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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

*A. Valinia, P. R. Shapiro, H. Martel, E. T. Vishniac (UT-Austin), J. V. Villumsen (MPI)*
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**