The environment of a galaxy cluster represents an ideal laboratory for testing the behaviour of the interstellar matter under extreme conditions. One of the most spectacular interactions is that of the galaxy with the hot tenuous gas which pervades the whole cluster (intracluster medium). A group of french and german scientists are leading a program to investigate galaxy evolution within the cluster environment. With the help of a numerical model together with interferometric observations new insights into the life of spiral galaxy within galaxy cluster could be gained. A major result of their simulations is that under certain conditions a considerable part of the interstellar matter, which is pushed out of the galaxy during the galaxy-cluster interaction, can fall back onto the galaxy's disk.

What is ram pressure stripping?

The simulations  The strength of ram pressure stripping depends crucially on galaxy orbits. Radial orbits allow galaxies to go deeper into the cluster potential where their velocity increases considerably and where the galaxy density and the density of the ICM is substantially higher. In a first step galaxy orbits within the galaxy cluster are modelled in order to recover temporal ram pressure profiles for different orbits. A three-dimensional simulation is used to follow the gas kinematics of a spiral galaxy falling into the Virgo cluster. The particles represent gas cloud complexes which are evolving in an analytically given gravitational potential of the galaxy; they can have inelastic collisions and are accelerated by ram pressure when the galaxy moves through the ICM.  The efficiency of ram pressure stripping depends on the eccentricity of the orbit and the inclination angle between the galaxy's disk and orbital plane. Several simulations with different orbits were made in order to quantify the effects of ram pressure stripping.  One of these simulations is shown in Figure 1. The galaxy is seen face-on and is moving down to the left, i.e. the wind is coming from the lower left corner (arrow). The length of the arrow is proportional to the strength of ram pressure. During the closest passage to the cluster center, the wind is maximum (t=0 yr).

Figure 1

The observations

Figure 2: NGC 4522.  Upper panel: Halpha velocity field. Lower panel: model velocity field. Figure 3: NGC 4654.  Upper panel: model gas distribution. Lower panel: HI gas distribution.

Confrontation between observations and simulations  Until now, interferometric observations of two galaxies in the Virgo cluster (NGC 4522 and NGC 4654) and one galaxy in the Coma cluster (NGC 4848) have been compared with snapshots of the simulations. The galaxies' HI deficiencies, projected distances from the cluster center, and radial velocities give hard constraints on the choice of the orbital parameters and the elapsed time since the closest approach to the cluster center (snapshot number).   NGC 4522  This galaxy is located ~1 Mpc in the south of the Virgo cluster center (M87). The ionized gas distribution (Figure 2 upper panel) and velocity field (Figure 2 lower panel) observed in the Halpha line correspond to that of an expanding ring structure due to the re-accretion of a part of the interstellar matter which was pushed by ram pressure. The angle between the disk and the orbital plane is ~30o.   NGC 4654  This galaxy is also located ~1 Mpc from M87. The south-eastern, extended, low surface density tail represents the same dynamical feature as for NGC 4522: an expanding asymmetric shell. Here, one side of the shell is open. This happens at late stages of the shell expansion when the galaxy is stripped edge-on. Figure 3 shows a comparison between the model (upper panel) and the observed (lower panel) atomic gas distribution.   NGC 4848  This Coma cluster galaxy has been observed in Halpha, CO, 20cm radio continuum, and HI (Figure 4 and 5). All gas distribution and kinematics are consistent with a scenario where the galaxy is emerging from the cluster center. Its orbit is highly eccentric, i.e. ram pressure stripping has been very efficient.     Figure 4: NGC 4848.  Top: greyscale: B band image; contours: CO(1-0). Bottom: greyscale: HI 21cm; contours: CO(1-0).

Figure 5: NGC 4848.  Left: greyscale: Halpha; contours: CO(1-0). Right: greyscale: 20cm continuum; contours: CO(1-0).

References:

1. Vollmer, B.; Cayatte, V.; Boselli, A.; Balkowski, C.; Duschl, W. J.,"Kinematics of the anemic cluster galaxy NGC 4548. Is stripping still active?", 1999, A&A, 349, 411 .
2. Vollmer, B.; Marcelin, M.; Amram, P.; Balkowski, C.; Cayatte, V.; Garrido, O., "The consequences of ram pressure stripping on the Virgo cluster spiral galaxy NGC 4522", 2000, A&A, 364, 532 .
3. Vollmer, B.; Braine, J.; Balkowski, C.; Cayatte, V.; Duschl, W. J., "12CO(1-0) observations of NGC 4848: A Coma galaxy after stripping", 2001, A&A, 364, 824 .
4. Vollmer, B.; Cayatte, V.; Balkowski, C.; Duschl, W.J., "Ram pressure stripping and galaxy orbits: The case of the Virgo cluster", 2001, ApJ, accepted for publication; astro-ph/0107237 .