25 research outputs found
Extended percolation analysis of the cosmic web
Aims. We develop an extended percolation method to allow the comparison of
geometrical properties of the real cosmic web with the simulated dark matter
web for an ensemble of over- and under-density systems. Methods. We scan
density fields of dark matter (DM) model and SDSS observational samples, and
find connected over- and underdensity regions in a large range of threshold
densities. Lengths, filling factors and numbers of largest clusters and voids
as functions of the threshold density are used as percolation functions.
Results. We find that percolation functions of DM models of different box sizes
are very similar to each other. This stability suggests that properties of the
cosmic web, as found in the present paper, can be applied to the cosmic web as
a whole. Percolation functions depend strongly on the smoothing length. At
smoothing length 1 Mpc the percolation threshold density for clusters
is , and for voids is , very different from percolation thresholds for random samples, . Conclusions. The extended percolation analysis is a
versatile method to study various geometrical properties of the cosmic web in a
wide range of parameters. Percolation functions of the SDSS sample are very
different from percolation functions of DM model samples. The SDSS sample has
only one large percolating void which fills almost the whole volume. The SDSS
sample contains numerous small isolated clusters at low threshold densities,
instead of one single percolating DM cluster. These differences are due to the
tenuous dark matter web, present in model samples, but absent in real
observational samples.Comment: 15 pages, 10 figures, Astronomy & Astrophysics (accepted
Dynamical state of superclusters of galaxies: do superclusters expand or have they started to collapse?
We investigate the dynamical state of superclusters in Lambda cold dark
matter (CDM) cosmological models, where the density parameter
and (the rms fluctuation on the Mpc
scale) is . To study the nonlinear regime, we use N-body simulations.
We define superclusters as maxima of the density field smoothed on the scale
Mpc. Smaller superclusters defined by the density field smoothed on
the scale Mpc are also investigated. We find the relations between
the radially averaged peculiar velocity and the density contrast in the
superclusters for different cosmological models. These relations can be used to
estimate the dynamical state of a supercluster on the basis of its density
contrast. In the simulations studied, all the superclusters defined with the
Mpc smoothing are expanding by the present epoch. Only a small
fraction of the superclusters defined with Mpc has already reached
their turnaround radius and these superclusters have started to collapse. In
the model with and , the number density of objects
which have started to collapse is Mpc. The results
for superclusters in the N-body simulations are compared with the spherical
collapse model. We find that the radial peculiar velocities in N-body
simulations are systematically smaller than those predicted by the spherical
collapse model (% for the Mpc superclusters).Comment: 10 pages, 8 figures, accepted for publication in MNRA
The rms peculiar velocity of galaxy clusters for different cluster masses and radii
We investigate the rms peculiar velocity of galaxy clusters in the Lambda
cold dark matter (CDM) and tau cold dark matter (CDM)
cosmological models using N-body simulations. Cluster velocities for different
cluster masses and radii are examined. To identify clusters in the simulations
we use two methods: the standard friends-of-friends (FOF) method and the
method, where the clusters are defined as the maxima of the density field
smoothed on the scale Mpc (DENSMAX). If we use the DENSMAX
method, the size of the selected clusters is similar for all clusters. We find
that the rms velocity of clusters defined with the DENSMAX method is almost
independent of the cluster density and similar to the linear theory
expectations. The rms velocity of FOF clusters decreases with the cluster mass
and radius. In the CDM model, the rms peculiar velocity of massive
clusters with an intercluster separation Mpc is 15%
smaller than the rms velocity of the clusters with a separation
Mpc.Comment: 10 pages, 7 figures, 4 tables, accepted for publication in MNRA
Evolution of superclusters and supercluster cocoons in various cosmologies
We investigate the evolution of superclusters and supercluster cocoons
(basins of attraction), and the influence of cosmological parameters to the
evolution. We perform numerical simulations of the evolution of the cosmic web
for different cosmological models: the LCDM model with a conventional value of
the dark energy (DE) density, the open model OCDM with no DE, the standard SCDM
model with no DE, and the Hyper-DE HCDM model with an enhanced DE density
value. We find ensembles of superclusters of these models for five evolutionary
stages, corresponding to the present epoch z = 0, and to redshifts z = 1, 3,
10, 30. We use diameters of the largest superclusters and the number of
superclusters as percolation functions to describe properties of the ensemble
of superclusters in the cosmic web. We analyse the size and mass distribution
of superclusters in models and in real Sloan Digital Sky Survey (SDSS) based
samples. In all models numbers and volumes of supercluster cocoons are
independent on cosmological epochs. Supercluster masses increase with time, and
geometrical sizes in comoving coordinates decrease with time, for all models.
LCDM, OCDM and HCDM models have almost similar percolation parameters. This
suggests that the essential parameter, which defines the evolution of
superclusters, is the matter density. The DE density influences the growth of
the amplitude of density perturbations, and the growth of masses of
superclusters, albeit significantly less strongly. The HCDM model has the
largest speed of the growth of the amplitude of density fluctuations, and the
largest growth of supercluster masses during the evolution. Geometrical
diameters and numbers of HCDM superclusters at high threshold densities are
larger than for LCDM and OCDM superclusters. SCDM model has about two times
more superclusters than other models; SCDM superclusters have smaller diameters
and masses.Comment: 14 pages, 10 figures (accepted by Astronomy & Astrophysics). arXiv
admin note: text overlap with arXiv:1901.0937
The biasing phenomenon
{We study biasing as a physical phenomenon by analysing geometrical and
clustering properties of density fields of matter and galaxies.} {Our goal is
to determine the bias function using a combination of geometrical and power
spectrum analysis of simulated and real data.} {We apply an algorithm based on
local densities of particles, , to form simulated biased models using
particles with . We calculate the bias function of model
samples as functions of the particle density limit . We compare the
biased models with Sloan Digital Sky Survey (SDSS) luminosity limited samples
of galaxies using the extended percolation method. We find density limits
of biased models, which correspond to luminosity limited SDSS
samples.} {Power spectra of biased model samples allow to estimate the bias
function of galaxies of luminosity . We find the estimated bias
parameter of galaxies, . } {The absence of
galaxy formation in low-density regions of the Universe is the dominant factor
of the biasing phenomenon. Second largest effect is the dependence of the bias
function on the luminosity of galaxies. Variations in gravitational and
physical processes during the formation and evolution of galaxies have the
smallest influence to the bias function. }Comment: 20 pages, 16 figures. Submitted to Astronomy & Astrophysic
Flux- and volume-limited groups/clusters for the SDSS galaxies: catalogues and mass estimation
We provide flux-limited and volume-limited galaxy group and cluster
catalogues, based on the spectroscopic sample of the SDSS data release 10
galaxies. We used a modified friends-of-friends (FoF) method with a variable
linking length in the transverse and radial directions to identify as many
realistic groups as possible. The flux-limited catalogue incorporates galaxies
down to m_r = 17.77 mag. It includes 588193 galaxies and 82458 groups. The
volume-limited catalogues are complete for absolute magnitudes down to M_r =
-18.0, -18.5, -19.0, -19.5, -20.0, -20.5, and -21.0; the completeness is
achieved within different spatial volumes, respectively. Our analysis shows
that flux-limited and volume-limited group samples are well compatible to each
other, especially for the larger groups/clusters. Dynamical mass estimates,
based on radial velocity dispersions and group extent in the sky, are added to
the extracted groups. The catalogues can be accessed via http://cosmodb.to.ee
and the Strasbourg Astronomical Data Center (CDS).Comment: 16 pages, 18 figures, 2 tables, accepted for publication in A&
Wavelet analysis of the formation of the cosmic web
According to the modern cosmological paradigm galaxies and galaxy systems
form from tiny density perturbations generated during the very early phase of
the evolution of the Universe. Using numerical simulations we study the
evolution of phases of density perturbations of different scales to understand
the formation and evolution of the cosmic web. We apply the wavelet analysis to
follow the evolution of high-density regions (clusters and superclusters) of
the cosmic web. We show that the positions of maxima and minima of density
waves (their spatial phases) almost do not change during the evolution of the
structure. Positions of extrema of density perturbations are the more stable,
the larger is the wavelength of perturbations. Combining observational and
simulation data we conclude that the skeleton of the cosmic web was present
already in an early stage of structure evolution.Comment: 12 pages, 8 figures, revised versio
Environmental Enhancement of DM Haloes
We study the properties of dark matter haloes of a LCDM model in different
environments. Using the distance of the 5th nearest neighbour as an
environmental density indicator, we show that haloes in a high density
environment are more massive, richer, have larger radii and larger velocity
dispersions than haloes in a low density environment. Haloes in high density
regions move with larger velocities, and are more spherical than haloes in low
density regions. In addition, low mass haloes in the vicinity of the most
massive haloes are themselves more massive, larger, and have larger rms
velocities and larger 3D velocities than low mass haloes far from massive
haloes. The velocities of low mass haloes near massive haloes increase with the
parent halo mass. Our results are in agreement with recent findings about
environmental effects for groups and clusters of galaxies from deep (SDSS and
LCRS) surveys.Comment: 9 pages, 7 figures, submitted for Astronomy and Astrophysic
Luminous superclusters: remnants from inflation
We derive the luminosity and multiplicity functions of superclusters compiled
for the 2dF Galaxy Redshift Survey, the Sloan Digital Sky Survey (Data Release
4), and for three samples of simulated superclusters. We find for all
supercluster samples Density Field (DF) clusters, which represent high-density
peaks of the class of Abell clusters, and use median luminosities/masses of
richness class 1 DF-clusters to calculate relative luminosity/mass functions.
We show that the fraction of very luminous (massive) superclusters in real
samples is more than tenfolds greater than in simulated samples. Superclusters
are generated by large-scale density perturbations which evolve very slowly.
The absence of very luminous superclusters in simulations can be explained
either by non-proper treatment of large-scale perturbations, or by some yet
unknown processes in the very early Universe.Comment: 6 pages, 3 Figures, submitted for Astronomy and Astrophysic