363 research outputs found
Unfolding the Hierarchy of Voids
We present a framework for the hierarchical identification and
characterization of voids based on the Watershed Void Finder. The Hierarchical
Void Finder is based on a generalization of the scale space of a density field
invoked in order to trace the hierarchical nature and structure of cosmological
voids. At each level of the hierarchy, the watershed transform is used to
identify the voids at that particular scale. By identifying the overlapping
regions between watershed basins in adjacent levels, the hierarchical void tree
is constructed. Applications on a hierarchical Voronoi model and on a set of
cosmological simulations illustrate its potential.Comment: 5 pages, 2 figure
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
A Dynamical Classification of the Cosmic Web
A dynamical classification of the cosmic web is proposed. The large scale
environment is classified into four web types: voids, sheets, filaments and
knots. The classification is based on the evaluation of the deformation tensor,
i.e. the Hessian of the gravitational potential, on a grid. The classification
is based on counting the number of eigenvalues above a certain threshold,
lambda_th at each grid point, where the case of zero, one, two or three such
eigenvalues corresponds to void, sheet, filament or a knot grid point. The
collection of neighboring grid points, friends-of-friends, of the same web
attribute constitutes voids, sheets, filaments and knots as web objects.
A simple dynamical consideration suggests that lambda_th should be
approximately unity, upon an appropriate scaling of the deformation tensor. The
algorithm has been applied and tested against a suite of (dark matter only)
cosmological N-body simulations. In particular, the dependence of the volume
and mass filling fractions on lambda_th and on the resolution has been
calculated for the four web types. Also, the percolation properties of voids
and filaments have been studied.
Our main findings are: (a) Already at lambda_th = 0.1 the resulting web
classification reproduces the visual impression of the cosmic web. (b) Between
0.2 < lambda_th < 0.4, a system of percolated voids coexists with a net of
interconected filaments. This suggests a reasonable choice for lambda_th as the
parameter that defines the cosmic web. (c) The dynamical nature of the
suggested classification provides a robust framework for incorporating
environmental information into galaxy formation models, and in particular the
semi-analytical ones.Comment: 11 pages, 6 figures, submitted to MNRA
Non-intersecting squared Bessel paths and multiple orthogonal polynomials for modified Bessel weights
We study a model of non-intersecting squared Bessel processes in the
confluent case: all paths start at time at the same positive value , remain positive, and are conditioned to end at time at . In
the limit , after appropriate rescaling, the paths fill out a
region in the -plane that we describe explicitly. In particular, the paths
initially stay away from the hard edge at , but at a certain critical
time the smallest paths hit the hard edge and from then on are stuck to
it. For we obtain the usual scaling limits from random matrix
theory, namely the sine, Airy, and Bessel kernels. A key fact is that the
positions of the paths at any time constitute a multiple orthogonal
polynomial ensemble, corresponding to a system of two modified Bessel-type
weights. As a consequence, there is a matrix valued
Riemann-Hilbert problem characterizing this model, that we analyze in the large
limit using the Deift-Zhou steepest descent method. There are some novel
ingredients in the Riemann-Hilbert analysis that are of independent interest.Comment: 59 pages, 11 figure
The Void Galaxy Survey: Optical Properties and H I Morphology and Kinematics
We have carefully selected a sample of 60 galaxies that reside in the deepest
underdensities of geometrically identified voids within the SDSS. HI imaging of
55 galaxies with the WSRT reveals morphological and kinematic signatures of
ongoing interactions and gas accretion. We probe a total volume of 485 Mpc^3
within the voids, with an angular resolution of 8 kpc at an average distance of
85 Mpc. We reach column density sensitivities of 5 x 10^19 cm^-2, corresponding
to an HI mass limit of 3 x 10^8 M_sun. We detect HI in 41 galaxies, with total
masses ranging from 1.7 x 10^8 to 5.5 x 10^9 M_sun. The upper limits on the 14
non-detections are not inconsistent with their luminosities, given their
expected HI mass to light ratios. We find that the void galaxies are generally
gas rich, low luminosity, blue disk galaxies, with optical and HI properties
that are not unusual for their luminosity and morphology. The sample spans a
range of absolute magnitudes (-16.1 > M_r > -20.4) and colors (0.06 < g-r <
0.87), and includes disk and irregular galaxies. We also identify three as
early type galaxies, all of which are not detected in HI. All galaxies have
stellar masses less than 3 x 10^10 M_sun, and many have kinematic and
morphological signs of ongoing gas accretion, suggesting that the void galaxy
population is still in the process of assembling. The small scale clustering in
the void, within 600 kpc and 200 km/s, is similar to that in higher density
regions, and we identify 18 HI rich neighboring galaxies in the voids. Most are
within 100 kpc and 100 km/s of the targeted galaxy, and we find no significant
population of HI rich low luminosity galaxies filling the voids, contrary to
what is predicted by simulations.Comment: 34 pages, 33 figures (including Atlas in Appendix), accepted for
publication in A
- …