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How the cosmic voids contribute to stalling and quenching the giant galaxies on their surfaces

Using TNG300-1 simulations, this study reveals that cosmic voids likely stall and quench giant galaxies on their surfaces by compressing adjacent matter to prevent radial accretion, a process evidenced by the strong perpendicular alignment of elliptical, red, and low-star-formation-rate galaxies with void centers.

Original authors: Geonwoo Kang, Jounghun Lee

Published 2026-02-13

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CC BY 4.0

Original authors: Geonwoo Kang, Jounghun Lee

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: The Universe as a Giant Sponge

Imagine the universe isn't just a random scattering of stars and galaxies. Instead, think of it as a giant, cosmic sponge or a web.

The Hubs: Dense clusters of galaxies (like city centers) are the "knots" where the web is tightest.
The Strings: Long chains of galaxies (filaments) connect these hubs.
The Holes: The vast, empty spaces in between are called cosmic voids. These are like giant bubbles in the sponge where almost nothing exists.

This paper asks a simple question: What happens to the galaxies that live right on the "skin" or surface of these giant empty bubbles?

The Discovery: The "Cosmic Wind" Effect

The authors used a super-computer simulation (a digital universe called TNG300) to watch how these galaxies behave. They found something surprising:

When a giant void expands (gets bigger), it doesn't just sit there. It acts like a giant vacuum cleaner or an inflating balloon pushing outward.

The Shape Shift (The "Squish"): As the void expands, it pushes against the matter on its surface. Imagine a balloon inflating inside a box of marbles. The marbles on the edge get pushed sideways. The paper found that galaxies on the edge of these voids get "squished" into elliptical shapes (like flattened eggs) rather than spirals (like pinwheels). Their long axes line up perfectly perpendicular to the center of the void, like fence posts standing around a round pond.
The Star Stop (The "Quenching"): Galaxies need to eat gas to make new stars. But the expansion of the void creates a "wind" that pushes gas away from the surface galaxies. It's like trying to catch rain while standing under a giant umbrella that is blowing air upward. The galaxies can't catch the gas, so they stop making new stars. They turn red (old stars) instead of blue (young stars) and become "stalled" or "quenched."

The Detective Work: Sorting the Galaxies

The researchers didn't just look at all galaxies; they acted like detectives sorting suspects into two groups to see who was affected by the "void wind."

They looked at galaxies with massive stars (big, heavy galaxies) on the edge of voids and split them up based on four traits:

Shape: Spiral (pinwheel) vs. Elliptical (smooth egg).
Color: Blue (young) vs. Red (old).
Activity: Making stars (active) vs. Stopped making stars (quiescent).
Age: Young vs. Old.

The Results:

The "Victims": The galaxies that were Elliptical, Red, and Quiet (not making stars) showed the strongest alignment with the void. They were the ones most clearly "squished" and "starved" by the void's expansion.
The "Survivors": The Spiral, Blue, and Active galaxies didn't show this strong alignment. They seemed to have arrived at the void's edge recently or were too young to have been affected yet.

The Analogy: Think of the void expansion as a slow-moving bulldozer. The galaxies that have been sitting there the longest (the old, red, elliptical ones) have been flattened and stopped by the bulldozer. The new arrivals (the young, blue spirals) haven't been hit by the bulldozer yet, so they still look normal.

Why This Matters: The "Cosmic Traffic Jam"

The paper suggests a new way galaxies die. Usually, we think galaxies stop making stars because they crash into other galaxies or get eaten by black holes.

But this paper proposes a new mechanism: The Void Expansion.
The empty space itself is expanding so fast that it compresses the matter around it, creating a "traffic jam" that prevents gas from falling into the galaxy. It's like a river drying up because the water is being pushed away by a rising tide.

The "What If" Scenario: Dark Energy

The authors end with a fascinating speculation. The expansion of the universe is driven by something called Dark Energy.

If Dark Energy is changing over time (which some new data suggests), then the "voids" might have expanded faster in the past.
If voids expanded faster in the past, they would have "stalled" and "quenched" galaxies much earlier in the universe's history.
This could explain a mystery: Why do we see massive, dead (red) galaxies in the very early universe? Maybe the cosmic voids were just really aggressive back then, turning off the star-formation switch early.

Summary in One Sentence

The universe's giant empty bubbles (voids) are expanding so fast that they push against the galaxies on their edges, squishing them into oval shapes and blowing away their gas supply, effectively turning off their star-making engines and turning them red and dead.

1. Problem Statement

The paper addresses the mechanism behind the transformation of galaxies located on the surfaces of cosmic voids. While the influence of the cosmic web (filaments, knots) on galaxy properties is well-studied, the specific role of cosmic voids remains less understood.

The Phenomenon: Giant galaxies ( $M_\star \geq 10^{10.5} h^{-1} M_\odot$ ) on void surfaces are observed to exhibit specific properties: elliptical morphologies, redder colors, lower specific star formation rates (sSFR), and older stellar ages.
The Hypothesis: The authors propose that the rapid expansion of cosmic voids compresses adjacent matter. This compression generates tangential motions that obstruct the radial infall of gas onto surface galaxies, effectively "stalling" their accretion and "quenching" their star formation.
The Observable Signature: This physical process should induce a specific intrinsic alignment: the major shape axes of these stalled galaxies should be perpendicular to the direction pointing toward the void center (which corresponds to the major principal axis of the local tidal field).

2. Methodology

Data Source

Simulation: The study utilizes the TNG300-1 run from the IllustrisTNG suite of cosmological hydrodynamical simulations.
Parameters: A flat $\Lambda$ CDM cosmology with a box size of $205 h^{-1}$ Mpc, tracking $2 \times 2500^3$ particles from $z=127$ to $z=0$ .
Redshifts: Analysis is performed at $z = 0, 0.5,$ and $1$.

Void Identification

Algorithm: The Void-Finder algorithm is used to identify voids as assemblies of maximal empty spheres fitting the underdense regions.
Surface Definition: "Void-surface galaxies" are defined as those located in a thin spherical shell surrounding the void, specifically where the distance $r$ from the void center satisfies $0.9 \leq r/R_v \leq 1.1$ (where $R_v$ is the effective void radius).
Control: To isolate the void effect, the authors compare these with galaxies inside the void ( $r < 0.9R_v$ ) and outside the void ( $r > 1.1R_v$ ).

Alignment Measurement

Shape Axis: The shape axis ( $\hat{u}$ ) of each galaxy is determined as the eigenvector corresponding to the largest eigenvalue of the inertia tensor of its stellar particles.
Alignment Metric: The angle $\theta$ is measured between the galaxy's shape axis ( $\hat{u}$ ) and the separation vector pointing to the void center ( $\hat{r}$ ).
Statistic: The probability density function (PDF), $p(\cos \theta)$ , is calculated. A value of $\langle \cos \theta \rangle < 0.5$ indicates perpendicular alignment.

Analytical Modeling

The numerical results are compared against an analytical one-parameter model (Lee [2]) which assumes a linear scaling between the covariance matrices of galaxy shape axes and local tidal tensors. The model uses a correlation parameter $d_t$ to quantify alignment strength.

Robustness Tests

Alternative Void Finder: Results are cross-checked using the VIDE (Void IDentification and Examination) algorithm, which uses Voronoi tessellation and watershed transforms.
Observational Effects: The study tests redshift-space distortions (using peculiar velocities) and 2D projection effects (simulating sky-plane observations) to assess feasibility for real-world surveys.
Sample Control: Galaxies are dichotomized by morphology, color, sSFR, and age, with strict control for stellar mass ( $M_\star$ ) distributions to ensure differences are not mass-driven.

3. Key Contributions & Results

A. Detection of Perpendicular Alignment

Significant Signal: A clear signal of perpendicular alignment ( $\langle \cos \theta \rangle \approx 0.47$ ) is detected only for giant galaxies ( $M_\star \geq 10^{10.5} h^{-1} M_\odot$ ) located specifically on the void surface ( $0.9 \leq r/R_v \leq 1.1$ ).
No Signal Elsewhere: Galaxies inside the void or far outside it show random orientations ( $\langle \cos \theta \rangle \approx 0.5$ ), confirming the alignment is a unique property of the void surface environment.
Model Fit: The numerical distribution $p(\cos \theta)$ is well-described by the analytical model with a best-fit parameter $d_t \approx 0.08$ at $z=0$ .

B. Dependence on Galaxy Properties

The alignment strength varies systematically with galaxy properties, supporting the "stalling/quenching" hypothesis:

Morphology: Elliptical galaxies show strong perpendicular alignment, while spiral galaxies show no significant alignment.
Color: Redder galaxies exhibit strong alignment; bluer galaxies do not.
Star Formation (sSFR): Galaxies with lower sSFR (quenched/stalled) show stronger alignment than those with higher sSFR.
Stellar Age: While older galaxies show slightly stronger alignment, the dependence on age is the weakest among the four properties.
- Crucial Finding: Even among elliptical galaxies, the alignment strength does not significantly differ between star-forming and quenched sub-samples, suggesting the alignment is driven by morphology (likely a result of long-term exposure) rather than just the current star-formation state.

C. Redshift Evolution

The property-dependent trends (stronger alignment for ellipticals, red, low-sSFR galaxies) persist at $z=0.5$ and $z=1$ .
This suggests the mechanism is active throughout cosmic history, not just a local ( $z=0$ ) phenomenon.
At $z=1$ , the dependence on stellar age vanishes, likely because the formation epochs of void-surface galaxies at that redshift are more uniform.

D. Robustness and Feasibility

Void-Finder vs. VIDE: The VIDE algorithm yields similar trends but with a lower signal-to-noise ratio. The authors suggest the spherical-shell assumption of the Void-Finder better aligns with the tidal field geometry required for this specific measurement.
Redshift Space: The alignment signal is robust against redshift-space distortions (only slightly weakened).
Projection Effects: Projecting 3D shapes to 2D (sky plane) significantly weakens the signal and increases errors. Detecting this in real observations will require stacking large samples of voids.

4. Significance and Implications

Mechanism for Quenching: The paper provides a numerical hint that void expansion is a physical mechanism capable of stalling gas accretion and quenching galaxies. The expansion compresses adjacent matter, creating tangential flows that prevent radial infall, effectively starving the galaxy of fuel.
Cosmic Web Influence: It highlights that voids are not just empty spaces but active agents in galaxy evolution, particularly for the "stalled" population of giant galaxies on their boundaries.
Dark Energy Probe: The authors speculate that the abundance of aligned, quenched elliptical galaxies on void surfaces could serve as a complementary probe for Dark Energy (DE). If DE is dynamical (changing equation of state), it would alter void formation and expansion rates, thereby changing the fraction of these aligned galaxies.
High-z Quenched Galaxies: The mechanism offers a potential explanation for the existence of massive, quenched galaxies at high redshifts in gas-rich environments, attributing their early quenching to early and rapid void expansion driven by dynamical DE.

Conclusion

The study establishes a robust link between the expansion of cosmic voids and the morphological and star-formation properties of giant galaxies on their surfaces. The observed perpendicular alignment serves as a kinematic fingerprint of the void's tidal field, confirming that void expansion actively suppresses gas accretion, leading to the stalling and quenching of these galaxies. This finding opens a new avenue for using void-galaxy alignments to constrain the nature of Dark Energy.