Galaxy Clusters


Galaxienverteilung Galaxies are not smoothly distributed across the sky (right-hand image). There are small regions of high galaxy density and large areas encompassing only relatively few galaxies. The distributions of galaxies on the sky shows a net-like structure in which thin walls and filaments surround large voids. Galaxy clusters are the nodes of this network.

The right-hand image shows part of the galaxy distribution measured by the 2dF Galaxy Redshift Survey.

Coma-Haufen Individual galaxy clusters contain some hundred up to a thousand galaxies. The right-hand picture shows the Coma cluster. Almost all objects are galaxies, most of them elliptical. Spiral galaxies are rare in galaxy clusters. Typical diametres of galaxy clusters are several mega-parsec (1 Mpc = 3.3 million light years).

The galaxies move within galaxy clusters with velocities up to approximately 1000 km/s. At that speed they need several billion years to cross typical galaxy clusters.

Fritz Zwicky found already in the 1930s that the mass of all galaxies in the Coma cluster together is by far insufficient for binding the galaxies to each other. From that he concluded that some 90% of the matter in galaxy clusters does not shine and this thus dark matter. Galaxy clusters have typical masses between 1014 und 1015 solar masses.

Hot Gas

Coma-Röntgen The first X-ray telescopes discovered that galaxy clusters are intense X-ray emitters. The X-ray radiation is not concentrated on individual galaxies but diffusely distributed across the clusters. This radiation is emitted by hot gas filling the galaxy clusters. It has temperatures from 107 to 108 degrees Kelvin. The right-hand image shows the X-ray emission of the Coma cluster.

The X-ray emission is very important for determining the mass distribution and the dynamics of the galaxy clusters. Modern X-ray telescopes like Chandra and XMM have found detailed structures in the cores of galaxy clusters.

A2163-SZ The cosmic microwave background (CMB) shines through the hot gas in galaxy clusters. In passing the gas, the photons of the CMB are scattered to higher energies. This leads to a small distortion of the Planck spectrum of the CMB because after scattering photons at low energies are missing which reappear at higher energies. The zero point of this so-called Sunyaev-Zel'dovich effect is at 217 GHz (approximately 1 mm wave length). At lower frequencies galaxy clusters cast shadows, while they shine at higher frequencies. This effect allows the detection of galaxy clusters out to very large distances.

The left-hand image shows the X-ray emission of the galaxy cluster Abell 2163 (colour) with the contours of the Sunyaev-Zel'dovich effect overlaid.

Magnetic Fields

Faraday-Rotation Galaxy clusters are pervaded by large-scale magnetic fields. They become visible because a magnetised plasma is optically bi-refringent. When propagating parallel to the magnetic field, left- and right-polarised light experience slightly different indices of refraction. This gives rise to a rotation of the polarisation direction of linearly polarised light. This so-called Faraday rotation is measurable because the degree of the rotation depends on the wave length of the polarised light. Measurements of the Faraday rotation are difficult but allow the magnetic field structure in the interiour of galaxy clusters to be studied. The right-hand image shows the Faraday rotation measure in a simulated galaxy cluster (red and blue encode different senses of rotation).

Galaxy clusters form by merging of smaller objects. Such merger processes are highly energetic and often occur at supersonic velocities. Accordingly, they drive shock waves into the intracluster plasma which accelerate electrons to relativistic velocities. Those electrons spiral around the magnetic field lines and emit synchrotron radiation which is measurable as diffuse radio emission in galaxy clusters.

Gravitational Lensing

Due to their large mass, galaxy clusters deflect light. They act as gravitational lenses.
Verantwortlich: Matthias Bartelmann