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 emission due to the heavy elements in the gas.

Cooling Flows

In the central parts of the cluster, the gas is able to radiate enough of its energy that it should cool and drop out of the hot phase at rates ofabout 10-100 times the mass of our sun per year.It appears that much of the energy radiated by the gas is replaced by some form of energy input into the gas. Currently it is thought that the energy input is due to Radio Galaxies produced by the massive central galaxy.
(See Figure 2).

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This optical and X-ray composite image shows Abell 2029, one of 26 galaxy clusters studied by Chandra, located one billion light years away. Visit "Chandra Discovery Sheds Light on Dark Energy" for full size version. Credit: Optical: National Optical Astronomy Observatory/Kitt Peak, X-ray: NASA/Chandra X-ray Center/IoA.

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 Local Universe (z ~ 0)

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 can expect to find that each of these galaxies respectively comprises roughly 50%, 40%, and 10% of the total population in the innermost part of the clusters. Outside the cluster, ellipticals and S0s each account for 10% of the number of galaxies, while spirals are 80% of this low density environment population. Again, we see this trend regardless of the differences in individual cluster richness.

At Intermediate Redshift (z ~ 0.5-1)

Despite the powerful morphological segregation we observe locally, this relationship does not hold as a function of redshift. While a similar morphology-density relation is present in centrally concentrated, regular clusters at z ~ 0.5 (about 5 billion years in the past), it is nearly absent in more irregular clusters. More importantly, there is a fundamental discrepancy between the overall number of S0s at this distance and the number we observe locally. There appears to be twice as many of these galaxies in clusters today as there were at z ~ 0.5, whose scarcity is accompanied by a proportionate increase in the spiral population. This situation is more exaggerated as we look back to z ~ 1 (8-9 billion years ago), where it becomes difficult to distinguish galaxies by appearance.

Possible Explanations

Clearly, a large-scale transformation of spiral galaxies to S0s must take place between z~1 and z~0, or from around 8-9 billion years ago to the present. The increasing prominence of gas-poor, dynamically relaxed early-types in cluster cores with time is likely a product of external agents that discourage the maintenance of delicate late-type spiral structure. These include full blown galaxy merging, high speed, ephemeral encounters between galaxies (“galaxy harassment”), ram pressure stripping of interstellar gas by the hot, ionized intracluster medium (ICM), and the gravitational stresses of the cluster environment itself. As a whole, these factors favor the eventual extinction of star formation and the appearances characteristic of early-type galaxies. However, the relative importance of all these processes is still uncertain, and individual scenarios admittedly provide some contradictions to observations, as seen below.

Galaxy mergers

While it is possible that two merging spirals can ultimately produce an elliptical galaxy, galaxies generally move too fast in clusters for them to “stick” efficiently. Still, this process may play a small role in producing a slight increase of elliptical galaxies over time.

Galaxy Harassment

These relatively minor events are more common than galaxy mergers; any particular galaxy in a sufficiently dense environment can be expected to interact with about 5 others in a period of a billion years. Minor interactions such as these are heavily dependent on a galaxy’s environment- galaxies in sparsely populated cluster outskirts do not enjoy the frequency of encounters that denser cluster interiors would. Unfortunately, while this accounts for the morphology-density relation in individual clusters, it does not explain why the relation exists for smaller, less dense clusters where harassment would not be important.

Ram Pressure Stripping

Like harassment, the potential strength of ram pressure stripping scales with cluster density- higher density environments have more ICM, and thus could strip away more of a galaxy’s gas and give it the subdued star formation rate of an S0. While this mechanism is an appealing solution, it cannot account for the actual morphological transformation of a spiral to an S0, only the quenching of its star formation. There are also problems with the long timescales required for this process. And like harassment, ram pressure stripping fails to justify the morphology-density relation across all densities of clusters.

Isolated Evolution

The efficacy of harassment and ram pressure stripping cannot be completely discounted in all cases. But many recent studies indicate that tidal interactions between individual galaxies and interactions between galaxies and the cluster’s gravity are dominant players in spiral-S0 transformations. In this case, galaxies naturally evolve on their own, from late to early type on the Hubble tuning fork, no matter where they are. Their proximity to the cluster center, with concomitant severity of ram pressure stripping and harassment, helps determine how rapidly this occurs.

Conclusion

It is worthwhile to note that all these trends listed are most pronounced for the most luminous galaxies observed. At faint magnitudes, the morphology-density relation is actually weaker than has been described. Since luminous and massive early-type galaxies are expected to dominate the highest density regions much more than smaller galaxies, this is to be expected.

Ultimately, the issue of equivalent population ratios for all types of local clusters, as well as the fundamental disk/bulge ratios of spirals and S0s, and how these galaxy components are transformed, remains unresolved. Further work remains in elucidating this problem.

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The "Ghost of Mirach" galaxy is shown in ultraviolet as seen by NASA's Galaxy Evolution Explorer. The Ghost of Mirach—a galaxy called NGC 404—seen as the whitish spot in the center of the images. Mirach is a red giant star that looms large in visible light. Because NGC 404 is lost in the glare of this star, it was nicknamed the Ghost of Mirach. But when the galaxy is viewed in ultraviolet light, it comes to "life," revealing a never-before-seen ring. This ring, seen in blue, contains new stars—a surprise considering that the galaxy was previously thought to be, essentially, dead. The field of view spans 55,000 light years across. The Ghost of Mirach is located 11 million light-years from Earth. The star Mirach is very close in comparison—it is only 200 light-years away and is visible with the naked eye. (Image Credit: NASA/JPL-Caltech/DSS.)