67 research outputs found
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
Merging and fragmentation in the Burgers dynamics
We explore the noiseless Burgers dynamics in the inviscid limit, the
so-called ``adhesion model'' in cosmology, in a regime where (almost) all the
fluid particles are embedded within point-like massive halos. Following
previous works, we focus our investigations on a ``geometrical'' model, where
the matter evolution within the shock manifold is defined from a geometrical
construction. This hypothesis is at variance with the assumption that the usual
continuity equation holds but, in the inviscid limit, both models agree in the
regular regions. Taking advantage of the formulation of the dynamics of this
``geometrical model'' in terms of Legendre transforms and convex hulls, we
study the evolution with time of the distribution of matter and the associated
partitions of the Lagrangian and Eulerian spaces. We describe how the halo mass
distribution derives from a triangulation in Lagrangian space, while the dual
Voronoi-like tessellation in Eulerian space gives the boundaries of empty
regions with shock nodes at their vertices. We then emphasize that this
dynamics actually leads to halo fragmentations for space dimensions greater or
equal to 2 (for the inviscid limit studied in this article). This is most
easily seen from the properties of the Lagrangian-space triangulation and we
illustrate this process in the two-dimensional (2D) case. In particular, we
explain how point-like halos only merge through three-body collisions while
two-body collisions always give rise to two new massive shock nodes (in 2D).
This generalizes to higher dimensions and we briefly illustrate the
three-dimensional (3D) case. This leads to a specific picture for the
continuous formation of massive halos through successive halo fragmentations
and mergings.Comment: 21 pages, final version published in Phys.Rev.
Voids as a Precision Probe of Dark Energy
A signature of the dark energy equation of state may be observed in the shape
of voids. We estimate the constraints on cosmological parameters that would be
determined from the ellipticity distribution of voids from future spectroscopic
surveys already planned for the study of large scale structure.
The constraints stem from the sensitivity of the distribution of ellipticity
to the cosmological parameters through the variance of fluctuations of the
density field smoothed at some length scale. This length scale can be chosen to
be of the order of the comoving radii of voids at very early times when the
fluctuations are Gaussian distributed. We use Fisher estimates to show that the
constraints from void ellipticities are promising. Combining these constraints
with other traditional methods results in the improvement of the Dark Energy
Task Force Figure of Merit on the dark energy parameters by an order of hundred
for future experiments. The estimates of these future constraints depend on a
number of systematic issues which require further study using simulations. We
outline these issues and study the impact of certain observational and
theoretical systematics on the forecasted constraints on dark energy
parameters.Comment: Submitted to PRD, 22 pages 9 figure
Minivoids in the Local Volume
We consider a sphere of 7.5 Mpc radius, which contains 355 galaxies with
accurately measured distances, to detect the nearest empty volumes. Using a
simple void detection algorithm, we found six large (mini)voids in Aquila,
Eridanus, Leo, Vela, Cepheus and Octans, each of more than 30 Mpc^3. Besides
them, 24 middle-size "bubbles" of more than 5 Mpc^3 volume are detected, as
well as 52 small "pores". The six largest minivoids occupy 58% of the
considered volume. Addition of the bubbles and pores to them increases the
total empty volume up to 75% and 81%, respectively. The detected local voids
look like oblong potatoes with typical axial ratios b/a = 0.75 and c/a = 0.62
(in the triaxial ellipsoide approximation). Being arranged by the size of their
volume, local voids follow power law of volumes-rankes dependence. A
correlation Gamma-function of the Local Volume galaxies follows a power low
with a formally calculated fractal dimension D = 1.5. We found that galaxies
surrounding the local minivoids do not differ significantly from other nearby
galaxies on their luminosity, but have appreciably higher hydrogen
mass-to-luminosity ratio and also higher star formation rate. We recognize an
effect of local expansion of typical minivoid to be \Delta H/H_0~(25+-15)%.Comment: 23 pages, 18 figures. Astrophysical Journal, accepte
The Size and Shape of Voids in Three-Dimensional Galaxy Surveys
The sizes and shapes of voids in a galaxy survey depend not only on the
physics of structure formation, but also on the sampling density of the survey
and on the algorithm used to define voids. Using an N-body simulation with a
CDM power spectrum, we study the properties of voids in samples with different
number densities of galaxies, both in redshift space and in real space. When
voids are defined as regions totally empty of galaxies, their characteristic
volume is strongly dependent on sampling density; when they are defined as
regions whose density is 0.2 times the mean galaxy density, the dependence is
less strong. We compare two void-finding algorithms, one in which voids are
nonoverlapping spheres, and one, based on the algorithm of Aikio and Mahonen,
which does not predefine the shape of a void. Regardless of the algorithm
chosen, the characteristic void size is larger in redshift space than in real
space, and is larger for low sampling densities than for high sampling
densities. We define an elongation statistic Q which measures the tendency of
voids to be stretched or squashed along the line of sight. Using this
statistic, we find that at sufficiently high sampling densities (comparable to
the number densities of galaxies brighter than L_*), large voids tend to be
slightly elongated along the line of sight in redshift space.Comment: LaTex, 21 pages (including 7 figures), ApJ, submitte
A Two-Temperature Model of the Intracluster Medium
We investigate evolution of the intracluster medium (ICM), considering the
relaxation process between the ions and electrons. According to the standard
scenario of structure formation, ICM is heated by the shock in the accretion
flow to the gravitational potential well of the dark halo. The shock primarily
heats the ions because the kinetic energy of an ion entering the shock is
larger than that of an electron by the ratio of masses. Then the electrons and
ions exchange the energy through coulomb collisions and reach the equilibrium.
From simple order estimation we find that the region where the electron
temperature is considerably lower than the ion temperature spreads out on a Mpc
scale. We then calculate the ion and electron temperature profiles by combining
the adiabatic model of two-temperature plasma by Fox & Loeb (1997) with
spherically symmetric N-body and hydrodynamic simulations based on three
different cosmological models. It is found that the electron temperature is
about a half of the mean temperature at radii 1 Mpc. This could lead to
an about 50 % underestimation in the total mass contained within 1 Mpc
when the electron temperature profiles are used. The polytropic indices of the
electron temperature profiles are whereas those of mean
temperature for Mpc. This result is consistent both
with the X-ray observations on electron temperature profiles and with some
theoretical and numerical predictions about mean temperature profiles.Comment: 20 pages with 6 figures. Accepted for publication in Ap
Voids in the Large-Scale Structure
Voids are the most prominent feature of the LSS of the universe. Still, they
have been generally ignored in quantitative analysis of it, essentially due to
the lack of an objective tool to identify and quantify the voids. To overcome
this, we present the Void-Finder algorithm, a novel tool for objectively
quantifying galaxy voids. The algorithm classifies galaxies as either wall- or
field-galaxies. Then it identifies voids in the wall-galaxy distribution. Voids
are defined as continuous volumes that do not contain any wall-galaxies. The
voids must be thicker than an adjustable limit, which is refined in successive
iterations. We test the algorithm using Voronoi tessellations. By appropriate
scaling of the parameters we apply it to the SSRS2 survey and to the IRAS 1.2
Jy. Both surveys show similar properties: ~50% of the volume is filled by the
voids, which have a scale of at least 40 Mpc, and a -0.9 under-density. Faint
galaxies populate the voids more than bright ones. These results suggest that
both optically and IRAS selected galaxies delineate the same LSS. Comparison
with the recovered mass distribution further suggests that the observed voids
in the galaxy distribution correspond well to under-dense regions in the mass
distribution. This confirms the gravitational origin of the voids.Comment: Submitted to ApJ; 33 pages, aaspp4 LaTeX file, using epsfig and
natbib, 1 table, 12 PS figures. Complete gzipped version is available at
http://shemesh.fiz.huji.ac.il/hagai/; uuencoded file is available at
http://shemesh.fiz.huji.ac.il/papers/ep3.uu or ftp://shemesh.fiz.huji.ac.i
Reconstruction Analysis of Galaxy Redshift Surveys: A Hybrid Reconstruction Method
In reconstruction analysis of galaxy redshift surveys, one works backwards
from the observed galaxy distribution to the primordial density field in the
same region, then evolves the primordial fluctuations forward in time with an
N-body code. This incorporates assumptions about the cosmological parameters,
the properties of primordial fluctuations, and the biasing relation between
galaxies and mass. These can be tested by comparing the reconstruction to the
observed galaxy distribution, and to peculiar velocity data. This paper
presents a hybrid reconstruction method that combines the `Gaussianization''
technique of Weinberg(1992) with the dynamical schemes of Nusser & Dekel(1992)
and Gramann(1993). We test the method on N-body simulations and on N-body mock
catalogs that mimic the depth and geometry of the Point Source Catalog Redshift
Survey and the Optical Redshift Survey. This method is more accurate than
Gaussianization or dynamical reconstruction alone. Matching the observed
morphology of clustering can limit the bias factor b, independent of Omega.
Matching the cluster velocity dispersions and z-space distortions of the
correlation function xi(s,mu) constrains the parameter beta=Omega^{0.6}/b.
Relative to linear or quasi-linear approximations, a fully non-linear
reconstruction makes more accurate predictions of xi(s,mu) for a given beta,
thus reducing the systematic biases of beta measurements and offering further
scope for breaking the degeneracy between Omega and b. It also circumvents the
cosmic variance noise that limits conventional analyses of xi(s,mu). It can
also improve the determination of Omega and b from joint analyses of redshift
& peculiar velocity surveys as it predicts the fully non-linear peculiar
velocity distribution at each point in z-space.Comment: 72 pages including 33 figures, submitted to Ap
Cavity evolution in relativistic self-gravitating fluids
We consider the evolution of cavities within spherically symmetric
relativistic fluids, under the assumption that proper radial distance between
neighboring fluid elements remains constant during their evolution (purely
areal evolution condition). The general formalism is deployed and solutions are
presented. Some of them satisfy Darmois conditions whereas others present
shells and must satisfy Israel conditions, on either one or both boundary
surfaces. Prospective applications of these results to some astrophysical
scenarios is suggested.Comment: 10 pages Revtex. To appear in Class. Quantum Grav
Sociology and hierarchy of voids: A study of seven nearby CAVITY galaxy voids and their dynamical CosmicFlows-3 environment
Context. The present study addresses a key question related to our
understanding of the relation between void galaxies and their environment: the
relationship between luminous and dark matter in and around voids. Aims. To
explore the extent to which local Universe voids are empty of matter, we study
the full (dark+luminous) matter content of seven nearby cosmic voids that are
fully contained within the CosmicFlows-3 volume. Methods. We obtained the
matter-density profiles of seven cosmic voids using two independent methods.
These were built from the galaxy redshift space two-point correlation function
in conjunction with peculiar velocity gradients from the CosmicFlows-3 dataset.
Results. The results are striking, because when the redshift survey is used,
all voids show a radial positive gradient of galaxies, while based on the
dynamical analysis, only three of these voids display a clear underdensity of
matter in their center. Conclusions. This work constitutes the most detailed
observational analysis of voids conducted so far, and shows that void emptiness
should be derived from dynamical information. From this limited study, the
Hercules void appears to be the best candidate for a local Universe pure
"pristine volume", expanding in three directions with no dark matter located in
that void.Comment: Submitted A\&A Nov 29, 2022 - AA/2022/45578 / Accepted March 3rd,
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