One of the nearest rich regular clusters is situated two degrees from the north galactic pole in Coma Berenices . It is ideally situated for observation, and the effects of galactic obscuration may be ignored. The cluster extends over several degrees, its total radius being difficult to define. Galaxies are usually assigned to a cluster if their rednhift is similar to that of the core, or central region of the cluster. Over 800 redshifts are available for the Coma cluster, the mean implying a recession velocity of 6888 km s-1 and the spread, or velocity dispersion – assumed due to the intrinsic velocities of member galaxies with respect to each other – being characterized by a value of 861 kms-1. For a Hubble constant, Ho, of 75kms-1Mpc-1,this implies a distance of about 90Mpc, and one minute of arc is about 25 kpc at that distance.

The distribution of galaxies within the Coma cluster mimics that of stars in an elliptical galaxy, or stars in a globular cluster. This is what might be expected from a system of self-gravitating objects which has had sufficient time to interact and share its kinetic energy amongst its members. Such a distribution is characterized by a core, which is of about 240 kpc radius for the Coma cluster. Counts of galaxies have shown galaxies with similar redshifts to that of the Coma cluster out to 12° from the centre. It is not clear just how these apply to the cluster, if at all, for they may belong to less rich neighbouring clusters. The Coma cluster contains over 1000 bright members in the central regions, and this estimate may increase to 10000 if galaxies similar to the lesser members of our Local Group are included. The luminosity function of galaxies in Coma and other clusters has been determined, and shows that low brightness members do not dominate the total mass or brightness The average separation between brighter members is about 200 kpc in the central regions, and decreases within the core. Two super-giant galaxies, an elliptical NCC 4889 and an SO galaxy, NGC 4874 dominate this core. It is devoid of spiral galaxies, and spirals are found only in the more outer regions. The main core population consists of elliptical and SO galaxies. The lack of gas in these core galaxies has in the past been attributed to galaxy collisions. These are somewhat unlikely, however, and perhaps more probable is the following mechanism, known as RAM-PRESSURE STRIPPING. The presence of a hot intracluster gas is indicated by the observed X-ray emission, and galaxies ploughing through this will be stripped of gas. Several of the more massive elliptical galaxies are active galaxies, and explosive events within them may act to clear them of much of their gas. Studies of the redshifts of member galaxies at differing distances from the core show that rotation of the whole cluster is small.

The Coma cluster contains an extended radio source, which is presumably due to synchrotron radiation. The source of the relativistic electrons responsible is not known, although it is possible that some originate in the massive central galaxies. NGC 4874 itself exhibits a complex radio source, and NGC 4869, another galaxy in the core is an example of a ‘head-tail’ radio source. An extended X-ray source (0.5° extent), centred on the core of the cluster has been detected by Earth-orbiting satellites. X-ray emission is expected from electrons scattering the microwave background, and more probably from thermal bremsstrahlung radiation from hot intracluster gas (temperatures of about 108K). X-ray spectral studies, although not well advanced, support this hypothesis. We
do not expect anyway that the formation of galaxies is completely efficient, and some residual gas which did not participate in the creation of galaxies seems quite reasonable.

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