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Session 54 - Large Scale Structure.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

[54.09] Gravitational Instability in Collisionless and Gaseous Cosmological Pancakes

The gravitational instability of cosmological pancakes in an Einstein-de Sitter universe is investigated numerically. A ``pancake'' is defined here to be the nonlinear outcome of the growth of a 1D, sinusoidal, plane-wave, adiabatic density perturbation. We have studied the stability and fragmentation of purely collisionless pancakes subject to either symmetric (density) or antisymmetric (bending) modes, with wavevectors transverse to that of the unperturbed pancake plane-wave and compared our results with those predicted by a thin sheet energy argument. The unstable wavenumbers are predicted by this argument to lie in the range k_Hk_H are stabilized by Hubble expansion while those with k>k_v are stabilized by the 1D velocity dispersion of the particles along the direction of pancake collapse within the region of shell-crossing. High resolution, 2D, numerical simulations by the Particle-Mesh (PM) method, however, show that perturbations with k>k_v are in fact unstable, too, since the purely 1D velocity dispersion of the unperturbed pancake is not enough to prevent particle orbits from gravitational focusing. Furthermore, our simulations suggest that the linear growth rate of instability scales as k^1/4 rather than as k^1/2 as predicted by the thin sheet analysis. After a brief period of linear growth for unstable modes, the onset of nonlinearity is signaled by a saturation of the growth rate and in the production of clumps with large overdensities relative to that of the unperturbed pancake. We have extended this analysis to pancakes composed of collisionless dark matter and gaseous baryons, by numerical simulation using Adaptive Smoothed Particle Hydrodynamics (ASPH), coupled to the PM gravity solver. We will discuss the hydrodynamical effects of pancake instability and the generation of vorticity due to curved shocks that arise in the baryonic clusters that form as a result of gravitational instability.

Program listing for Tuesday