In the early seventies several papers appeared considering the
effect of the ICM on galaxies in clusters, e.g. ram pressure stripping
(Gunn and Gott 1972),
evaporation
(Cowie and Songaila
1977)
and turbulent viscosity and evaporation
(Nulsen 1982).
Not much work was done though to connect theory with observations.
The first paper that specifically looks at observational
characteristics is the paper by
Stevens, Acreman and
Ponman (1999).
This paper focusses on the impact of the ICM on an elliptical galaxy
with a hot ISM and calculates the observational signatures of this
in the hot ICM, predicting bow-shocks, wakes and tails.
Some, but still precious few, examples of structures that could be
interpreted like this exist in the X-ray literature
(Stevens et al. 1999).
Observationally, there is much more evidence for the impact
of the ICM on the cool ISM of disk galaxies. From a view point of
galaxy evolution this is also the more important question. In clusters
star formation rates are known to evolve rapidly between intermediate
redshifts and the local universe
(Poggianti et al 1999,
Balogh et al 1999).
A mechanism is required to bring star formation almost completely
to a halt and a major issue is whether ram pressure stripping could
do this to disk galaxies. The original analytical estimates of
Gunn and Gott (1972)
predict that gas gets stripped from a galaxy up to
a stripping radius within which the restoring force from the disk
exceeds the ram pressure. The first numerical simulations
(Abadi, Moore and Bower
(1999))
using a 3 dimensional SPH/N-body
simulation to study ram pressure stripping of gas from spiral galaxies
orbiting in clusters confirm that gas
in disk galaxies gets stripped up to the stripping radius estimated by
Gunn and Gott (1972).
At small radii the potential provided by the
bulge component contributes considerably. They estimate that a galaxy
passing through the center of Coma would have its gaseous
disk truncated to 
4
kpc, losing about 80% of its gas.
However the process is in general not efficient enough to account for the
rapid and widespread truncation of star formation observed in cluster
galaxies.
Quilis, Moore and Bower
(2000)
use a finite difference code
to achieve higher resolution in order to be able to include complex
turbulent and viscous stripping at the interface of cold and hot
gaseous components as well as the formation of bow shocks in the ICM ahead
of the galaxy. From only a few selected runs on galaxies with holes
in the central gas distribution they reverse the conclusion of
Abadi et al. (1999)
and state that ICM - ISM interaction could explain
the morphology of S0 galaxies and the rapid truncation of star formation
implied by spectroscopic observations. The main difference with the
Abadi et al. result is the use of a complex multi phase structure of the
ISM. They show that the presence of holes and bubbles
in the diffuse H I can greatly enhance the stripping efficiency.
As the ICM streams through the holes in the ISM it ablates the edges and
prevents stripped gas from falling back.
Schulz and Struck
(2001)
in a comprehensive study using SPH, an adaptive mesh HYDRA code, and
including radiative cooling, confirm that low column density gas
is promptly removed from the disk. They also find that the onset of the
ICM wind has a profound effect on the gas in the disk, that does
not get stripped. The remnant disk is compressed and slightly displaced
relative to the halo center. This can trigger gravitational instability,
angular momentum gets transported outward and the disk compresses further
forming a ring. This makes the inner disk resistant to further
stripping, but presumably susceptible to global starbursts.
These various simulations appear to more or less agree on the effects
of the ISM. All of the above work modelled the ICM as a constant wind.
Vollmer et al. (2001)
took a different approach. Using an N-body/sticky-particle
code they simulate galaxies in radial orbits through the gravitational
potential of the Virgo cluster. The galaxies thus experience a time
variable
ram pressure and maximal damage to their gaseous disks only becomes apparent
well after closest approach to the Virgo Cluster
center. Thus if we see galaxies with truncated H I disks or distorted
velocity fields they are likely to be on their way out from the center.
They also find that a considerable part of the stripped total gas mass
remains bound to the galaxy and falls back onto the galactic disk
after the stripping event, possibly causing a central starburst. The
results of
Schulz and Struck
(2001) and
Vollmer et al (2001)
are the first to produce
simultaneously stripping in the outer parts and a mechanism to
enhance star formation in the inner parts. This may help use up any
remaining gas in the central regions and it could possibly do some
secular bulge building. There is observational evidence for stripped
H I disks with enhanced central H I surface densities
(Cayatte et al. 1994)
and there are several lines of evidence that the most recent episode
of star formation in cluster galaxies occurred in the central parts (e.g.
Rose et al. 2001).