RAM PRESSURE STRIPPING FROM ELLIPTIC GALAXIES .1. THE MASS CONTAINED IN THE GALAXY

We examine the physical assumptions and the basic hydrodynamic equations of the ram pressure stripping model for elliptical galaxies in clusters of galaxies. Our equations differ from those of Gaetz, Salpeter, and Shaviv in the treatment of the motion of stars in the galaxy. We solved the model equations numerically in two dimensions and extended the range of parameters beyond the range assumed by Gaetz et al. We find the following: 1. The formation of a bow shock in a cluster galaxy is due to the replenished gas from stellar mass loss. A typical galaxy without replenishment moving through the cluster gas does not form a shock. 2. The velocity dispersion of the stars in the galaxy has a large influence on the injection temperature of the replenished gas. The inclusion of the stellar velocity dispersion in the injection temperature of the gas leads to injection temperatures which are higher than those estimated in previous ram pressure stripping calculations. The higher injection temperature affects the results substantially. 3. The physical conditions are such that the time scale for proton-electron equilibration may be longer than the dynamical one, and a two fluid model for the plasma is required. The major differences between the numerical results found in the two models are (1) in the rate of star formation and (2) in the flow morphology outside the galaxy. In particular, the morphology of the temperature distribution in the wake of the galaxy is significantly different in the two models. The electrons in the two fluid model are hotter than the electrons in the single fluid models. On the other hand, the protons in the two fluid model are significantly cooler than the protons in the single fluid model. Still, the differences in the mass of gas inside the galaxy (irrespective of its temperature) between the results yielded by these two approximations are very small. 4. We show that the velocity at infinity affects weakly the amount of gas contained in the galaxy, yet it determines the efficiency of ram pressure stripping. 5. We provide simple fitting formulae for the amount of mass contained within the galaxy as a function of the external and internal parameters. 6. The parameters of the cluster environment affect only weakly the amount of mass contained in the galaxy. The mass of gas in the moving galaxy is more sensitive to the galactic parameters. 7. We discuss the effect of stellar density distribution and show that it has a major effect on the amount of mass contained in the galaxy.