com for more stories that turn complex research into clear, honest explanations. This post was written for you by FreeAstroScience.com, a project devoted to explaining hard science in simple language and keeping wonder—and reason—alive.
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