Abstract
AbstractA particle method for the simulation of the global evolution of cold gas in galactic potentials is described. It incorporates an efficient artificial viscosity, based on the smoothed particle hydrodynamics approach, which conserves both linear and angular momenta and in the continuum limit is equivalent to a Navier-Stokes viscosity in a compressible gas. Parameters of this viscosity have been calibrated in such a way as to make a gas disk, embedded in an axisymmetric galactic potential and originally inclined to its equatorial plane, settle to the preferred plane on the time scale characterizing the differential precession. After a careful analysis of the previously published settling times we find that our results are consistent with Steiman-Cameron’s (1984) lower limits, and we argue that Simonson’s (1982) time scales have been seriously underestimated. We also demonstrate that two dimensional methods based on rigid rings do not model the evolution of a differentially precessing settling gas disk realistically and cannot be used to study the morphology of gas distribution.
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