Galaktikų spiečiai ir superspiečiai

Galaxy clusters and superclusters

The largest gravitationally bound systems forming the cosmic web and influencing the cluster member galaxies

Galaxies in the cosmos are not isolated. They gather into clusters – enormous structures made up of hundreds or even thousands of galaxies bound by gravity. On a larger scale, superclusters exist, connecting many clusters along the filaments of the cosmic web. These massive structures dominate the densest parts of the Universe, determine the distribution of galaxies, and influence every cluster galaxy. This article explores what galaxy clusters and superclusters are, how they form, and why they are important for understanding large-scale cosmology and galaxy evolution.

1. Definition of clusters and superclusters

1.1 Galaxy clusters: the core of the cosmic web

Galaxy clusters are gravitationally bound systems containing from several tens to thousands of galaxies. Cluster total masses typically reach ∼1014–1015 M⊙. Besides galaxies, they contain:

  1. Dark matter halos: Dark matter makes up the majority of the cluster's mass (~80–90%).
  2. Hot intracluster medium (ICM): Diffuse, extremely hot gas (temperature 107–108 K) emitting in the X-ray range.
  3. Interacting galaxies: Cluster galaxies experience gas stripping as they move through the hot medium (ram-pressure stripping), "harassment," or mergers, due to the high collision rate.

Clusters are often detected by searching for high galaxy concentrations in optical surveys, observing ICM X-ray emission, or using the 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 fully gravitationally bound; rather, they are loose associations of galaxy clusters and groups connected by filaments. They span from several tens to hundreds of megaparsecs, showing the largest-scale structure of the Universe and the densest nodes of the cosmic web. Although some parts of a supercluster may be gravitationally bound, not all regions of these structures will be stably collapsed over cosmic timescales if they are not fully formed.

2. Cluster formation and evolution

2.1 Hierarchical growth in the ΛCDM model

According to the modern cosmological model (ΛCDM), dark matter halos grow hierarchically: smaller halos form first and merge, eventually creating galaxy groups and clusters. The main stages are:

  1. Early density fluctuations: Small density differences formed after inflation gradually "fade away."
  2. Group stage: Galaxies first gather into groups (~1013 M⊙), which later accrete additional halos.
  3. Cluster stage: When groups merge, clusters form with a gravitational potential deep enough to retain the hot ICM.

The largest cluster halos can continue to grow by accreting more galaxies or merging with other clusters, forming the most massive gravitationally bound structures in the Universe [1].

2.2 Intracluster medium and heating

When groups merge into clusters, infalling gas is shock-heated to virial temperatures reaching tens of millions of degrees, creating an X-ray source — the hot intracluster medium (ICM). This plasma significantly affects cluster galaxies, e.g., through ram-pressure stripping.

2.3 Relaxed and unrelaxed clusters

Some clusters that experienced major mergers earlier are called "relaxed", with smooth X-ray emission and a single deep gravitational potential. Others show obvious substructures indicating ongoing or recent collisions — shock fronts in the ICM or multiple separate galaxy concentrations indicate an unrelaxed cluster (e.g., the "Bullet Cluster") [2].

3. Observational characteristics

3.1 X-ray radiation

The hot ICM in clusters is a strong X-ray source. Telescopes like Chandra and XMM-Newton observe:

  • Thermal free-free radiation (bremsstrahlung): Hot electrons radiating in the X-ray range.
  • Chemical abundances: Spectral lines showing heavy elements (O, Fe, Si) dispersed by supernovae in cluster galaxies.
  • Cluster profiles: Gas density and temperature distributions, allowing reconstruction of mass distribution and merger history.

3.2 Optical surveys

A dense concentration of red, elliptical galaxies in the cluster center is characteristic of clusters. Spectral studies help detect rich clusters (e.g., Coma) by the concentrated confirmed members' redshift. Often, the cluster center hosts a massive "Brightest Cluster Galaxy" (BCG), indicating a deep gravitational well.

3.3 Sunyaev–Zel’dovich (SZ) effect

Hot ICM electrons can interact with cosmic microwave background photons, giving them slightly more energy. This creates the distinctive SZ effect, reducing the CMB intensity along the cluster line. This method allows detecting clusters almost independently of their distance [3].

4. Impact on cluster galaxies

4.1 Gas "stripping" (ram-pressure) and quenching

When a galaxy moves at high speed through dense hot ICM, gas is "stripped off". This results in the loss of star formation fuel, producing gas-deficient, "red and inactive" elliptical or S0 galaxies.

4.2 "Harassment" and tidal interactions

In dense cluster environments, close galaxy flybys can disrupt stellar disks, forming warps or bars. Such repeated "harassment" dynamics eventually heat the spiral stellar component and transform it into a lenticular (S0) [4].

4.3 BCG and bright members

The brightest cluster galaxies (BCG), usually located near the cluster center, can grow significantly through "galactic cannibalism" — accreting satellites or merging with other massive members. They feature very extended stellar halos and often especially massive black holes, emitting powerful radio jets or AGN activity.

5. Superclusters and the cosmic web

5.1 Filaments and voids

Superclusters connect clusters through galaxy and dark matter filaments, while voids fill the sparser gaps. This network "fabric" arises from large-scale dark matter distribution shaped by initial density fluctuations [5].

5.2 Examples of superclusters

  • Local supercluster (LSC): Includes the Virgo cluster, the Local Group (where the Milky Way is), and other nearby groups.
  • Shapley supercluster: One of the most massive in the local Universe (~200 Mpc away).
  • Sloan Great Wall: A gigantic supercluster structure discovered in the Sloan Digital Sky Survey.

5.3 Gravitational connectivity?

Many superclusters are not fully virialized — they may "disperse" due to the expansion of the Universe. Only some denser parts of superclusters eventually collapse into future cluster halos. Due to accelerating expansion, large-scale filaments may be "stretched" and thinned out, gradually isolating them from the environment over cosmic timescales.

6. Cluster cosmology

6.1 Cluster mass function

By counting clusters as a function of mass and redshift, cosmologists test:

  1. Matter density (Ωm): Higher density means more clusters.
  2. Dark energy: The rate of structure growth (including clusters) depends on the properties of dark energy.
  3. σ8: The amplitude of initial density fluctuations determines how quickly clusters form [6].

X-ray and SZ studies allow precise determination of cluster masses, thus providing strict constraints on cosmological parameters.

6.2 Gravitational lensing

Cluster-scale gravitational lensing also helps estimate the cluster mass. Strong lensing forms huge arc-shaped sources or multiple images, while weak lensing slightly distorts the shapes of background galaxies. These measurements confirm that ordinary (visible) matter makes up only a small fraction of the cluster mass — dark matter dominates.

6.3 Baryon fraction and CMB

The ratio of gas mass (baryons) to total cluster mass indicates a universal baryon fraction, which we compare with cosmic microwave background (CMB) data. These studies consistently confirm the ΛCDM model and refine the Universe's baryon budget [7].

7. Evolution of clusters and superclusters over time

7.1 High redshift protoclusters

Observing distant (high-z) galaxies reveals protoclusters – dense young galaxy concentrations soon to "collapse" into full clusters. Some bright star-forming galaxies or AGN at z∼2–3 are found in such dense regions, predicting present-day massive clusters. JWST and large ground-based telescopes increasingly detect these protoclusters, identifying small sky areas with the richest galaxy "redshift groups" and active star formation.

7.2 Cluster mergers

Clusters can merge with each other, forming extremely massive systems – "cluster collisions" generate shock fronts in the ICM (e.g., the "Bullet Cluster") and reveal subhalo structures. These are the largest gravitationally bound events in the Universe, releasing enormous amounts of energy that heat the gas and rearrange galaxies.

7.3 The future of superclusters

As the Universe expands (with dark energy dominating), it is likely that many superclusters will never collapse. In the future, cluster mergers will still occur, forming huge virialized halos, but the largest filament parts may stretch and thin out, eventually isolating these mega-structures as "separate Universes."

8. 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), containing the giant elliptical M87. It belongs to the Local supercluster.
  • Bullet Cluster (1E 0657-558): Demonstrates the collision of two clusters, where X-ray gas is displaced from dark matter concentrations (determined by lensing) — a key proof of dark matter's existence [8].
  • Shapley supercluster: One of the largest known superclusters, extending ~200 Mpc, composed of a network of connected clusters.

9. Summary and future prospects

Galaxy clusters – the largest gravitationally bound systems – are the densest nodes of the cosmic web, showing how large-scale matter is organized. They host complex interactions between galaxies, dark matter, and hot intracluster medium, leading to morphological changes and star formation "quenching" in clusters. Meanwhile, superclusters represent an even broader arrangement of these massive nodes and filaments, depicting the cosmic web's framework.

By observing cluster masses, analyzing X-ray and SZ emission, and evaluating gravitational lensing, scientists determine key cosmological parameters, including dark matter density and dark energy properties. 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 detection of protoclusters in early epochs and more detailed tracking of how supercluster-scale structures evolve in the rapidly expanding Universe.

Although the galaxies themselves are amazing, their collective structure in massive clusters and extended superclusters shows that cosmic evolution is a common phenomenon where environment, gravitational clustering, and feedback merge to create the largest known structures in the Universe.

Links and further reading

  1. White, S. D. M., & Rees, M. J. (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.
  2. 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.
  3. Sunyaev, R. A., & Zeldovich, Y. B. (1970). “The Interaction of Matter and Radiation in Expanding Universe.” Astrophysics and Space Science, 7, 3–19.
  4. Moore, B., Lake, G., & Katz, N. (1998). “Morphological transformation from galaxy harassment.” The Astrophysical Journal, 495, 139–149.
  5. Bond, J. R., Kofman, L., & Pogosyan, D. (1996). “How filaments are woven into the cosmic web.” Nature, 380, 603–606.
  6. Allen, S. W., Evrard, A. E., & Mantz, A. B. (2011). “Cosmological Parameters from Observations of Galaxy Clusters.” Annual Review of Astronomy and Astrophysics, 49, 409–470.
  7. Vikhlinin, A., et al. (2009). “Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints.” The Astrophysical Journal, 692, 1060–1074.
  8. 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.
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