SEGMENTING the UNIVERSE

One of the most striking aspects of redshift surveys is that there are huge regions of space totally devoid of galaxies. These are the cosmic voids, which grew hierarchically from the initial density troughs in the primordial fluctuation field. In a hierarchical structure formation scenario smaller voids build up larger ones; the void hierarchy. In this paper we study the void evolution and merging history of large voids and void ensembles. We do this by means of constrained realizations of Gaussian fields, which are our initial fields for N-body simulations. Our direct goal was to test the theoretical model of Sheth & van de Weygaert (2004). In this model they proposed that small voids can either merge into larger ones or collapse into larger overdense regions. Preliminary analysis confirms this model, where smaller voids either merge away in large voids, analogue to build up of large haloes or small voids can collapse away. The model assumes that small voids collapse, if they are embedded in larger spherical overdensities. Our results, however, indicate that this condition might be even too stringent and that void-collapse also occurs when the local deformation, caused by the tidal field, is strong enough that it can shear and tear away voids into filaments or walls. In order to track this Lagrangian evolution of small voids and its surrounding in an objective manner we propose the usage of the Watershed transformation. This tool combines the capability of detecting filaments and voids simultaneous in a parameter free way. In combination with the DTFE interpolation method we regard it as an excellent tool to extract and visualize the cosmic foam. Here we present the 2 dimensional version with applications to slices out of a large simulation. In upcoming work it will be extended to three dimensions and we expect it to use as a foam extractor.