We can now begin to answer the question posed in the introduction: are interactions with the ICM important for the evolution of disk galaxies. The answer is a definite yes for disk galaxies in cluster environments. We see individual galaxies that show all the signs of an ongoing interaction, signs that are predicted by detailed hydrodynamical simulations. We see trends in galaxy properties, such as the truncated gaseous disks in the center of Virgo and Coma, and truncated starformation disks in Virgo (Koopmann and Kenney 2002) that can be reproduced in simulations of ICM interactions with the cold ISM in disk galaxies. These interactions should produce a population of early type non star-forming disk galaxies with a range of bulge to disk ratios, as was predicted by Dressler and Gunn (1983) and as has been found in Virgo (Koopmann and Kenney 2002). However other effects play a role too in clusters. Galaxies are found that experience both stripping and gravitational interactions (Koopmann and Kenney 2002; Vollmer 2003) and examples of gravitationally induced star formation have also been found (Koopmann and Kenney 2002; Rose et al. 2001; Sakai et al. 2002). However the dominant environmental effect on cluster disk galaxies is a reduction of the star formation rate, which goes hand in hand with hydrogen deficiency, and for most galaxies this is due to ram pressure stripping (Koopmann and Kenney 2002).
On larger scales
Solanes et al 2001
found evidence that the H I deficiency
goes out as far as 2 RA. Although this is surprisingly far, the
fact that the deficient galaxies at large distances from the cluster
tend to be on radial orbits, makes it plausible that the cause of the
deficiency is ram pressure stripping as well. The extent of the H I
deficiency fits in nicely with the results of
Balogh et al. (1998),
who find that the star formation rate as measured by the [O II]
equivalent width is depressed in clusters out to two R200
(
2 RA) and
more recently the analyses of the 2dF and the SDSS survey results
(Lewis et al. 2002;
Gomez et al. 2003;
Nichol, this conference), which indicate
that star formation rates begin to drop between one and two virial radii.
These results are somewhat at odds with the
H I imaging results where evidence is found that the groups in the
outskirts of clusters are very gas rich. Many examples of ongoing
interactions are found in these locations. In a simple scenario the
interactions would bring gas to large distances from the galaxies,
which could then easily be stripped as the galaxies fall into the denser
ICM.
It is an intriguing possibility that the impact and reach of the ICM is closely related to the dynamical state of the cluster. Cluster-(sub)cluster merging can give rise to bulk motions, shocks and temperature structure within the ICM. In merging clusters observational evidence has been found for large velocities in the ICM (Dupke and Bregman 2001), enhanced star formation (Miller and Owen 2003; Miller, this conference) and distortion of radio sources by the ICM motions (Bliton et al 1998). If stripping would mostly depend on the motions of the ICM and the dynamical state of the cluster, it would more easily explain why the effects are seen far out into the infall region. The radial orbits measured for the more distant H I deficient galaxies would then reflect the infall direction of the most recent accretion event in the cluster.
Acknowledgments. I am grateful to the organizers of this conference for inviting me to give this review. I thank Jose Solanes, Marc Verheijen, Hector Bravo-Alfaro, Dwarakanath, Jeff Kenney, Bianca Poggianti, Raja Guhathakurta, Ann Zabludoff, David Schiminovich, Monica Valluri, Eric Wilcots and Bernd Vollmer for help with figures and many discussions on these topics. I thank the Kapteyn Institute, where part of this work was done, for their hospitality. This research was supported by an NSF grant to Columbia University and a NWO bezoekers beurs to Groningen.