Cosmic Web Connectivity: How It Shapes Galaxy Evolution

Introduction

The universe was not perfectly uniform at the beginning; some regions had higher density than others.

Over time these higher-density regions attracted most of the matter and began forming galaxies where matter concentration was greatest. The resulting large-scale structure—the “cosmic web”—connects observed galaxy clusters with a network of filaments. Figure 1 shows a model of this structure.

Illustration and video

Figure 1: Skeleton of a cosmic web traced by an algorithm run on a sample of observed galaxies. The far right shows the complete web; the left images show close-up portions. Blue areas indicate higher density.

Watch a short explanatory video: Cosmic web overview on YouTube.

Key points about the cosmic web

1. The early universe had small density fluctuations; regions with higher density became sites of galaxy formation.
2. High-density regions are connected by filaments forming the cosmic web; a galaxy’s “connectivity” is the number of filaments attached to it, and higher connectivity correlates with greater galaxy mass.
3. Connectivity is related to star formation: galaxies with higher connectivity generally show lower star formation rates.
4. Active galactic nucleus (AGN) activity is more effective at quenching star formation in galaxies that have high connectivity.
5. Connectivity influences a galaxy’s evolution, affecting star formation, total mass and morphology.

At the crossroads

The cosmic web is a way to represent the density field of the Universe and can be traced using the observed distribution of galaxies. Similar algorithms are applied to both observations and simulations to identify connected structures. The densest, most massive galaxies sit at the crossroads of multiple filaments (nodes), similar to the blue regions in Figure 1. Previous studies also show that a galaxy’s mass increases with the number of connected filaments. Recent research examined whether connectivity is also related to star formation and whether it can suppress star formation.

Forming stars—or not

Samples and simulations

To test this, scientists analyzed observed and simulated galaxies that occupy the densest nodes of the cosmic web. Observed galaxies were taken from the Sloan Digital Sky Survey (SDSS) data release, numbering ≈ 7×10^5 (~700,000). Two independent cosmological simulations were also used for comparison; simulation samples are much smaller because of the computational cost of simulating large galaxy populations.

Connectivity vs star formation

These galaxies have a median connectivity of three filaments. For the observed sample, researchers compared connectivity to star formation rate (inferred from stellar age distributions). Figure 2 shows that as connectivity increases, the excess star formation rate decreases for all galaxy types. The same trend appears in the simulations, where star formation is a direct output parameter.

Figure 2: Comparison for observed galaxies of average connectivity and excess star formation rate relative to the average for that galaxy type. The black lines show all galaxies. Top panel: blue = star-forming galaxies; red = passive galaxies (not actively forming a significant number of stars). Bottom panel: blue = spherical/irregular galaxies; red = elliptical galaxies. The horizontal axis indicates increasing star formation to the right. The plots show that as connectivity increases, star formation decreases.

This result is counterintuitive because filaments can funnel cold gas into galaxies, which might be expected to stimulate star formation. Scientists propose several explanations: gas arriving along multiple filaments may be more chaotic than gas funneled along a single coherent filament; filaments may be inefficient at delivering gas to the inner, star-forming regions of galaxies; or galaxies with higher connectivity may experience fewer gas-rich mergers. Regardless of the mechanism, observations and simulations both show that greater connectivity correlates with reduced star formation.

Transitioning to old age

Researchers also examined the relationship between connectivity and AGN activity. AGN feedback is an important quenching mechanism: energetic output from an AGN can expel star-forming gas and halt star formation, helping a galaxy transition from an active, star-forming state to a passive, older state. By comparing two similar simulations—one that includes AGN feedback and one that does not—scientists found that AGN are more effective at quenching star formation in galaxies with high connectivity. In those galaxies, AGN can receive more fuel and therefore produce more energetic feedback.

Implications for galaxy evolution

Overall, connectivity emerges as an important factor in galaxy formation: it affects when stars stop forming and influences a galaxy’s mass and morphology in both observations and simulations. Because galaxy evolution unfolds over millions to billions of years, studies like this provide crucial snapshots that improve our understanding of cosmic history.

References

Arnouts, S.; Codis, S.; Davé, R.; Devriendt, J.; Dubois, Y.; Hwang, H. S.; Kraljic, K. (Institute for Astronomy, University of Edinburgh); Laigle, C.; Musso, M.; Peirani, S.; Pichon, C.; Pogosyan, D.; Slyz, A.; Treyer, M.

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