COMPRESSIVE TIDAL HEATING OF A DISK GALAXY IN A RICH CLUSTER

The effects of the mean tidal field of a cluster of galaxies on the internal dynamics of a disk galaxy traveling through it are studied in the restricted three-body framework. In the model adopted the galaxy experiences a tidal field that is compressive within the core of the cluster. The effect of the cluster tidal field on the disk galaxy in this region of the cluster resembles the phenomenon of compressive shocking of globular clusters by the Galactic disk. A disk parallel to its orbital plane in the cluster develops a transient, two-armed spiral pattern. A disk which is perpendicular or inclined to the orbital plane is transiently compressed and the originally circular disk is deformed into an ellipse. Also, the planar random velocities of all components in the disk increase after the galaxy passes through the core of the cluster. The low-velocity dispersion (5-10 km s-1) gas clouds experience a relatively larger increase in random velocity than the hotter stellar components (20-35 km s-1). A strong tidal field can increase the planar random velocities of all particles to between 50 and 70 km s-1. The increase in planar velocities results in a strong anisotropy between the planar and vertical velocity dispersions. We argue that this will make the disk unstable to the ''fire-hose instability'' which leads to bending modes in the disk and which will thicken the disk slightly. The mean tidal fields in rich clusters were probably stronger during the epoch of cluster formation and relaxation than they are in present-day relaxed clusters. It is suggested that the activity in spiral galaxies in high redshift clusters (the Butcher-Oemler clusters) could have been triggered by these strong tidal fields.