11.3 Galaxy clusters and large-scale structure

Galaxy clusters are massive gatherings of galaxies, hot gas, and dark matter. They're like cosmic cities, teeming with activity and holding clues about the universe's structure and evolution.

These clusters play a crucial role in understanding the cosmos. Scientists use various methods to detect and study them, revealing insights into dark matter, cosmic expansion, and the formation of large-scale structures in the universe.

Galaxy Clusters and Large-Scale Structure

Properties of galaxy clusters

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• Galaxy clusters form gravitationally bound groups containing hundreds to thousands of galaxies spanning several megaparsecs with masses ranging from $10^{14}$ to $10^{15}$ solar masses

• Intracluster medium (ICM) consists of hot diffuse gas at $10^7$ to $10^8$ K primarily composed of ionized hydrogen and helium emitting X-rays through thermal bremsstrahlung

• Dark matter dominates cluster mass accounting for 80-85% of total mass inferred through gravitational effects on visible matter extending beyond visible cluster

• Galaxy population predominantly includes elliptical and lenticular galaxies often with a massive central elliptical galaxy (cD galaxy) undergoing frequent interactions and mergers

Detection methods for clusters

• X-ray observations detect thermal emission from hot ICM providing gas temperature and density data allowing mass estimation through hydrostatic equilibrium (Chandra, XMM-Newton telescopes)

• Optical observations identify clusters through galaxy overdensities using red sequence method to detect similarly colored galaxy concentrations and spectroscopic measurements for redshifts and velocity dispersions

• Gravitational lensing analyzes strong lensing producing highly distorted background galaxy images and weak lensing examining slight distortions in galaxy shapes providing direct total mass distribution measurement

• Sunyaev-Zel'dovich (SZ) effect detects distortion of cosmic microwave background radiation by hot ICM allowing high-redshift cluster detection independent of distance

Cosmic web and large-scale structure

• Cosmic web describes large-scale matter distribution in the universe consisting of filaments walls and voids resulting from gravitational clustering over cosmic time

• Filaments form thread-like structures of dark matter and galaxies connecting clusters and superclusters extending over hundreds of megaparsecs

• Walls or sheets create flattened planar structures in the cosmic web formed by intersecting filaments

• Voids represent large underdense regions between filaments and walls typically 10-100 megaparsecs in diameter containing few or no galaxies

• Structure formation driven by gravitational instability begins with small density fluctuations in early universe following hierarchical clustering where smaller structures form first then merge into larger ones

Clusters as cosmological probes

• Galaxy clusters trace structure growth over cosmic time sensitive to universe expansion history

• Cluster dynamics and gravitational lensing provide evidence for dark matter and test alternative gravity theories

• Evolution of cluster abundance constrains dark energy while SZ effect surveys detect high-redshift clusters

• Observed baryon fraction in clusters constrains cosmological parameters

• Cluster mass function evolution tests $\Lambda$CDM model while merger rates probe hierarchical structure formation

• SZ effect distorts CMB spectrum providing independent measure of cosmic expansion

• Combining SZ effect and X-ray observations offers Hubble constant measurement independent of cosmic distance ladder