Since gravity is the dominant force in the formation of galaxies and galaxy systems, the composition by mass of these objects is of great importance in understanding their behaviour. There are three main contributors to the mass of groups and clusters of galaxies, namely dark matter (80-85%); hot,  X-ray emitting gas (10-15%); and stars (2-5%).  The dominance of the dark matter component ensures that the properties of these systems are largely driven by gravity. Gravity is a scale-free force, and therefore the behaviour of a dark matter dominated halo is determined mostly by its mass. This implies that galaxy clusters of different masses resemble simple scaled versions of each other.  As a result, observable parameters such as the luminosity or gas temperature of these objects are expected to follow very predictable relationships, because these quantities reflect the depth of the cluster gravitational potential well (i.e. the total mass).

The following images show the X-rays emitted by the hot gas, as well as the corresponding optical view of the galaxies embedded in this gas. The first case is a large cluster of galaxies, and the panels below that show two typical galaxy groups. Despite being roughly 100 times less massive than the cluster, the groups of galaxies resemble a smaller-scaled version of it.

X-ray (blue colour table) and optical images of 2 galaxy groups. (Credit: NASA)

Numerical simulations of clusters show that the dark matter component behaves in a self-similar way, aside from slight variations between halos formed at different epochs, when the mean density of the Universe was different. However, most observations of real clusters don't generally measure the mass directly, but instead study the radiation emitted by luminous material which is confined by gravity, thereby tracing the mass.  Unlike dark matter, however, luminous matter (often referred to as "baryons") is subject to a range of complex physical processes, that alter the extent to which it tracks gravity. For this reason the scaling properties of real clusters are different from simple predictions. For example, the X-ray luminosities of poor clusters and groups are known to be significantly lower than expected, and such systems are found to contain proportionately much less hot gas in their inner regions than massive clusters.  Such behaviour points to the influence of physical processes originating within galaxies that can interact with and modify the hot gas (also known as the intracluster medium, or ICM) within the cluster or group. Examples of such feedback interactions include outbursts from active galactic nuclei (AGN) or supernove-driven galaxy winds, which can both heat and displace the ICM.

Since the energy injected by feedback mechanisms does not depend on the mass of a cluster, the impact it has will vary for different clusters. For instance, a large AGN outburst will have a very big effect on the ICM in a small group, where it is capable of displacing large amounts of material, effectively blowing the gas right out of the inner regions and dramatically reducing the amount of remaining gas. However, the same outburst occuring in a large cluster would have a much lesser impact, resulting in a smaller fractional decrease in the mass (and hence luminosity) of the gas near the AGN. This differential behaviour is revealed when the properties of clusters and groups of different masses are compared, across a large number of objects. Consequently the scaling properties of galaxy systems can be used to shed light on the nature of feedback mechanisms and their implications for galaxy formation and evolution.

Early analysis of cluster scaling relations was limited to simple global properties, such as the total X-ray luminosity of the hot gas and its mean temperature (expressed in units of keV). However, with better X-ray telescopes, it has been possible to study the ICM in more detail across a large number of systems.  The following two graphs show how the hot gas varies in temperature and density for a sample of 20 clusters observed with the Chandra X-ray telescope.  The temperatures range from massive clusters (blue), through small clusters (green) and down to groups of galaxies (red), with half the sample showing signs of strong cooling in their cores. The gas density varies considerably compared to the case of self-similarity, where the profiles would appear stacked on top of each other. Clusters with cool cores have denser gas than those without, and in general the density of the gas decreases for smaller (and hence cooler) clusters. This behaviour can be explained by the actions of galaxy feedback processess,  which have energised the gas and caused it to puff up more within groups, due to their weaker gravity.

     
        Gas temperature and density profiles for a sample of 20 clusters, colour-coded according to their mean gas temperature.

The area of cluster scaling relations is a key subject of ongoing research at Birmingham, and we are currently analysing a sample of galaxy clusters and groups observed with the Chandra and XMM-Newton X-ray telescopes, which provide a much more detailed view of the intracluster medium. This study will provide new insights into the close mutual interaction between galaxies and the intracluster medium surrounding them, which will lead to a better understanding of the formation and evolution of structure in the Universe.

Researchers:  Alastair Sanderson, Trevor Ponman, Rowan Temple, Paul Russell