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It has long been known that in the local universe the mix of morphological types differs in different galactic environments with ellipticals and S0's dominating in the densest clusters and spirals dominating the field population (Hubble and Humason 1931). This so called density-morphology relation has been quantified by Oemler (1974) and Dressler (1980) and is found to extend over five orders of magnitude in space density (Postman and Geller 1984). Whether this relation arises at formation (nature) or is caused by density driven evolutionary effects (nurture) remains a matter of debate. More recent studies of clusters of galaxies at intermediate redshifts show that both the morphological mix and the star formation rate strongly evolve with redshift (Poggianti et al. 1999; Dressler et al. 1997; Fasano et al. 2000). In particular the fraction of S0's goes down and the spiral fraction and star formation rate go up with increasing redshift. There are many physical mechanisms at work in clusters or during the growth of clusters that could affect the star formation rate and possibly transform spiral galaxies into S0's. In this review I will limit myself to the role that the hot intracluster medium (ICM) may play.

The first suggestion that an interaction between the ICM and disk galaxies may affect the evolution of these galaxies was made immediately after the first detection of an ICM in clusters (Gursky et al. 1971). In a seminal paper on "the infall of matter into clusters" Gunn and Gott (1972) discuss what might happen if there is any intergalactic gas left after the clusters has collapsed. The interstellar material in a galaxy would feel the ram pressure of the intracluster medium as it moves through the cluster. A simple estimate of the effect assumes that the outer disk gas gets stripped off when the local restoring force in the disk is smaller than the ram pressure. Thus disks gets stripped up to the so called stripping radius where the forces balance. They estimate that for a galaxy moving at the typical velocity of 1700 km/s through the Coma cluster the ISM would be stripped in one pass. This would explain why so few normal spirals are seen in nearby clusters. In particular it would explain the existence of so many gas poor, non star forming disk galaxies first noticed by Spitzer and Baade (1951) and later dubbed anemics by van den Bergh (1976).

Ram pressure stripping is but one way in which the ICM may affect the ISM. The effects of viscosity, thermal conduction and turbulence on the flow of hot gas past a galaxy were considered by Nulsen (1982), who concluded that turbulent viscous stripping will be an important mechanism for gas loss from cluster galaxies. While the above mentioned mechanisms would work to remove gas from galaxies and thus slow down their evolution, an alternative possiblity is that an interaction with the ICM compresses the ISM and leads to ram pressure induced star formation (Dressler and Gunn, 1983; Gavazzi et al. 1995).

On the observational side there has long been evidence that spiral galaxies in clusters have less neutral atomic hydrogen than galaxies of the same morphological type in the field (for a review see Haynes, Giovanelli and Chincarini 1984). The CO content however does not seem to depend on environment (Stark et al. 1986; Kenney and Young 1989). Both single dish observations and synthesis imaging results of the Virgo cluster show that the HI disks of galaxies in projection close to the cluster center are much smaller than the H I disks of galaxies in the outer parts (Giovanelli and Haynes, 1983; Warmels 1988a, b, c; Cayatte et al. 1990, 1994). All of these phenomena could easily be interpreted in terms of ram pressure stripping. Dressler (1986) made this even more plausible by pointing out that the gas deficient galaxies seem statistically to be mostly on radial orbits which would carry them into the dense environment of the cluster core. However nature turned out to be more complicated than that. In a comprehensive analysis of HI data on six nearby clusters Magri et al. (1988) conclude that the data can not be used to distinguish between inbred and evolutionary gas deficiency mechanisms or among different environmental effects. Although H I deficiency varies with projected radius from the cluster center, with the most H I poor objects close to the cluster centers, no correlation is found between deficiency and (relative radial velocity)2, as would be expected from ram pressure stripping.

In more recent years a number of developments have taken place. First there was a flurry of activity on the theoretical front, for the first time detailed numerical simulations on the effects of ram pressure stripping appeared. Since then both improved statistics on HI deficiency and detailed multiwavelengths observations of cluster galaxies undergoing trauma appeared. More recently detailed comparisons have been made between individual systems and numerical simulations. Finally synthesis imaging of neutral hydrogen no longer needs to be limited to a few selected systems in nearby clusters and results of volume limited surveys of entire clusters at redshifts between 0 and 0.2 have started to appear in the literature. In this review I will first discuss what we have learned about the statistical properties of the H I content of cluster galaxies. Then I will review some of the recent numerical work that has been done and compare these with observational results. After that I will discuss what we have learned from imaging surveys, and in conclusion I will discuss the importance of the ICM interaction for galaxy evolution.

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