Galaxy clusters
Statistics have shown that the distribution of galaxies in space is far from the uniform "sea" first envisaged, with many (perhaps most) galaxies arrayed in groups, clusters with thousands of members, superclusters, and even larger sheets and fingers stretching as far across the Universe as we can reliably map. Clusters come in a variety of kinds, just as galaxies do. The richest and densest clusters are round assemblages, while sparser clusters have flattened or irregular shapes. The cluster environment is reflected in its galaxy content - dense environments like cluster cores are populated almost solely by elliptical and S0 galaxies, nearly devoid of gas and star formation. Less extreme environments can host, as well, spiral and irregular galaxies. This so-called morphology-density relation has engendered a classic heredity-environment question - were spiral galaxies never formed in those regions which would one day be rich clusters, or are they somehow destroyed or transformed in such clusters?
The jury is still out, though there is strong evidence that in some clusters spirals were once numerous and have been tranformed by external factors into elliptical or S0 systems. One such transforming mechanism is via galaxy mergers, which, while not common at the high speeds typical of cluster encounters today, might have been more common early on. A second transforming mechanism could be provided if clusters contain some kind of external medium - intergalactic gas. Such a medium was indeed discovered by early X-ray astronomy satellites, and is known to be ubiquitous in clusters and even galaxy groups. Random motions in the cluster heat this gas to temperatures of 10,000,000 standard units, making it visible only by its own X-ray emission. This gas typically has as much mass as do stars in the visible galaxies, and as galaxies move through it, will provide an external wind. This would in principle be strong enough to sweep gas out of a spiral galaxy, and a gas-free spiral will cease star formation and quickly look like an S0. Detailed observations in local clusters in fact show that spirals nearest the center seem to have lost the outermost parts of their gas distributions.
Detailed studies of the distance and redshifts of nearby galaxies have added another dynamic aspect to our understanding of clusters - they are still growing. At greater and greater distances, the gravity of a cluster takes longer to affect the motions of its surrounding galaxies, so that galaxies at larger distances will eventually turn around against the expansion of the Universe and fall into the cluster. Our own local group has a detectable motion toward the core of the Local Supercluster, and such large-scale motions can be found near many nearby clusters. In this sense, the cosmic epoch of cluster formation is now.
