galaxy cluster

History

Astronomers noticed clusters as early as compiling catalogs of nebulae. In the 1930s, Fritz Zwicky measured the velocities of galaxies in the Coma cluster and found that the visible mass was insufficient to keep them from flying apart. Thus arose the hypothesis of dark matter—invisible substance.

How it works

Just as water droplets in a cloud coalesce into larger drops, galaxies attract each other and form clusters. Gravity holds them together, but there is not enough visible mass, so additional invisible mass—dark matter—is assumed to exist.

History

Astronomers noticed clusters as early as compiling catalogs of nebulae. In the 1930s, Fritz Zwicky measured the velocities of galaxies in the Coma cluster and found that the visible mass was insufficient to keep them from flying apart. Thus arose the hypothesis of dark matter—invisible substance.

How it works

Just as water droplets in a cloud coalesce into larger drops, galaxies attract each other and form clusters. Gravity holds them together, but there is not enough visible mass, so additional invisible mass—dark matter—is assumed to exist.

History

The Coma cluster was catalogued by Charles Messier in 1781 as a nebulous object. In 1933, Fritz Zwicky, applying the virial theorem to the velocity dispersion of galaxies in this cluster, showed that the dynamical mass far exceeds the visible mass and proposed the term 'dark matter'. In the 1970s, X-ray satellites (Uhuru, Einstein) discovered the hot intergalactic gas, which became an important tool for studying clusters. By the 1980s, it was clear that the baryonic component (stars and gas) is only a fraction of the total mass, with dark matter dominating. Vera Rubin earlier confirmed the need for dark matter on galactic scales.

How it works

The cluster is held together by gravity, described by Newton's law of universal gravitation and Einstein's general relativity. The velocity dispersion of galaxies (about 800 km/s) and the temperature of hot gas measure the depth of the gravitational potential: kinetic energy of gas particles ~ kT is comparable to potential, kT ~ μ m_p σ², where μ is the mean molecular mass, m_p the proton mass. The gas radiates via bremsstrahlung and spectral lines of highly ionized ions (e.g., iron Fe XXV–XXVI), allowing determination of temperature and chemical composition. Gravitational lensing of background galaxies provides an independent method to map the mass distribution. Cosmological simulations (Illustris, Millennium) within the cold dark matter model (ΛCDM) reproduce cluster formation.

Open problems & fun facts

The nature of dark matter; Physics of the intracluster medium: heating and cooling of gas, feedback mechanisms from active galactic nuclei; Accurate determination of mass and density profiles of dark matter; The cusp problem—discrepancy between observed and simulated central density profiles

💡 The mass of a cluster can be estimated from the distortion of images of distant galaxies—the gravitational lensing effect, akin to looking through an uneven glass.