A galaxy cluster is an assembly of a few hundreds to a few thousands of galaxies of all morphological types (irregular, spiral, lenticular and elliptical). The inner regions of a galaxy cluster (or intra-cluster medium, ICM) is filled with a very hot gas at a temperature of several millions of degrees that can be observed by its X-ray emission. When a galaxy crosses a cluster, its morphology, its kinematics and its spectral properties are strongly perturbed by various processes called environmental effects. Two important mechanisms come into play in the evolution of the galaxy : the tidal interactions and ram pressure stripping by the intra-cluster medium. Indeed, a galaxy undergoes several gravitational interactions across the cluster because it is disturbed by the gravity fields from other galaxies in the cluster and from the cluster itself. This kind of interaction acts on a long time-scale (several interactions during several billion years). As a result its morphology is strongly perturbed, leading to the possible creation of stellar tails outside the galaxy plane or the triggering of bursts of star formation from hydrogen gas clouds in the galaxy interstellar medium (ISM).
Furthermore, the hot gas from the intracluster medium exerts a pressure on a galaxy ISM. This pressure acts like a wind that blows the ISM gas and thus is called ram pressure stripping. The faster the galaxy moves with respect to the ICM and the higher the density of the hot gas is (generally towards the most central parts of the cluster), the stronger the stripping will be and the easier the galaxy will lose its gas. This type of interaction is effective on a time-scale of about a few tens of million years, which is much shorter than for the gravitational interactions. An important consequence of this effect is that the stellar formation becomes less and less effective because the ISM gas tank is gradually emptied. The team of researchers led by Laurent Chemin, Chantal Balkowski and Veronique Cayatte carried out the observations of a sample of thirty galaxies of the Virgo Cluster (the nearest cluster to the Milky Way located at 50 million light-years) with the aim of studying how the morphological, kinematical and dynamical properties of the ionized gas of the galaxies depend on their environment.
Figure 1 : Velocity fields of 30 spiral galaxies of the Virgo cluster. Click on the image to enlarge it
The team of researchers led by Laurent Chemin, Chantal Balkowski and Veronique Cayatte carried out the observations of a sample of thirty galaxies of the Virgo Cluster (the nearest cluster to the Milky Way located at 50 million light-years) with the aim of studying how the morphological, kinematical and dynamical properties of the ionized gas of the galaxies depend on their environment. Figure 1 : Velocity fields of 30 spiral galaxies of the Virgo cluster. Click on the image to enlarge it The observations have been performed at the following telescopes : the 1.93-m of the Observatoire de Haute-Provence, the 1.6-m of the Observatoire du mont Mégantic (Canada), the ESO 3.6-m (Chile) and the CFHT 3.6-m (Hawaii) from 2000 to 2005. A typical exposure lasts two hours. The data were acquired by using the Fabry-Perot interferometry technique with the very sensitive photon-counting camera (FaNTOmM) from the Université de Montréal. With such an instrument it is possible to obtain emission maps of the ionised gas (Halpha line at 656.3 nm) of the ISM and velocity fields of galaxies with very high angular and spectral accuracies (of the order of an arcsecond and one ten of km/s respectively). An Halpha emission-line map allows to study the distribution of star-forming regions in galaxies while a velocity field allows to measure its kinematical properties and its rotation. Figure 1 displays the 30 velocity fields of the observed Virgo cluster galaxies and Figure 2 illustrates two examples of Halpha emission-line maps and velocity fields for the spiral galaxies NGC 4254 (Messier 99) and NGC 4654.
Figure 2 : Halpha images and velocity fields of two Virgo cluster galaxies. Emission-line maps (left hand panels) and velocity fields (right hand panels) of the ionised gas (Halpha line) for the galaxies NGC 4254 (upper line) and NGC 4654 (bottom line). The Halpha emission is dominated by star forming regions along the spiral arms. The colour gradation of a velocity field shows the rotation of the gas around the center of a galaxy. The blue colour indicates that the rotation is done in the direction of the observer (the gas approaches) while the red colour indicates that the rotation is done in an opposite direction (the gas moves away). Click on the image to enlarge it
The velocity fields allow to study the rotation of galaxies, their mass, the properties of their dark matter halo, their kinematical disturbances (non-circular motion) as a function of their environment (their position in the cluster, the local density of galaxies, the density of the ICM gas...). This will be used to understand how the environmental effects intervene in the dynamical evolution of spiral galaxies in clusters. The observations have noticeably helped to cover the complete kinematics of NGC 4438 for the first time. NGC 4438 is a typical example of a galaxy undergoing effects from the environment (Figure 3) It is located very close to the center of the Virgo Cluster and has the most disturbed morphology among all the galaxies of the cluster. Figure 3 shows large stellar tails outside the galactic disc (left hand image) and filaments of ionised gas that mostly extend to the West of the disc (middle image). The kinematics of NGC 4438 also appears very perturbed (right hand image) compared with a relatively regular velocity field (like the ones shown in Figure 1). There are strong reasons to think that NGC 4438 underwent a tidal interaction with a companion (NGC 4435) and is undergoing the effects of ram pressure stripping by the intra-cluster medium.
Figure 3 : The galaxy NGC 4438 : the prototype of galaxy which is strongly perturbed by environmental effects. It can be noticed : 1- the stellar tails to the North and South-West of the galactic plane (left hand image, obtained in the B-band) which result from a tidal interaction with the companion NGC 4435 and 2- the extended ionised gas filaments (central image) which are caused by ram pressure stripping from the ICM. The right hand image is the velocity field of NGC 4438. Click on the image to enlarge it