24 research outputs found
A Hierarchy of Voids
We present a model for the distribution of void sizes and its evolution
within the context of hierarchical scenarios of gravitational structure
formation. For a proper description of the hierarchical buildup of the system
of voids in the matter distribution, not only the "void-in-void" problem should
be taken into account, but also that of the "void-in-cloud" issue. Within the
context of the excursion set formulation of an evolving void hierarchy is one
involving a "two-barrier" excursion problem, unlike the "one-barrier" problem
for the dark halo evolution. This leads to voids having a peaked size
distribution at any cosmic epoch, centered on a characteristic void size that
evolves self-similarly in time, this in distinct contrast to the distribution
of virialized halo masses which do not have a small-scale cut-off.Comment: 6 pages, 4 figures, to appear in the Proceedings of IAU Colloquium
195: "Outskirts of Galaxy Clusters: intense life in the suburbs", Torino
Italy, March 200
A Cosmic Watershed: the WVF Void Detection Technique
On megaparsec scales the Universe is permeated by an intricate filigree of
clusters, filaments, sheets and voids, the Cosmic Web. For the understanding of
its dynamical and hierarchical history it is crucial to identify objectively
its complex morphological components. One of the most characteristic aspects is
that of the dominant underdense Voids, the product of a hierarchical process
driven by the collapse of minor voids in addition to the merging of large ones.
In this study we present an objective void finder technique which involves a
minimum of assumptions about the scale, structure and shape of voids. Our void
finding method, the Watershed Void Finder (WVF), is based upon the Watershed
Transform, a well-known technique for the segmentation of images. Importantly,
the technique has the potential to trace the existing manifestations of a void
hierarchy. The basic watershed transform is augmented by a variety of
correction procedures to remove spurious structure resulting from sampling
noise. This study contains a detailed description of the WVF. We demonstrate
how it is able to trace and identify, relatively parameter free, voids and
their surrounding (filamentary and planar) boundaries. We test the technique on
a set of Kinematic Voronoi models, heuristic spatial models for a cellular
distribution of matter. Comparison of the WVF segmentations of low noise and
high noise Voronoi models with the quantitatively known spatial characteristics
of the intrinsic Voronoi tessellation shows that the size and shape of the
voids are succesfully retrieved. WVF manages to even reproduce the full void
size distribution function.Comment: 24 pages, 15 figures, MNRAS accepted, for full resolution, see
http://www.astro.rug.nl/~weygaert/tim1publication/watershed.pd
The Cosmically Depressed: Life, Sociology and Identity of Voids
We review and discuss aspects of Cosmic Voids that form the background for
our Void Galaxy Survey (see accompanying paper by Stanonik et al.). Following a
sketch of the general characteristics of void formation and evolution, we
describe the influence of the environment on their development and structure
and the characteristic hierarchical buildup of the cosmic void population. In
order to be able to study the resulting tenuous void substructure and the
galaxies populating the interior of voids, we subsequently set out to describe
our parameter free tessellation-based watershed void finding technique. It
allows us to trace the outline, shape and size of voids in galaxy redshift
surveys. The application of this technique enables us to find galaxies in the
deepest troughs of the cosmic galaxy distribution, and has formed the basis of
our void galaxy program.Comment: 10 pages, 4 figures, proceedings "Galaxies in Isolation" (May 2009,
Granada, Spain), eds. L. Verdes-Montenegro, ASP (this is a colour, extended
and combined version; accompanying paper to Stanonik et al., arXiv:0909.2869,
in same volume
Alignments of Voids in the Cosmic Web
We investigate the shapes and mutual alignment of voids in the large scale
matter distribution of a LCDM cosmology simulation. The voids are identified
using the novel WVF void finder technique. The identified voids are quite
nonspherical and slightly prolate, with axis ratios in the order of c:b:a
approx. 0.5:0.7:1. Their orientations are strongly correlated with significant
alignments spanning scales >30 Mpc/h.
We also find an intimate link between the cosmic tidal field and the void
orientations. Over a very wide range of scales we find a coherent and strong
alignment of the voids with the tidal field computed from the smoothed density
distribution. This orientation-tide alignment remains significant on scales
exceeding twice the typical void size, which shows that the long range external
field is responsible for the alignment of the voids. This confirms the view
that the large scale tidal force field is the main agent for the large scale
spatial organization of the Cosmic Web.Comment: 10 pages, 4 figures, submitted to MNRAS, for high resolution version,
see http://www.astro.rug.nl/~weygaert/tim1publication/voidshape.pd
The Spine of the Cosmic Web
We present the SpineWeb framework for the topological analysis of the Cosmic
Web and the identification of its walls, filaments and cluster nodes. Based on
the watershed segmentation of the cosmic density field, the SpineWeb method
invokes the local adjacency properties of the boundaries between the watershed
basins to trace the critical points in the density field and the separatrices
defined by them. The separatrices are classified into walls and the spine, the
network of filaments and nodes in the matter distribution. Testing the method
with a heuristic Voronoi model yields outstanding results. Following the
discussion of the test results, we apply the SpineWeb method to a set of
cosmological N-body simulations. The latter illustrates the potential for
studying the structure and dynamics of the Cosmic Web.Comment: Accepted for publication HIGH-RES version:
http://skysrv.pha.jhu.edu/~miguel/SpineWeb
The Aspen-Amsterdam void finder comparison project
Despite a history that dates back at least a quarter of a century, studies of voids in the large-scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void-finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of 13 different void finders constructed using particles, haloes, and semi-analytical model galaxies extracted from a subvolume of the Millennium simulation. This study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 per cent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than mB=−20 is found to be smaller than about −0.8 by all our void finding algorithm