"What goes around comes
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
The best place to study the gas removal due to ram pressure is the
cluster as it is the closest cluster which can be observed in great
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).
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
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).
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.
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.
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 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.
Contact : Chantal.Balkowski
(Département DAEC, Observatoire de Paris, France)
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
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
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
Vollmer, B.; Cayatte, V.; Balkowski, C.; Duschl, W.J., "Ram pressure stripping
and galaxy orbits: The case of the Virgo cluster", 2001, ApJ, accepted
Bernd Vollmer (Max-Planck-Institut
fuer Radioastronomie, Bonn, Germany)
(Département DAEC, Observatoire de Paris, France)