42 research outputs found
Clustering statistics in cosmology
The main tools in cosmology for comparing theoretical models with the
observations of the galaxy distribution are statistical. We will review the
applications of spatial statistics to the description of the large-scale
structure of the universe. Special topics discussed in this talk will be:
description of the galaxy samples, selection effects and biases, correlation
functions, Fourier analysis, nearest neighbor statistics, Minkowski functionals
and structure statistics. Special attention will be devoted to scaling laws and
the use of the lacunarity measures in the description of the cosmic texture.Comment: 15 pages, 7 figures, uses spie.cls (included). This paper will be
published in SPIE Proceedings Vol. 4847, 2002, "Astronomical Data Analysis
II," J.-L. Stark and F. Murtagh, eds., and is made available as an electronic
preprint with permission of SPI
Multi-scale morphology of the galaxy distribution
Many statistical methods have been proposed in the last years for analyzing
the spatial distribution of galaxies. Very few of them, however, can handle
properly the border effects of complex observational sample volumes. In this
paper, we first show how to calculate the Minkowski Functionals (MF) taking
into account these border effects. Then we present a multiscale extension of
the MF which gives us more information about how the galaxies are spatially
distributed. A range of examples using Gaussian random fields illustrate the
results. Finally we have applied the Multiscale Minkowski Functionals (MMF) to
the 2dF Galaxy Redshift Survey data. The MMF clearly indicates an evolution of
morphology with scale. We also compare the 2dF real catalog with mock catalogs
and found that Lambda-CDM simulations roughly fit the data, except at the
finest scale.Comment: 17 pages, 19 figures, accepted for publication in MNRA
Joint constraints on galaxy bias and through the N-pdf of the galaxy number density
We present a full description of the N-probability density function of the
galaxy number density fluctuations. This N-pdf is given in terms, on the one
hand, of the cold dark matter correlations and, on the other hand, of the
galaxy bias parameter. The method relies on the assumption commonly adopted
that the dark matter density fluctuations follow a local non-linear
transformation of the initial energy density perturbations. The N-pdf of the
galaxy number density fluctuations allows for an optimal estimation of the bias
parameter (e.g., via maximum-likelihood estimation, or Bayesian inference if
there exists any a priori information on the bias parameter), and of those
parameters defining the dark matter correlations, in particular its amplitude
(). It also provides the proper framework to perform model selection
between two competitive hypotheses. The parameters estimation capabilities of
the N-pdf are proved by SDSS-like simulations (both ideal log-normal
simulations and mocks obtained from Las Damas simulations), showing that our
estimator is unbiased. We apply our formalism to the 7th release of the SDSS
main sample (for a volume-limited subset with absolute magnitudes ). We obtain and , for galaxy number density fluctuations in cells of a size of
Mpc. Different model selection criteria show that galaxy biasing is
clearly favoured.Comment: 25 pages, 9 figures, 2 tables. v2: Substantial revision, adding the
joint constraints with \sigma_8 and testing with Las Damas mocks. Matches
version accepted for publication in JCA
Infalling groups and galaxy transformations in the cluster A2142
We study galaxy populations and search for possible merging substructures in
the rich galaxy cluster A2142. Normal mixture modelling revealed in A2142
several infalling galaxy groups and subclusters. The projected phase space
diagram was used to analyse the dynamics of the cluster and study the
distribution of various galaxy populations in the cluster and subclusters. The
cluster, supercluster, BCGs, and one infalling subcluster are aligned. Their
orientation is correlated with the alignment of the radio and X-ray haloes of
the cluster. Galaxies in the centre of the main cluster at the clustercentric
distances have older stellar populations (with the median age
of ~Gyrs) than galaxies at larger clustercentric distances.
Star-forming and recently quenched galaxies are located mostly in the infall
region at the clustercentric distances ,
where the median age of stellar populations of galaxies is about ~Gyrs.
Galaxies in A2142 have higher stellar masses, lower star formation rates, and
redder colours than galaxies in other rich groups. The total mass in infalling
groups and subclusters is ,
approximately half of the mass of the cluster, sufficient for the mass growth
of the cluster from redshift (half-mass epoch) to the present. The
cluster A2142 may have formed as a result of past and present mergers and
infallen groups, predominantly along the supercluster axis. Mergers cause
complex radio and X-ray structure of the cluster and affect the properties of
galaxies in the cluster, especially in the infall region. Explaining the
differences between galaxy populations, mass, and richness of A2142, and other
groups and clusters may lead to better insight about the formation and
evolution of rich galaxy clusters.Comment: 16 pages, 13 figures, A&A, in pres
Unusual A2142 supercluster with a collapsing core: distribution of light and mass
We study the distribution, masses, and dynamical properties of galaxy groups
in the A2142 supercluster. We analyse the global luminosity density
distribution in the supercluster and divide the supercluster into the
high-density core and the low-density outskirts regions. We find galaxy groups
and filaments in the regions of different global density, calculate their
masses and mass-to-light ratios and analyse their dynamical state with several
1D and 3D statistics. We use the spherical collapse model to study the
dynamical state of the supercluster. We show that in A2142 supercluster groups
and clusters with at least ten member galaxies lie along an almost straight
line forming a 50 Mpc/h long main body of the supercluster. The A2142
supercluster has a very high density core surrounded by lower-density outskirt
regions. The total estimated mass of the supercluster is M_est = 6.2
10^{15}M_sun. More than a half of groups with at least ten member galaxies in
the supercluster lie in the high-density core of the supercluster, centered at
the rich X-ray cluster A2142. Most of the galaxy groups in the core region are
multimodal. In the outskirts of the supercluster, the number of groups is
larger than in the core, and groups are poorer. The orientation of the cluster
A2142 axis follows the orientations of its X-ray substructures and radio halo,
and is aligned along the supercluster axis. The high-density core of the
supercluster with the global density D8 > 17 and perhaps with D8 > 13 may have
reached the turnaround radius and started to collapse. A2142 supercluster with
luminous, collapsing core and straight body is an unusual object among galaxy
superclusters. In the course of the future evolution the supercluster may be
split into several separate systems.Comment: 13 pages, 9 figures, Astronomy and Astrophysics, in press. References
update
Tracing high redshift cosmic web with quasar systems
We trace the cosmic web at redshifts 1.0 <= z <= 1.8 using the quasar data
from the SDSS DR7 QSO catalogue (Schneider et al. 2010). We apply a
friend-of-friend (FoF) algorithm to the quasar and random catalogues to
determine systems at a series of linking lengths, and analyse richness and
sizes of these systems. At the linking lengths l <= 30 Mpc/h the number of
quasar systems is larger than the number of systems detected in random
catalogues, and systems themselves have smaller diameters than random systems.
The diameters of quasar systems are comparable to the sizes of poor galaxy
superclusters in the local Universe, the richest quasar systems have four
members. The mean space density of quasar systems is close to the mean space
density of local rich superclusters. At intermediate linking lengths (40 <= l
<= 70 Mpc/h) the richness and length of quasar systems are similar to those
derived from random catalogues. Quasar system diameters are similar to the
sizes of rich superclusters and supercluster chains in the local Universe. At
the linking length 70 Mpc/h the richest systems of quasars have diameters
exceeding 500 Mpc/h. The percolating system which penetrate the whole sample
volume appears in quasar sample at smaller linking length than in random
samples (85 Mpc/h). Quasar luminosities in systems are not correlated with the
system richness. Quasar system catalogues at our web pages
http://www.aai.ee/~maret/QSOsystems.html serve as a database to search for
superclusters of galaxies and to trace the cosmic web at high redshifts.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic