The Shape of the First Collapsed Objects

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Abstract
Since the early seventies, there was a conjecture that the first collapse of a selfgravitating dust--like medium (appropriate approximation for nonbaryonic dark matter) results in the formation of a ``pancake" object, that is a thin surface. The conjecture has been based on the Zel'dovich approximate solution of the nonlinear gravitational instability of a generic smooth density perturbation. Recent works cast doubt on the Zel'dovich conjecture, suggesting that the first collapse might be point--like or filament--like rather than pancake--like. Our $N$--body simulations show first pancake collapse. We can reject with 97\% confidence the Bayesian prior that the other kinds of collapse are more or equally probable.Comment: 12 pages (excluding figures); LaTeX (revtex style); figures may be obtained via anonymous ftp at ftp://kusmos.phsx.ukans.edu/preprints/sergei/shapes/ to be published in in Phys. Rev. Lett. Revisions include more detail on the nature of the objects and the initial condition

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