The largest gravitationally bound systems, forming a cosmic web and affecting the cluster member galaxies
Galaxies are not alone in space. They form clusters. swarm – giant formations consisting of hundreds or even thousands of galaxies, bound together by a common gravity. On an even larger scale, there are superclusters, connecting many clusters in the threads of a cosmic web. These giant structures dominate the densest parts of the Universe, determine the arrangement of galaxies, and influence each galaxy in the cluster. In this article, we will explore what galaxy clusters and superclusters are, how they form, and why they are important for understanding large-scale cosmology and the evolution of galaxies.
1. Definition of clusters and superclusters
1.1 Galaxy clusters: the core of the cosmic web
Galaxy cluster – this gravitationally bound a system that can contain from a few tens to thousands of galaxies. The total mass of clusters is typically ∼1014–1015 M⊙In addition to galaxies, they contain:
- Dark matter halos: Most of the cluster mass (~80–90) %) is made up of dark matter.
- Hot intercluster medium (ICM): Diluted, superheated gas (temperature 107–108 K), emitting in the X-ray range.
- Interacting galaxies: Cluster galaxies experience ram-pressure stripping, harassment, or mergers because the collision frequency is high.
Clusters are often discovered by searching for large concentrations of galaxies in optical surveys, by observing the ICM in X-rays, or by using Sunyaev–Zel'dovich effect – the distortion of cosmic microwave background photons by hot electrons in the cluster.
1.2 Superclusters: Looser, Larger Structures
Superclusters are not completely gravitationally bound, rather it is free associations of galaxy clusters and groups connected by filaments. They extend from a few tens to hundreds of megaparsecs, showing the largest-scale structure of the Universe and the densest knots of the cosmic web. Although some parts of a supercluster may be interconnected, not all regions of these formations will be stably aligned on cosmic timescales if they are not fully formed.
2. Formation and development of swarms
2.1 Hierarchical growth in the ΛCDM model
According to the modern cosmological model (ΛCDM), dark matter halos grow hierarchically: smaller halos are formed first, which merge, eventually forming galaxy groups and clusters. The main stages are:
- Early density fluctuations: The small differences in density that formed after inflation are gradually "collapsing".
- Group stage: Galaxies first gather into groups (~1013 M⊙), which are later joined by additional halos.
- Swarm stage: As groups merge, clusters form in which the gravitational potential is deep enough to maintain a hot ICM.
The largest cluster halos can continue to grow, either by annexing more galaxies or merging with other clusters, forming the most massive gravitationally bound entities in the Universe [1].
2.2 Interstellar medium and heating
As the groups merge into clusters, the infalling gas is heated to virial temperatures of tens of millions of degrees, creating X-rays source — the hot intercluster medium (ICM). This plasma significantly affects the cluster galaxies, e.g. through ram pressure stripping impact.
2.3 Organized and unorganized swarms
Some clusters that have undergone major mergers in the past are called "relaxed", with a uniform X-ray emission and a single deep gravitational potential. Others exhibit obvious substructures indicating ongoing or recent collisions - shock fronts in the ICM or several separate galaxy clusters suggest an unrelaxed cluster (e.g., the "Bullet Cluster") [2].
3. Observation features
3.1 X-ray radiation
Hot ICM in clusters is strong X-ray source. Telescopes as Chandra and XMM-Newton watches:
- Thermal free charge radiation (bremsstrahlung): Hot electrons emitting in the X-ray range.
- Chemical abundance: Spectral lines showing heavy elements (O, Fe, Si) scattered in supernova cluster galaxies.
- Swarm profiles: Gas density and temperature distribution, allowing reconstruction of mass distribution and merger history.
3.2 Optical surveys
Dense red, elliptical galaxies concentration at the center of the cluster is characteristic of clusters. Spectral studies help to detect rich swarm (e.g. Coma) by the compacted redshift of confirmed members. Often we find a massive "Brightest Cluster Galaxy" (BCG) at the center of the cluster, indicating a deep gravitational well.
3.3 Sunyaev–Zel'dovich (SZ) effect
Hot electrons in the ICM can interact with photons in the cosmic microwave background, giving them a little more energy. This creates a unique SZ effect, which reduces the CMB intensity along the cluster line. This method allows clusters to be detected almost independently of their distance [3].
4. Impact on cluster galaxies
4.1 Ram-pressure and extinguishing
When a galaxy moves at high speed through a dense, hot ICM, gas is "torn off"This deprives the star formation fuel, resulting in gas-starved, "red and inactive" elliptical or S0 galaxies.
4.2 Harassment and tidal interactions
In dense cluster environments, close galaxy passes can disrupt stellar disks, forming bulges or bars. This repetitive harassment dynamic eventually heats the stellar part of the spiral and turns it into a lenticular (S0) [4].
4.3 BCG and prominent members
The brightest cluster galaxies (BCGs), usually located near the cluster center, can grow significantly through "galactic cannibalism"—by acquiring satellites or merging with other large members. They are characterized by very extended stellar halos and often extremely massive black holes that emit powerful radio jets or AGN activity.
5. Superclusters and the cosmic web
5.1 Threads and voids
Superclusters connect clusters via galaxies and dark matter filaments, and emptiness (voids) fill the rarer gaps. This network “fabric” arises from the large-scale distribution of dark matter, which determined the initial density fluctuations [5].
5.2 Examples of superclusters
- Local Supercluster (LSC): Includes the Virgo Cluster, Our Group (where the Milky Way is located), and other nearby groups.
- Shapley Supercluster: One of the most massive in the local Universe (~200 Mpc away).
- Sloan Great Wall: Giant supercluster structure found in Sloan Digital Sky Survey.
5.3 Gravitational entanglement?
Many superclusters are not fully virialized – they may “spread out” due to the expansion of the Universe. Only some of the denser parts of superclusters eventually collapse into the halo of future clusters. Due to the accelerating expansion, large-scale filaments may be destined to “stretch out” and become rarefied, gradually separating them from their surroundings over cosmic timescales.
6. Swarm cosmology
6.1 Swarm mass function
By calculating clusters as a function of mass and redshift, cosmologists test:
- The density of matter (Ωm): Higher density means more swarms.
- Dark energy: The growth rate of structure (including clusters) depends on the properties of dark energy.
- σ8: The amplitude of the initial density fluctuations determines how quickly swarms form [6].
X-ray and SZ studies allow for precise determination of cluster masses, thus providing tight constraints on cosmological parameters.
6.2 Gravitational lensing
Cluster-scale gravitational lensing also helps estimate the cluster's mass. Strong lensing forms giant arc-shaped sources or multiple images, and weak lensing slightly distorts the shapes of the background galaxies. These measurements confirm that ordinary (visible) matter makes up only a small fraction of the clusters' mass—dark matter dominates.
6.3 Baryon fraction and the KMB
The ratio of the gas mass (baryons) to the total cluster mass indicates the universal baryon fraction, which we compare with the cosmic microwave background (CMB) data. These studies consistently confirm the ΛCDM model and refine the baryon balance of the Universe [7].
7. The evolution of clusters and superclusters over time
7.1 High-redshift protospheric galaxies
When observing distant (high-z) galaxies, we find protospiders – dense clusters of young galaxies that may soon “collapse” into full-fledged clusters. Some bright star-forming galaxies or AGNs at z∼2–3 are found in such dense regions, foreshadowing today’s massive clusters. JWST and large ground-based telescopes are increasingly detecting these protoclusters, identifying small regions of the sky with the most abundant “redshift groups” of galaxies and active star formation.
7.2 Mergers within the clusters themselves
Swarms can join together to form extremely massive systems – "swarm collisions" generates shock fronts in the ICM (e.g., the Bullet Cluster) and reveals subhalo structures. These are the largest gravitationally bound events in the Universe, releasing enormous amounts of energy that heat gas and rearrange galaxies.
7.3 The future of superclusters
As the expansion of the Universe increases (with dark energy dominating), it is likely that a significant fraction of superclusters will never collapse. In the future, cluster mergers will still occur, forming giant virialized halos, but the largest parts of the filaments may stretch and thin out, eventually separating these mega-structures as "separate Universes."
8. The most famous examples of clusters and superclusters
- Coma Cluster (Abell 1656): A massive, rich cluster (~300 million light-years away), famous for its many elliptical and S0 galaxies.
- Virgo Cluster: The nearest rich cluster (~55 million light-years away), containing the gigantic elliptical M87. It belongs to the Local Supercluster.
- Bullet Swarm (1E 0657-558): Demonstrates a collision of two clusters, where X-ray gas is displaced from dark matter clumps (detected by lensing) — important evidence for the existence of dark matter [8].
- Shapley Supercluster: One of the largest known superclusters, spanning a distance of ~200 Mpc, is composed of a network of interconnected clusters.
9. Summary and future prospects
Galaxy clusters – the largest gravitationally bound systems – are the densest nodes in the cosmic web, showing how large-scale matter organizes. They are the site of complex interactions between galaxies, dark matter, and the hot intercluster medium, leading to morphological changes and the "quenching" of star formation in clusters. Meanwhile superclusters conveys an even broader arrangement of these massive nodes and threads, representing the framework of the cosmic web.
By observing the masses of clusters, analyzing X-ray and SZ emission, and assessing gravitational lensing, scientists determine key cosmological parameters, including the density of dark matter or the properties of dark energy. Future projects (e.g., LSST, Euclid, Roman Space Telescope) will provide thousands of new cluster discoveries, further refining cosmic models. At the same time, deep observations will allow us to detect protoclusters at early epochs and follow in more detail how supercluster-scale structures evolve in the rapidly expanding Universe.
While galaxies themselves are magnificent, their collective structure in massive clusters and extended superclusters shows that cosmic evolution is a general phenomenon where environment, gravitational attraction, and feedback combine to create the largest structures in the Universe known to us.
References and further reading
- White, SDM, & Rees, MJ (1978). "Core condensation in heavy halos - A two-stage theory for galaxy formation and the missing satellite problem." Monthly Notices of the Royal Astronomical Society, 183, 341–358.
- Markevitch, M., et al. (2002). "Direct Constraints on the Dark Matter Self-Interaction Cross Section from the Merging Galaxy Cluster 1E 0657–56.” The Astrophysical Journal, 567, L27–L30.
- Sunyaev, RA, & Zeldovich, YB (1970). "The Interaction of Matter and Radiation in the Expanding Universe." Astrophysics and Space Science, 7, 3–19.
- Moore, B., Lake, G., & Katz, N. (1998). "Morphological transformation from galaxy harassment." The Astrophysical Journal, 495, 139–149.
- Bond, JR, Kofman, L., & Pogosyan, D. (1996). "How filaments are woven into the cosmic web." Nature, 380, 603–606.
- Allen, SW, Evrard, AE, & Mantz, AB (2011). "Cosmological Parameters from Observations of Galaxy Clusters." Annual Review of Astronomy and Astrophysics, 49, 409–470.
- Vikhlinin, A., et al. (2009). "Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints." The Astrophysical Journal, 692, 1060–1074.
- Clowe, D., et al. (2004). "Weak-lensing mass reconstruction of the interacting cluster 1E 0657–558: Direct evidence for the existence of dark matter.” The Astrophysical Journal, 604, 596–603.