The evolution of cluster galaxies

The Virgo cluster represents an ideal laboratory to study environmental effects on cluster galaxies. Virgo is the closest cluster in the northern hemisphere which can be observed in great detail (1~kpc). It is a dynamically young, spiral-rich cluster. Most of the spiral galaxies have entered the cluster only recently  (within several Gyr). It is known since the eighties that galaxies in nearby clusters are different from field galaxies of the same size and morphological type: they are redder, have less atomic hydrogen (HI) and have truncated HI disks. The cluster environment thus changes the properties of the cluster members. One can distinguish two classes of interactions between the cluster and a galaxy:

• Gravitational interactions with the cluster potential
• Gravitational interaction with other cluster galaxies (galaxy ``harassment'').
• Interaction of the interstellar medium and the intracluster medium (ram pressure stripping).

To study the properties of Virgo spiral galaxies, three different tracers are generally used: Halpha to trace the recent massive star formation, CO(1--0) as a tracer of the molecular gas and HI for the atomic gas. Observations of Virgo spiral galaxies show truncated gas disks making them gas deficient. These galaxies also show truncated star formation within their disks. Within the truncation radius most of the disks show normal gas surface densities and star formation rates.

We use a numerical N-body code, which includes a collisional and a non-collisional component, to study the effects of ram pressure and gravitational interactions on Virgo cluster spirals. In addition we observe these galaxies in HI at the VLA and the Effelsberg 100m telescope, in CO at the IRAM 30m or the IRAM Plateau de Bure interferometer.

A recent development:
observations of the polarized radio continuum radiation can give important and complementary information on the gas dynamics of perturbed Virgo spiral galaxies. This radiation traces the ordered large scale (~1 kpc) magnetic field in galactic interstellar medium which is very sensitive to (i) compression and (ii) shear motions. Both motions  are difficult to detect in radial velocity fields. We solve the induction equation using the velocity fields of our numerical simulations to calculate the evolution of the large scale magnetic field. In a second step the polarized radio continuum emission is calculated and maps of its evolution are made that can be directly compared to observations.

By comparing directly the results of our numerical simulations to multi-wavelength observations we can diagnostic which interaction is responsible for the observed characteristics of a galaxy and deduce the interaction parameters. In this way we study the role of the different interactions for the evolution of a spiral galaxy in its cluster environment.

DSS image of the Virgo cluster spiral galaxy NGC 4438 which had a recent encounter with its companion NGC 4435.

The almost edge-on Virgo cluster spiral galaxy NGC 4522. Greyscale: optical R band image which shows the stellar disk. Contours: atomic hydrogen (HI) distribution. The gas disk is heavily truncated and there is extraplanar gas which has been pushed to the west by ram pressure (from Kenney et al. 2004, AJ, 127, 3361).