9.4 Galaxy clusters and large-scale structure
3 min read•july 22, 2024
Galaxy clusters are cosmic giants, housing hundreds to thousands of galaxies bound by gravity. These massive structures span millions of light-years and contain hot gas that emits X-rays, revealing their immense size and power.
Dark matter plays a starring role in cluster formation, providing the gravitational glue that holds everything together. As clusters grow and merge, they create a vast of , walls, and , shaping the universe's large-scale structure.
Galaxy Clusters
Properties of galaxy clusters
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- Contain hundreds to thousands of galaxies gravitationally bound together (Virgo Cluster, Coma Cluster)
- Span sizes of 2-10 Mpc (megaparsecs), larger than individual galaxies but smaller than large-scale structures
- Have masses ranging from to solar masses, making them the most massive gravitationally bound objects
- Exhibit high galaxy density compared to the field, with galaxies packed closely together
- Contain a significant amount of hot, X-ray emitting intracluster medium (ICM) filling the space between galaxies
- Often host a massive, central dominant galaxy (cD galaxy) at the center of the cluster's gravitational potential well
- Classified by richness based on the number of galaxies within the cluster
- Abell richness classes range from 0 (30-49 galaxies) to 5 (more than 300 galaxies)
- Classified by morphology based on the distribution of galaxies within the cluster
- Regular clusters have symmetric, centrally concentrated galaxy distributions
- Irregular clusters have asymmetric distributions with multiple concentration centers
- Classified by X-ray luminosity based on the intensity of X-ray emission from the ICM, indicating the mass and temperature of the cluster
Dark matter in cluster formation
- Dark matter dominates the mass content of galaxy clusters, accounting for approximately 80-90% of the total cluster mass
- Provides the gravitational potential well that holds the cluster together, allowing galaxies and gas to be bound
- Plays a crucial role in the process
- Smaller dark matter halos merge and grow over time
- These growing halos attract galaxies and gas to form larger structures
- Enables mergers between galaxy clusters, leading to the formation of more massive clusters
- Induces shocks and turbulence in the ICM during mergers, heating the gas
- Causes effects where clusters act as lenses, distorting the images of background galaxies (Abell 2218)
- Allows for the mapping of the dark matter distribution within clusters
Large-scale structure of universe
- The large-scale structure is a complex network of galaxies and clusters formed through the gravitational amplification of primordial density fluctuations
- Filaments are long, thin, and relatively dense regions of galaxies and gas that connect galaxy clusters and form the "cosmic web" (Perseus-Pisces Supercluster)
- Walls (or sheets) are large, two-dimensional structures of galaxies often found between filaments and voids (Sloan Great Wall)
- Voids are vast, underdense regions with few galaxies that occupy the majority of the volume in the universe (Boötes void)
- Surrounded by filaments and walls, creating a foam-like appearance on large scales
Galaxy clusters as cosmological probes
- Galaxy clusters serve as powerful cosmological probes since their properties and distribution are sensitive to the underlying cosmological parameters
- The cluster mass function, which describes the number density of clusters as a function of mass and , depends on the matter density, dark energy, and growth of structure
- Comparing observed and predicted mass functions constrains cosmological models
- The baryon fraction in clusters, the ratio of baryonic matter (gas and stars) to total matter, should closely match the cosmic baryon fraction
- Deviations can indicate modifications to the standard cosmological model
- The Sunyaev-Zel'dovich (SZ) effect is a distortion of the cosmic microwave background (CMB) spectrum caused by hot cluster gas
- Independent of redshift, allowing for the detection of high-z clusters
- Used to study the evolution of cluster properties over cosmic time
- Observations of cluster dynamics and mergers test the properties of dark matter and gas physics
- The Bullet Cluster shows an offset between gas and dark matter, supporting the concept of collisionless dark matter
Key Terms to Review (18)
Cluster mergers: Cluster mergers refer to the process where two or more galaxy clusters collide and combine into a single, larger cluster. This dynamic interaction can lead to significant changes in the structure and evolution of the clusters involved, influencing the distribution of galaxies, dark matter, and hot gas within the merged system.
Cold dark matter theory: Cold dark matter theory posits that the universe contains a significant amount of unseen matter that does not emit, absorb, or reflect light, and moves slowly compared to the speed of light. This theory is essential for explaining the formation and structure of galaxies and galaxy clusters, helping to account for the gravitational effects observed in the universe that cannot be explained by visible matter alone.
Cosmic Web: The cosmic web is the large-scale structure of the universe, composed of galaxies, galaxy clusters, and vast voids interconnected by filaments of dark matter and gas. This intricate network reveals how matter is distributed in the universe and plays a crucial role in understanding the formation and evolution of cosmic structures over time.
Dark energy density: Dark energy density refers to the amount of dark energy present in a given volume of space. It plays a crucial role in the accelerated expansion of the universe, affecting the formation and dynamics of large-scale structures, such as galaxy clusters, and influencing observations that suggest an unseen force driving this acceleration.
Dark matter halo: A dark matter halo is a theoretical structure that surrounds galaxies and galaxy clusters, composed predominantly of dark matter which interacts only via gravity and possibly weak interactions. These halos are crucial for explaining the observed rotation curves of galaxies and the formation of large-scale structures in the universe, helping to account for the mass discrepancy between visible matter and the gravitational forces at play.
Edwin Hubble: Edwin Hubble was an American astronomer who played a pivotal role in the development of modern cosmology, particularly known for discovering that the universe is expanding. His work provided crucial evidence for the Big Bang theory and established the relationship between redshift and distance, transforming our understanding of the cosmos.
Filaments: Filaments are massive, thread-like structures in the universe that form part of the cosmic web, connecting galaxy clusters and superclusters. These structures are critical in understanding the large-scale distribution of matter and energy in the universe, as they define the locations of galaxies and dark matter. Filaments are surrounded by vast voids and are interspersed with sheets of galaxies, creating a complex network that influences the formation and evolution of cosmic structures.
Gravitational Lensing: Gravitational lensing is the phenomenon where the light from a distant object, such as a galaxy or quasar, is bent around a massive object, like a galaxy cluster, due to the effects of gravity. This bending of light can create multiple images, magnify the brightness of the source, and provide valuable insights into the distribution of mass in the universe, especially dark matter and its role in cosmic structure.
Hierarchical Structure Formation: Hierarchical structure formation is a model that explains how cosmic structures, like galaxies and galaxy clusters, develop and evolve over time. It posits that smaller structures form first and then merge together to create larger structures, leading to the universe's complex web-like arrangement. This concept highlights the role of gravitational interactions and cosmic expansion in shaping large-scale structures in the universe.
Lambda cold dark matter model: The lambda cold dark matter (ΛCDM) model is the leading cosmological model that describes the large-scale structure and evolution of the universe. It incorporates the effects of dark energy (represented by the cosmological constant lambda, Λ) and cold dark matter, which together account for the observed phenomena in the universe such as galaxy formation, cosmic expansion, and the cosmic web.
Luminosity Function: The luminosity function is a statistical tool used in astronomy to describe the distribution of luminosities among a group of stars or galaxies. It provides insights into the number of objects per unit volume as a function of their brightness, allowing astronomers to understand the characteristics and evolution of galaxies, especially within clusters and the large-scale structure of the universe.
Massive galaxy cluster collisions: Massive galaxy cluster collisions refer to the high-energy events that occur when two or more galaxy clusters collide and interact gravitationally. These collisions can lead to significant transformations in the structure and dynamics of the clusters, often resulting in the merging of galaxies, the heating of gas, and the formation of new cosmic structures, highlighting the role of such events in the large-scale structure of the universe.
Poor Clusters: Poor clusters are galaxy groups that contain a smaller number of galaxies compared to rich clusters, typically having fewer than 50 member galaxies. These clusters are often less densely populated and exhibit a lower overall mass, making them less prominent in the large-scale structure of the universe. While they may not be as massive or as densely packed as their richer counterparts, poor clusters still play an important role in understanding galaxy evolution and the distribution of dark matter.
Redshift: Redshift is the phenomenon where light from an object moving away from an observer is stretched to longer wavelengths, making it appear redder. This effect is crucial in understanding the universe's expansion and provides essential insights into the formation of galaxies, the evidence for the Big Bang, and the large-scale structure of the cosmos.
Rich clusters: Rich clusters are large groupings of galaxies that contain a high number of members, typically several hundred to several thousand. These clusters are significant in the study of galaxy formation and evolution as they represent regions of the universe where gravity has pulled together a substantial amount of mass, leading to dense environments that influence galaxy interactions and the cosmic web structure.
Vera Rubin: Vera Rubin was an influential American astronomer known for her pioneering work on the rotation curves of galaxies, which provided strong evidence for the existence of dark matter. Her groundbreaking observations revealed that galaxies rotate at such speeds that they would fly apart if only visible matter were present, highlighting the need for unseen mass to account for this phenomenon and shaping our understanding of dark matter's role in the universe.
Voids: Voids are large, nearly empty regions in the universe that exist between clusters of galaxies and other large-scale structures. These expansive spaces are critical to understanding the distribution of matter and energy in the cosmos, revealing a significant aspect of the universe's overall structure and evolution.
X-ray observations: X-ray observations involve detecting and analyzing high-energy electromagnetic radiation emitted by cosmic sources, providing crucial insights into the nature of astronomical objects. These observations help astronomers study extreme environments, such as those found in galaxy clusters and large-scale structures, revealing the presence of hot gas, dark matter, and the dynamics of galaxies.