clusters of galaxies
clusters of galaxiesGroups of galaxies that may contain up to a few thousand members. The majority of galaxies appear to occur in clusters or in smaller groups such as doubles or triples. Our own Galaxy is a member of a small irregular cluster – the Local Group. The nearest large cluster is the Virgo cluster. The densest clusters, which typically contain a thousand or more members, are apparently roughly spherical and consist almost entirely of elliptical and S0 galaxies. Irregularly shaped clusters may be large, as with the Virgo cluster, or small, but tend to be less dense and to contain all types of galaxies. Adjacent clusters are loosely grouped into larger superclusters.
Large clusters with an unusually high concentration of galaxies in the center are called rich clusters. Several thousand are listed in the Abell Catalog, examples being the Coma cluster and Perseus cluster. The comparison of the cluster mass derived from dynamical studies of its constituent galaxies, and that inferred by summing the contribution of luminous objects in the system shows that 90% of the mass of the cluster must be in the form of dark matter. This is known as the missing mass. A rich cluster contains on average about a thousand galaxies, and has a total mass of 1015 solar masses within a radius of a megaparsec. X-ray observations show that clusters contain a large amount of hot (up to 108 K) gas; the mass of the gas is not, however, sufficient to explain the missing mass. In the irregularly shaped clusters the gas is associated with individual galaxies, but in the regular clusters it forms a large pool between the galaxies, the intracluster medium. This provides evidence that the regular clusters are more dynamically relaxed: their galaxies have interacted so often that their gases are stripped off and enrich the intracluster medium to about one-third solar metallicity.
The hot intracluster gas loses energy through the radiation of X-rays, and in the core of the cluster where the gas is most dense its cooling time is substantially less than the presumed age of the cluster. Within this cooling radius the cooling gas flows inward to maintain the pressure required to support the weight of the outer hot atmosphere, thus forming a cooling flow. Cooling flows are detected from X-ray spectra and images of clusters and occur in 70–90% of all clusters. As the gas cools from X-ray temperatures it separates from the flow, and typically several hundreds of solar masses a year are deposited. Only a small percentage of the cooled matter goes into detectable star formation; the bulk of it is thought to form very low mass stars and large clouds of neutral hydrogen. Over the lifetime of the cluster, the cooling flow will contribute a significant mass to the centrally located cD galaxy , the most massive type of galaxy known.
Clusters evolve rapidly through the hierarchical merging of small clusters to form today's rich clusters. Merging of two equal subclusters leads to systems such as the Coma cluster with two central dominant galaxies and no cooling flow. Cosmologically distant clusters of galaxies show a higher proportion of blue galaxies than do those at the present day, a feature known as the Butcher–Oemler effect, discovered by H. Butcher and A. Oemler in 1978. The galaxies are blue because of enhanced star formation, which is thought to be triggered either by ram pressure stripping by the intracluster medium or by galaxy harassment.
Measurements of the gas and galaxy content of clusters, coupled with limits on cosmic nucleosynthesis, suggest a low value of the cosmological density parameter, ω. This is consistent with the lack of strong evolution in the X-ray luminosity of clusters.