Glossary

4.4 Galaxy clusters and gravitational lensing

9 min read•august 20, 2024

Galaxy clusters, the largest gravitationally bound structures in the universe, offer crucial insights into cosmic evolution and dark matter. These massive structures act as powerful gravitational lenses, bending light from distant objects and providing a unique tool for studying the cosmos.

Gravitational lensing by galaxy clusters allows astronomers to probe the distribution of matter, detect high-redshift galaxies, and constrain cosmological parameters. This phenomenon, a consequence of 's general relativity, manifests in various forms, from producing multiple images to causing subtle distortions in background galaxies.

Galaxy cluster properties

  • Galaxy clusters are the largest gravitationally bound structures in the universe, consisting of hundreds to thousands of galaxies
  • The properties of galaxy clusters provide valuable insights into the formation and evolution of large-scale structures and the nature of dark matter
  • Studying galaxy clusters helps constrain cosmological parameters and test theories of gravity

Richness of galaxy clusters

Top images from around the web for Richness of galaxy clusters

  • Virgo Cluster galaxies Archives - Universe Today View original

    Is this image relevant?

  • The Distribution of Galaxies in Space | Astronomy View original

    Is this image relevant?

  • Virgo Cluster galaxies Archives - Universe Today View original

    Is this image relevant?

1 of 3

Top images from around the web for Richness of galaxy clusters

  • Virgo Cluster galaxies Archives - Universe Today View original

    Is this image relevant?

  • The Distribution of Galaxies in Space | Astronomy View original

    Is this image relevant?

  • Virgo Cluster galaxies Archives - Universe Today View original

    Is this image relevant?

1 of 3
  • Richness refers to the number of galaxies within a cluster, typically measured within a certain radius from the cluster center
  • Clusters are classified as rich (more than 50 galaxies) or poor (fewer than 50 galaxies) based on their richness
  • The richness of a cluster correlates with its mass, with richer clusters being more massive
  • Examples of include the Coma Cluster and the Virgo Cluster

Morphological types in clusters

  • Galaxy clusters contain a mix of morphological types, including elliptical, spiral, and irregular galaxies
  • The morphological composition of clusters varies with cluster richness and distance from the cluster center
  • Rich clusters tend to have a higher fraction of , while have a higher fraction of
  • The morphology-density relation describes the increasing fraction of elliptical galaxies in denser environments (cluster cores)

Intracluster medium

  • The intracluster medium (ICM) is the hot, diffuse gas that fills the space between galaxies in a cluster
  • The ICM is composed mainly of ionized hydrogen and helium, with temperatures ranging from 10^7 to 10^8 K
  • The ICM emits X-rays due to thermal bremsstrahlung radiation, allowing clusters to be detected in X-ray surveys
  • The properties of the ICM, such as temperature and metallicity, provide information about the history of star formation and gas enrichment in clusters

Dark matter content

  • Galaxy clusters are dominated by dark matter, which makes up ~80-90% of their total mass
  • The presence of dark matter is inferred from the gravitational effects it has on the visible matter in clusters
  • The dark matter distribution in clusters can be studied using gravitational lensing and X-ray observations
  • The Bullet Cluster is a famous example of a cluster merger that provides strong evidence for the existence of dark matter

Gravitational lensing basics

  • Gravitational lensing is the bending of light by massive objects, such as galaxies and galaxy clusters
  • Lensing occurs when the light from a distant source passes near a massive foreground object, causing the light to be deflected and distorted
  • Gravitational lensing is a powerful tool for studying the distribution of matter in the universe, including both visible and dark matter

Deflection of light

  • In the presence of a massive object, light follows a curved path due to the distortion of spacetime
  • The deflection angle depends on the mass of the lens and the impact parameter (distance of closest approach)
  • For a point mass lens, the deflection angle is given by α=4GMc2b\alpha = \frac{4GM}{c^2b}, where MM is the mass of the lens, cc is the speed of light, and bb is the impact parameter

Einstein's general relativity

  • Gravitational lensing is a consequence of Einstein's theory of general relativity, which describes gravity as the curvature of spacetime
  • In general relativity, massive objects cause spacetime to curve, and light follows the straightest possible path (geodesic) in this curved spacetime
  • The amount of deflection predicted by general relativity is twice that predicted by Newtonian gravity

Gravitational potential wells

  • Massive objects create gravitational potential wells in spacetime, which act as lenses for light
  • The shape of the potential well determines the type and strength of the lensing effect
  • For a point mass lens, the potential well is spherically symmetric, resulting in a circular Einstein ring if the source, lens, and observer are perfectly aligned

Lens equation

  • The lens equation relates the positions of the source, lens, and image in a gravitational lensing system
  • For a thin lens, the lens equation is given by β=θ−DLSDSα\beta = \theta - \frac{D_{LS}}{D_S}\alpha, where β\beta is the angular position of the source, θ\theta is the angular position of the image, DLSD_{LS} is the distance between the lens and source, DSD_S is the distance to the source, and α\alpha is the deflection angle
  • The lens equation can be used to determine the magnification and distortion of

Types of gravitational lensing

  • Gravitational lensing can be classified into different types based on the strength of the lensing effect and the nature of the lens
  • The main types of gravitational lensing are strong lensing, weak lensing, and microlensing
  • Each type of lensing provides unique insights into the distribution of matter and the properties of the lens

Strong vs weak lensing

  • Strong lensing occurs when the lens is massive enough and the alignment is close enough to produce multiple images, arcs, or rings
  • Weak lensing refers to the subtle distortions of background galaxy shapes due to the gravitational influence of intervening matter
  • Strong lensing is rare and requires a precise alignment, while weak lensing is more common and can be studied statistically

Microlensing

  • Microlensing occurs when a compact object (star, planet, or black hole) passes in front of a background star, causing a temporary brightening of the background star
  • The duration and shape of the microlensing light curve depend on the mass and velocity of the lens
  • Microlensing is used to detect exoplanets and study the distribution of dark matter in the Milky Way (MACHOs)

Giant arcs and arclets

  • Giant arcs are highly elongated and distorted images of background galaxies that are strongly lensed by galaxy clusters
  • Arclets are smaller, less distorted images of background galaxies that are weakly lensed by clusters
  • The presence of giant arcs and arclets in a cluster indicates a high mass concentration and provides a way to map the mass distribution

Multiple images and time delays

  • Strong lensing can produce multiple images of the same background source, with each image following a different path through the lens
  • The multiple images can have different brightnesses and arrive at different times due to the different path lengths and gravitational time dilation
  • Measuring the time delays between multiple images can be used to determine the Hubble constant and study the mass distribution of the lens

Lensing by galaxy clusters

  • Galaxy clusters are the most massive gravitationally bound structures in the universe and are powerful gravitational lenses
  • Cluster lensing provides a unique opportunity to study the distribution of dark matter and test theories of gravity on large scales
  • Lensing by clusters can magnify and distort the images of background galaxies, allowing the study of high-redshift galaxies that would otherwise be too faint to observe

Cluster mass determination

  • Gravitational lensing is a direct probe of the total mass distribution in galaxy clusters, including both visible and dark matter
  • Strong lensing can be used to measure the mass within the Einstein radius, while weak lensing provides a measure of the mass distribution on larger scales
  • Combining lensing mass estimates with X-ray and galaxy velocity dispersion measurements provides a comprehensive view of cluster mass profiles

Magnification and distortion

  • Cluster lensing can magnify the apparent size and brightness of background galaxies, making them easier to detect and study
  • The magnification effect is strongest near the critical lines, where the lensing equation diverges
  • Weak lensing by clusters causes a coherent distortion of background galaxy shapes, known as shear, which can be measured statistically to map the mass distribution

Hubble constant from time delays

  • Measuring the time delays between multiple images of a strongly lensed quasar can provide an independent estimate of the Hubble constant
  • The time delay depends on the difference in path lengths and the gravitational potential of the lens, which in turn depends on the mass distribution and cosmological parameters
  • Time delay measurements from multiple cluster lenses can help resolve the tension between local and cosmological measurements of the Hubble constant

Probing dark matter distribution

  • Gravitational lensing by clusters is sensitive to the total mass distribution, making it a powerful probe of dark matter
  • The distribution of dark matter in clusters can be mapped using strong and weak lensing, revealing the presence of substructure and the concentration of the dark matter halo
  • Comparing the lensing mass distribution with the distribution of visible matter (galaxies and gas) provides insights into the nature of dark matter and its interaction with baryonic matter

Observational techniques

  • Studying gravitational lensing by galaxy clusters requires a combination of observational techniques across different wavelengths
  • Optical and near-infrared imaging is used to detect lensed galaxies and arcs, while radio observations can reveal lensed quasars and active galactic nuclei
  • High-resolution imaging from space-based telescopes and adaptive optics on ground-based telescopes is essential for resolving the details of lensed images

Optical vs radio observations

  • Optical observations are the primary means of detecting lensed galaxies and arcs in clusters
  • Deep optical imaging with telescopes like the Hubble Space Telescope can reveal faint lensed galaxies and the detailed structure of giant arcs
  • Radio observations are particularly useful for studying lensed quasars and active galactic nuclei, as they are not affected by dust obscuration

Hubble Space Telescope imaging

  • The Hubble Space Telescope (HST) has been instrumental in the study of cluster lensing due to its high resolution and sensitivity
  • HST imaging has revealed numerous examples of strongly lensed galaxies, giant arcs, and multiple image systems in clusters
  • The Frontier Fields program used HST to image six massive galaxy clusters, providing a deep view of the distant universe through the lensing effect

Adaptive optics and interferometry

  • Adaptive optics (AO) systems on ground-based telescopes can correct for atmospheric distortions, providing high-resolution imaging of lensed galaxies
  • AO imaging is particularly useful for studying the detailed structure of giant arcs and the properties of the lensed galaxies
  • Interferometric techniques, such as the Atacama Large Millimeter/submillimeter Array (ALMA), can provide high-resolution imaging of lensed galaxies in the submillimeter wavelengths

Lensing surveys and databases

  • Large-scale surveys, such as the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES), have identified numerous galaxy clusters and lensing systems
  • Dedicated lensing surveys, like the Cluster Lensing And Supernova survey with Hubble (CLASH) and the Reionization Lensing Cluster Survey (RELICS), have provided detailed observations of selected clusters
  • Online databases, such as the Masterlens project and the Hubble Source Catalog, provide access to data and images of known lensing systems

Applications and implications

  • Gravitational lensing by galaxy clusters has a wide range of applications in cosmology and astrophysics
  • Cluster lensing provides a powerful tool for constraining cosmological parameters, studying galaxy evolution, and testing theories of gravity
  • The magnification effect of cluster lensing allows the study of high-redshift galaxies that would otherwise be too faint to observe

Constraining cosmological parameters

  • The strength and frequency of cluster lensing depend on the geometry and expansion history of the universe, making it a probe of cosmological parameters
  • The number and distribution of giant arcs in clusters can be used to constrain the matter density and dark energy equation of state
  • Time delay measurements from strongly lensed quasars in clusters provide an independent estimate of the Hubble constant

High-redshift galaxy detection

  • Cluster lensing magnifies the apparent brightness of background galaxies, making it possible to detect and study galaxies at high redshifts (z > 6)
  • The magnification effect is particularly useful for studying the faint end of the galaxy luminosity function and the properties of the first galaxies in the early universe
  • Lensing clusters have been used as "cosmic telescopes" to discover some of the most distant galaxies known, such as the galaxy MACS1149-JD1 at z = 9.11

Studying galaxy evolution

  • Cluster lensing provides a means of studying the properties and evolution of galaxies over a wide range of redshifts
  • The magnification effect allows detailed studies of the morphology, star formation rates, and chemical composition of lensed galaxies
  • Comparing the properties of lensed galaxies at different redshifts can provide insights into the growth and evolution of galaxies over cosmic time

Testing modified gravity theories

  • Gravitational lensing is a direct consequence of the theory of general relativity, making it a powerful test of gravity on cosmological scales
  • The strength and distribution of cluster lensing can be used to test alternative theories of gravity, such as modified Newtonian dynamics (MOND) and scalar-tensor theories
  • Deviations from the predictions of general relativity in cluster lensing observations could provide evidence for new physics beyond the standard model

Key Terms to Review (18)

Abell: Abell refers to a catalog of galaxy clusters, compiled by George O. Abell in the 1950s, which includes over 4,000 entries. This catalog is significant for studying the large-scale structure of the universe, as it provides a systematic way to identify and classify galaxy clusters, which are crucial for understanding cosmic evolution and gravitational lensing phenomena.
Cold dark matter: Cold dark matter (CDM) is a hypothetical form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects. This type of dark matter is believed to clump together slowly, forming structures such as halos around galaxies, and plays a critical role in the formation and evolution of cosmic structures. It influences the behavior of galaxies and galaxy clusters, providing a framework for understanding the large-scale structure of the universe.
Cosmic Web: The cosmic web is the large-scale structure of the universe, characterized by a vast network of galaxies, clusters, and superclusters interconnected by filaments of dark matter and gas, along with vast voids in between. This intricate structure highlights the distribution of matter and energy in the universe and plays a crucial role in understanding its formation and evolution.
Einstein: Einstein refers to Albert Einstein, the theoretical physicist who developed the theory of relativity, fundamentally changing our understanding of space, time, and gravity. His work has profound implications in cosmology, especially regarding massive objects like galaxy clusters and the way light interacts with them through gravitational lensing, revealing the influence of gravity on light paths and supporting the existence of dark matter.
Einstein Rings: Einstein Rings are a fascinating astronomical phenomenon that occurs when the gravitational field of a massive object, like a galaxy cluster, acts as a lens, bending the light from a more distant source, such as a background galaxy. This bending can create a circular ring-like appearance around the foreground object if the alignment is just right. They provide powerful evidence for gravitational lensing and allow astronomers to study the distribution of dark matter and the geometry of the universe.
Elliptical Galaxies: Elliptical galaxies are a type of galaxy characterized by their smooth, featureless light profiles and an elliptical shape. They generally contain older stars, little to no gas or dust, and exhibit minimal star formation compared to spiral galaxies. Understanding their formation and evolution provides insights into the processes that govern galaxy development and structure.
Hierarchical formation: Hierarchical formation refers to the process in which structures, such as galaxies, develop through a series of merging events, creating larger systems from smaller components. This concept explains how smaller galaxies and star systems come together over time to form larger and more complex galaxies, revealing insights into the evolution of cosmic structures. It plays a crucial role in understanding the formation and evolution of different galaxy types, the interactions between galaxies, and the large-scale structure of the universe.
Lensed images: Lensed images are distorted or multiple images of astronomical objects created when their light passes through the gravitational field of a massive foreground object, such as a galaxy cluster. This phenomenon, known as gravitational lensing, occurs because the mass of the foreground object bends the path of light, causing the background objects to appear magnified, elongated, or even duplicated. Lensed images provide valuable information about both the foreground and background objects, allowing astronomers to study dark matter and the distribution of galaxies.
Poor Clusters: Poor clusters are galaxy clusters that contain a relatively small number of galaxies, typically fewer than 50, and have lower overall mass compared to rich clusters. These clusters are often characterized by a more diffuse distribution of galaxies and a less prominent central galaxy, resulting in less gravitational binding and interaction between the member galaxies. Their significance lies in understanding the formation and evolution of galaxies within different environments.
Redshift surveys: Redshift surveys are astronomical studies that measure the redshift of light from galaxies to determine their distance and velocity relative to Earth. By analyzing how light stretches to longer wavelengths as galaxies move away, these surveys help map the large-scale structure of the universe, providing insights into galaxy clusters, voids, baryon acoustic oscillations, and the overall cosmic web.
Rich clusters: Rich clusters are large groups of galaxies that contain a high number of members, typically over a hundred, bound together by gravity. These clusters are important as they can provide insights into the formation and evolution of galaxies and the large-scale structure of the universe, as well as how they interact with their environment through gravitational lensing effects.
Spiral Galaxies: Spiral galaxies are a type of galaxy characterized by their distinct spiral arms that radiate from a central bulge, containing stars, gas, and dust. These galaxies are significant because their structure and dynamics provide insights into stellar formation, galactic evolution, and the gravitational forces at play within the universe.
Strong lensing: Strong lensing occurs when a massive object, like a galaxy or galaxy cluster, significantly distorts and magnifies the light from a background object due to its gravitational field. This effect can produce multiple images, arcs, or even rings of the background source, allowing astronomers to study both the foreground mass and the distant background objects in greater detail. Understanding strong lensing is crucial for mapping dark matter and gaining insights into the universe's structure.
Structure Formation Theory: Structure formation theory is a framework in cosmology that describes how matter in the universe, primarily dark matter and baryonic matter, has evolved over time to form the large-scale structures we observe today, such as galaxies, clusters of galaxies, and cosmic filaments. This theory helps us understand the growth of these structures under the influence of gravitational forces, leading to the current arrangement of matter across the universe.
Voids: Voids are vast, empty spaces in the universe where very few galaxies and matter exist, creating a striking contrast to the denser regions filled with galaxy clusters and filaments. These large-scale structures play a crucial role in the overall distribution of matter in the cosmos and are key to understanding cosmic evolution. The presence of voids is significant when examining gravitational lensing, large-scale structure surveys, and cosmological parameters, as they help astronomers infer the distribution of dark matter and the expansion of the universe.
Weak lensing: Weak lensing is a phenomenon where the gravitational field of a massive object, such as a galaxy cluster, subtly distorts the shapes of distant background galaxies. This distortion is caused by the bending of light as it travels through the gravitational field, allowing astronomers to study the distribution of dark matter and the large-scale structure of the universe. It provides crucial insights into cosmic structures and their formation, linking well with gravitational lensing concepts and techniques.
X-ray astronomy: X-ray astronomy is the study of astronomical objects that emit X-rays, a type of high-energy radiation. It enables scientists to observe and understand extreme cosmic phenomena such as supernovae, black holes, and neutron stars by analyzing their X-ray emissions. This field of astronomy plays a crucial role in revealing the high-energy processes and interactions occurring in the universe, particularly in environments where gravity is strong and temperatures are extremely high.
λcdm model: The λcdm model, or Lambda Cold Dark Matter model, is the prevailing cosmological model that describes the large-scale structure and evolution of the Universe. It incorporates dark energy (represented by Lambda, λ) which is responsible for the accelerated expansion of the Universe, along with cold dark matter (cdm) that explains the formation and clustering of galaxies and cosmic structures over time.
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Back
Practice QuizGlossary
Practice QuizGlossary
Next guide