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C. Struck, S. Schulz (Iowa State)
When disk galaxies fall into groups or clusters containing a substantial intracluster medium (ICM) parts of the gas disk can be promptly removed by ram pressure stripping. Observations and models both suggest that passage through the dense ICM in the cores of large clusters like Coma will result in large amounts of prompt stripping (e.g., Abadi et al. MNRAS, 308, 947). However, orbits in the outskirts of clusters or through clusters of mass comparable to or less than Virgo, much of the gas disk will remain. Ram pressure still provides a significant pressure perturbation in most of the unstripped disk. The pressure on one side of a gas disk will displace the disk by a small amount relative to the (collisionless) halo potential center, until equilibrium is attained. In equilibrium, the gravitational pull toward the halo center cancels part of the external pressure force, while the opposite side of the disk feels a comparable gravity excess. Thus, the disk is squeezed from sides. These forces are equivalent to the weight of additional mass in the disk, so they can be included in the cylindrically symmetric gravitational stability criterion as an increase in the 'effective' gas column density. Since most star-forming galaxy disks have surface densities near the instability threshold, the external pressure induces instability. These effects, including the displacement, are radius dependent, so the gas disk bends and flocculent spirals form. In the outer disk gas is swept from low column density regions between spirals. As the spirals propagate and shear they are eventually swept as well. The spirals transport angular momentum and as the inner disk loses angular momentum it is compressed. Increased star formation is the likely result. We have carried out new N-body, adaptive mesh SPH simulations that demonstrate these processes.