"What goes around comes around..."

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 theoretical 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 mapping of the gas content of spiral galaxies in the Virgo cluster showed that the HI disks of cluster spiral galaxies are disturbed and considerably reduced. However, their molecular content seems to be unchanged. These observational results indicate that the gas removal due to the rapid motion of the galaxy within the intracluster medium is responsible for the HI deficiency and the disturbed gas disks of the cluster spirals.
When a gas rich spiral galaxy falls into a galaxy cluster for the first time, it can lose its gas during interactions with the cluster environment. There is a tremendeous amount of hot (~107 K), tenuous (~10-4 cm-3) intracluster gas (several 1013 solar masses). Within the inertial system of the infalling galaxy, the interstellar matter is exposed to a wind which is due to its rapid motion within the hot gas. This wind pushes the galaxy's gas to the opposite direction of the galaxy's motion within the cluster. This effect is called ram pressure stripping.
This effect has been recognizedin the middle of the 70s. Nevertheless, it has not yet been unambiguously shown where and when the galaxies lose their gas.
The best place to study the gas removal due to ram pressure is the Virgo cluster as it is the closest cluster which can be observed in great detail.

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 N-body code is used to simulate 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

In order to compare the simulations with reality, observations of the interstellar matter of cluster spiral galaxies are necessary. Only interferometric observations are used (Fabry-Perot at the OHP, CFHT, IRAM PdB, VLA), because only the comparison of the model/observed gas distribution and velocity field gives enough constraints to determine if the model describes well the observed galaxies.
Furthermore, observations in different wavelengths are needed (Halpha, HI, CO) in order to investigate how the different gas phases (ionized, atomic, molecular gas) behave under the influence of ram pressure.

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. 
Left: greyscale: B band image; contours: CO(1-0). Right: 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).

The future 

The detailed comparison of multiple wavelength observations for other Virgo galaxies are in progress. Since the dynamics of the galaxy is known for single galaxy, it is possible to implement basic physics of the interstellar matter, i.e. star formation and phase transitions. Only the close interplay between observations and theoretical models will allow us to gain new insights into the evolution of spiral galaxies in clusters.
It is planned to extend the numerical code:
* to include a recipe for star formation;
* to include 50000 particle to simulate the halo, disk and bulge component of the galaxy. This code is now being tested. 


  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 .
Contact : Chantal.Balkowski (Département DAEC, Observatoire de Paris, France)
Bernd Vollmer (Max-Planck-Institut fuer Radioastronomie, Bonn, Germany)
Veronique.Cayatte (Département DAEC, Observatoire de Paris, France)