Rich cluster of galaxies have central masses of 10,000 × the mass of the Milky Way, and as such are both the most massive collapsed systems and also some of the intrinsically rarest systems in the Universe. Most galaxies inhabit the field, crudely defined as the lower-density regions outside clusters and voids. Although these field galaxies will themselves lie in structures, typically in groups of a few galaxies. The Local Group comprises our Galaxy and Andromeda (M31), as well as a number of smaller galaxies, in a single gravitationally-bound structure. Nevertheless, the Local Group resides on the edge of a moderate-sized cluster in Virgo, and the group is in the process of falling into this cluster. Within the next few billion years the surroundings of our Galaxy will look very different. The most massive nearby cluster is the Coma cluster (left) the central regions of which are dominated by two super-massive elliptical galaxies. The extreme conditions found in such rich clusters including very high densities of galaxies, as well as large amounts of very hot gas which emit at X-ray wavelengths, making clusters some of the most luminous X-ray sources in the sky (see below). In addition to galaxies and hot gas, galaxy clusters contain populations of stars which are not bound to individual galaxies, these have been stripped out of galaxies in the cluster by tidal interactions. As we will see below the vastly different conditions found in the centres of clusters, compared to the lower density field, are associated with differences in the properties of the galaxies in the two environments and in particular with differences in their morphologies.
The image here shows a view of the core of the Virgo cluster in the X-ray waveband and illustrates the structure of the hot, gravitationally-bound gas in the cluster's potential well. This potential well is sufficiently deep that the gas between the galaxies within the cluster is compressed and heated to high enough temperature that emits radiation at X-ray wavelengths. Images of this X-ray radiation illustrate the very extended potential well of the cluster, which contains two major peaks, associated with sub-groups of galaxies within the cluster. Nevertheless, the X-ray image still appears much smoother than the distribution of the individual galaxies. Moreover, the temperature and the distribution of the X-ray gas can be used to estimate the mass of the cluster (assuming that the hot X-ray gas behaves as an ideal gas) and in all cases this has been found to significantly exceed the mass contained within the galaxies. These observations are one of the strongest pieces of evidence for dark matter on large scales in the Universe. Observing at X-ray wavelengths requires the use satellites to get above the absorption from the Earth's atmosphere, which is opaque in the X-ray band.
The image to the left illustrates a view of the central regions, a 1 Mpc across, of a distant rich cluster. This is a true colour image, constructed from individual exposures through blue (B), visual (V) and infrared (I) filters, and the large numbers of yellow galaxies are the luminous cluster galaxies (the large interacting spiral in the upper-right hand corner is foreground of the cluster). The strong central concentration of galaxies in the cluster can be readily seen, and the cluster centre is further highlighted by the massive dominant galaxy lying at the bottom of the cluster's potential well. This massive elliptical galaxy has accreted an extended envelope of stars stripped from other galaxies passing through the cluster core. The central parts of the cluster are sufficiently massive to bend the space-time, this deflects the paths of photons passing through the core of the cluster in a phenomenum called `gravitational-lensing'. The deflection means that the images of background galaxies seen through the cluster appear to be distorted into concentric arcs around the cluster. A number of gravitationally-lensed arcs are visible around the central galaxy in this cluster, these are star-forming galaxies at high redshift and are therefore blue, appearing conspicious against the red, central cluster galaxy. Arcs are also visible around a secondary sub-clump within the cluster below the central galaxy.
Clusters of galaxies are of particular interest for testing models of galaxy evolution in high density regions. The possibility that galaxy properties, such as luminosity or morphology, can be altered by interactions within rich clusters has led to considerable theoretical work on predicting the expected effects of these interactions on model galaxies. The figure here shows such a simulation, where the motions and interactions of galaxies have been followed through the formation and collapse of a rich cluster. The density of the mass in the cluster is colour coded with the densest regions having red and yellow colours, and less dense regions blue and black. The simulation, although it simply follows Newton's laws, relies on highly sophisticated code running on the most powerful, parrallel supercomputers available. Direct comparisons of the observations and simulations of rich clusters of galaxies allow astronomers to test and refine their models of the growth of structure in the Universe.
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