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The effects of ram pressure stripping on spirals in rich clusters are examined using the TREESPH code, which can handle the large dynamical range in density between the galactic gas and the intracluster medium (ICM). In the simulation, we model the galactic gas as a smoothly distributed single phase medium, so the results are appropriate to the outer HI disks. Typically, significant gas ablation occurs in the outer galaxy, while the dense central region of the gaseous disk suffers little or no effect. The cutoff radius can be estimated by balancing the gravitational force in the disk against the ram pressure force of the ICM. In extreme cases ($\rho_{icm} \geq 10^{-3} {\rm H/cm^3}$, $V \geq 1500 {\rm km/s}$, where $V$ is the velocity of the spiral moving face--on through the ICM), a typical spiral will lose all of its atomic gas.
In addition to stripping the gas from cluster spirals, the process of ram pressure also affects their dynamics. In rich clusters where ICM contributes an appreciable fraction of the total mass, the momentum loss timescale due to gas drag is comparable to the momentum loss timescale due to dynamical friction. Neither process, however, is effective enough to produce a velocity bias through a significant settling of spiral galaxies relative to the dark matter. We find that a simple analytical estimate for the gas drag timescale agrees well with the results of numerical simulations. One should note, however, that dynamical evolution of clusters may have been different at high redshifts when galaxies presumably contained a larger fraction of gas. Therefore, future work should address the dynamical effects of gas drag in the context of cluster formation and evolution.
