Intergalactic Medium

Although intergalactic space is very close to a total vacuum, surrounding and stretching between galaxies there is a rarefied plasma that is thought to possess a filamentary structure and that is slightly denser than the average density in the Universe. This material is called the intergalactic medium (IGM) and is mostly ionized hydrogen, i.e. a plasma consisting of equal numbers of electrons and protons. The IGM is thought to exist at a density of 10 to 100 times the average density of the Universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the Universe in rich clusters of galaxies.

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Galaxies: Morphology-Density Relation Last Updated on 2008-03-18 00:00:00 The morphology-density relation describes how different types of galaxies tend to be arranged in clusters. In general, bulge-dominated early-type galaxies, Ellipticals and S0s, preferentially inhabit the central, densest areas of galaxy clusters. Meanwhile, disk-dominated late types tend to be scattered in the more sparsely populated regions of these clusters. This relation is valid for wide variations in shape and richness of clusters. However, this correlation of galaxy shape and the environment it inhabits changes as we look back in time, highlighting some important physical mechanisms at work in clusters and painting a richly dynamic tableau of galaxy evolution as a whole. In the early 1980s, it was established that the distribution of galaxy morphologies varied smoothly from the densest regions to the outskirts. In counting the numbers of elipticals, S0s, and spiral galaxies, one... More »
Intracluster Medium Last Updated on 2007-12-16 00:00:00 The intracluster medium (ICM) is a hot ionized gas which fills the space between the galaxies in Galaxy Clusters (See Figure 1). The ICM is composed of gas which fell into the cluster and gas which has been removed from cluster galaxies. The gas expelled from galaxies has been enriched by elements produced inside stars, and the resulting metalicity of the ICM is about 1/3 that of our sun. The gas is removed from the galaxies either by galaxy scale winds powered by supernovae or by stripping due to the pressure of the ICM as the galaxies move through the cluster. About 2/3 of the baryonic matter in a cluster of galaxies is in the hot gas in the ICM and the rest is in the galaxies. The density is in the range 10-3 to 10-4 particles/cm3, and the temperature is roughly tenmillion degrees. This is hot enough that the gas emits X-rays via thermal Bremstrahlung radiation. There is also line... More »