Continuous Fields and Discrete Samples: Reconstruction through Delaunay Tessellations

Here we introduce the Delaunay Density Estimator Method. Its purpose is rendering a fully volume-covering reconstruction of a density field from a set of discrete data points sampling this field. Reconstructing density or intensity fields from a set of irregularly sampled data is a recurring key issue in operations on astronomical data sets, both in an observational context as well as in the context of numerical simulations. Our technique is based upon the stochastic geometric concept of the Delaunay tessellation generated by the point set. We shortly describe the method, and illustrate its virtues by means of an application to an N-body simulation of cosmic structure formation. The presented technique is a fully adaptive method: automatically it probes high density regions at maximum possible resolution, while low density regions are recovered as moderately varying regions devoid of the often irritating shot-noise effects. Of equal importance is its capability to sharply and undilutedly recover anisotropic density features like filaments and walls. The prominence of such features at a range of resolution levels within a hierarchical clustering scenario as the example of the standard CDM scenario is shown to be impressively recovered by our scheme.Comment: 4 pages, 2 figures, accepted for publication in Astronomy & Astrophysics Letter

Rotation of Cosmic Voids and Void-Spin Statistics

We present a theoretical study of void spins and their correlation properties. The concept of the spin angular momentum for an unbound void is introduced to quantify the effect of the tidal field on the distribution of matter that make up the void. Both the analytical and numerical approaches are used for our study. Analytically, we adopt the linear tidal torque model to evaluate the void spin-spin and spin-density correlations, assuming that a void forms in the initial region where the inertia momentum and the tidal shear tensors are maximally uncorrelated with each other. Numerically, we use the Millennium run galaxy catalog to find voids and calculate their spin statistics. The numerical results turn out to be in excellent agreement with the analytic predictions, both of which consistently show that there are strong spatial alignments between the spin axes of neighbor voids and strong anti-alignments between the void spin axes and the directions to the nearest voids. We expect that our work will provide a deeper insight into the origin and properties of voids and the large scale structure.Comment: accepted version, ApJ in press, the concept of void spins explained, typos correcte

Tidal fields and structure formation

The role of tidal shear in the formation of structure in the Universe is explored. To illustrate the possible and sometimes dramatic impact of tidal fields we focus on the evolution of voids. We firstly analyze the role of tidal fields both in the highly symmetric situation of an initially homogeneous ellipsoidal underdensity embedded in an artificially imposed tidal field. In addition, we present selfconsistent case studies consisting of N-body simulations that start from constrained Gaussian initial conditions in which the matter distribution is appropriately sculpted to yield an a priori specified tidal field. We conclude that voids may indeed be induced to collapse. Also, we present evidence for the strong relation between tidal fields and filaments in the mass distribution