ESA Science & Technology - Clusters of galaxies: probing their formation

The most widely accepted picture of cosmic evolution is based on a model of structure formation where small structures form first and these then build the larger structures. Galaxy clusters are the largest and most recently formed objects in the known Universe. As such they are key elements in the structure formation process and important probes for testing cosmological models. Observations of galaxy clusters with X-ray satellites have revealed large amounts of very hot gas trapped between the galaxies. A record of how this gas has been heated and cooled over a long period of time is stored in the X-ray emission; this provides crucial information about how the galaxy cluster formed.

In these XMM-Newton images of three galaxy clusters the colours provide an indication of the temperature of the internal gas of the clusters, and the brightness is a measure of the gas density. From the temperature and density distribution, the physically very important parameters of pressure and entropy can be derived. Entropy is a measure of the heating and cooling history of a physical system. These three images illustrate the use of entropy distribution in the X-ray luminous gas as a way of identifying various physical processes. Entropy has the unique property of decreasing with radiative cooling, increasing due to heating processes, but staying constant with compression or expansion under energy conservation. The latter ensures that a ‘fossil record’ of any heating or cooling is kept even if the gas subsequently changes its pressure adiabatically (under energy conservation).