396 research outputs found

### The Correlation Function of Clusters of Galaxies and the Amplitude of Mass Fluctuations in the Universe

We show that if a sample of galaxy clusters is complete above some mass
threshold, then hierarchical clustering theories for structure formation
predict its autocorrelation function to be determined purely by the cluster
abundance and by the spectrum of linear density fluctuations. Thus if the shape
of the initial fluctuation spectrum is known, its amplitude $\sigma_8$ can be
estimated directly from the correlation length of a cluster sample in a way
which is independent of the value of $\Omega_0$. If the cluster mass
corresponding to the sample threshold is also known, it provides an independent
estimate of the quantity $\sigma_8\Omega_0^{0.6}$. Thus cluster data should
allow both $\sigma_8$ and $\Omega_0$ to be determined observationally. We
explore these questions using N-body simulations together with a simple but
accurate analytical model based on extensions of Press-Schechter theory.
Applying our results to currently available data we find that if the linear
fluctuation spectrum has a shape similar to that suggested by the APM galaxy
survey, then a correlation length $r_0$ in excess of 20\mpch for Abell
clusters would require $\sigma_8>1$, while r_0<15\mpch would require
$\sigma_8<0.5$. With conventional estimates of the relevant mass threshold
these imply \Omega_0\la 0.3 and \Omega_0\ga 1 respectively.Comment: Latex, 25 pages (including 8 PS figures). The PS-file of the paper is
also available via anonymous ftp at:
ftp://ibm-3.mpa-garching.mpg.de/pub/jing/xicc.ps . Submitted to MNRAS. In the
replaced version, a typo in Eq.(1a) is fixe

### QUASARS AND LARGE SCALE STRUCTURE OF THE UNIVERSE

The majority of bright distant quasars $(z {}^{>}_{\sim} 1)$ may form in
massive mergers appearing in compact galaxy groups in/and young clusters. The
expected tests are (i) large correlation signal for medium-$z$ QSOs $(1 < z <
2)$ and (ii) direct search for quasar groups (QGs) indicating positions of
distant pre-superclusters which later will evolve to the "systems" like the
local Great Attractor or Shapley concentration. We discuss large QGs with more
than ten members within regions $\le l_{LS} \sim 100-150 \, h^{-1} Mpc$,
tracing the enhanced density regions at $z {}^{<}_{\sim} 2$. These early large
scale structures (i) provide a natural way to "bias" the distribution of Abell
clusters, and (ii) suggest that the spectrum of primordial density
perturbations is nearly flat at scales encompassing both cluster and GAs, $l =
\pi k^{-1} \in(10,100)h^{-1} Mpc: \Delta^{2}_{k} \sim k^{3}P(k) \sim
k^{\gamma}, \gamma = 1^{+0.6}_{-0.4}$.Comment: 5 pages, uuencoded Z-compressed postscript, contribution to the
Proceedings of Rencontres de Moriond 1995 "Clustering in the Universe

### Three-Point Correlations in Weak Lensing Surveys: Model Predictions and Applications

We use the halo model of clustering to compute two- and three-point
correlation functions for weak lensing, and apply them in a new statistical
technique to measure properties of massive halos. We present analytical results
on the eight shear three-point correlation functions constructed using
combination of the two shear components at each vertex of a triangle. We
compare the amplitude and configuration dependence of the functions with
ray-tracing simulations and find excellent agreement for different scales and
models. These results are promising, since shear statistics are easier to
measure than the convergence. In addition, the symmetry properties of the shear
three-point functions provide a new and precise way of disentangling the
lensing E-mode from the B-mode due to possible systematic errors.
We develop an approach based on correlation functions to measure the
properties of galaxy-group and cluster halos from lensing surveys. Shear
correlations on small scales arise from the lensing matter within halos of mass
M > 10^13 solar masses. Thus the measurement of two- and three-point
correlations can be used to extract information on halo density profiles,
primarily the inner slope and halo concentration. We demonstrate the
feasibility of such an analysis for forthcoming surveys. We include covariances
in the correlation functions due to sample variance and intrinsic ellipticity
noise to show that 10% accuracy on profile parameters is achievable with
surveys like the CFHT Legacy survey, and significantly better with future
surveys. Our statistical approach is complementary to the standard approach of
identifying individual objects in survey data and measuring their properties.Comment: 30 pages, 21 figures. Corrected typos in equations (23) and (28).
Matches version for publication in MNRA

### Primordial non-Gaussianity in the Bispectrum of the Halo Density Field

The bispectrum vanishes for linear Gaussian fields and is thus a sensitive
probe of non-linearities and non-Gaussianities in the cosmic density field.
Hence, a detection of the bispectrum in the halo density field would enable
tight constraints on non-Gaussian processes in the early Universe and allow
inference of the dynamics driving inflation. We present a tree level derivation
of the halo bispectrum arising from non-linear clustering, non-linear biasing
and primordial non-Gaussianity. A diagrammatic description is developed to
provide an intuitive understanding of the contributing terms and their
dependence on scale, shape and the non-Gaussianity parameter fNL. We compute
the terms based on a multivariate bias expansion and the peak-background split
method and show that non-Gaussian modifications to the bias parameters lead to
amplifications of the tree level bispectrum that were ignored in previous
studies. Our results are in a good agreement with published simulation
measurements of the halo bispectrum. Finally, we estimate the expected signal
to noise on fNL and show that the constraint obtainable from the bispectrum
analysis significantly exceeds the one obtainable from the power spectrum
analysis.Comment: 34 pages, 15 figures, (v3): matches JCAP published versio

### Relativistic effects and primordial non-Gaussianity in the galaxy bias

When dealing with observables, one needs to generalize the bias relation
between the observed galaxy fluctuation field to the underlying matter
distribution in a gauge-invariant way. We provide such relation at second-order
in perturbation theory adopting the local Eulerian bias model and starting from
the observationally motivated uniform-redshift gauge. Our computation includes
the presence of primordial non-Gaussianity. We show that large scale-dependent
relativistic effects in the Eulerian bias arise independently from the presence
of some primordial non-Gaussianity. Furthermore, the Eulerian bias inherits
from the primordial non-Gaussianity not only a scale-dependence, but also a
modulation with the angle of observation when sources with different biases are
correlated.Comment: 12 pages, LaTeX file; version accepted for publication in JCA

### Order parameter model for unstable multilane traffic flow

We discuss a phenomenological approach to the description of unstable vehicle
motion on multilane highways that explains in a simple way the observed
sequence of the phase transitions "free flow -> synchronized motion -> jam" as
well as the hysteresis in the transition "free flow synchronized motion".
We introduce a new variable called order parameter that accounts for possible
correlations in the vehicle motion at different lanes. So, it is principally
due to the "many-body" effects in the car interaction, which enables us to
regard it as an additional independent state variable of traffic flow. Basing
on the latest experimental data (cond-mat/9905216) we assume that these
correlations are due to a small group of "fast" drivers. Taking into account
the general properties of the driver behavior we write the governing equation
for the order parameter. In this context we analyze the instability of
homogeneous traffic flow manifesting itself in both of the mentioned above
phase transitions where, in addition, the transition "synchronized motion ->
jam" also exhibits a similar hysteresis. Besides, the jam is characterized by
the vehicle flows at different lanes being independent of one another. We
specify a certain simplified model in order to study the general features of
the car cluster self-formation under the phase transition "free flow
synchronized motion". In particular, we show that the main local parameters of
the developed cluster are determined by the state characteristics of vehicle
motion only.Comment: REVTeX 3.1, 10 pages with 10 PostScript figure

### Constraining Primordial Non-Gaussianity with High-Redshift Probes

We present an analysis of the constraints on the amplitude of primordial
non-Gaussianity of local type described by the dimensionless parameter $f_{\rm
NL}$. These constraints are set by the auto-correlation functions (ACFs) of two
large scale structure probes, the radio sources from NRAO VLA Sky Survey (NVSS)
and the quasar catalogue of Sloan Digital Sky Survey Release Six (SDSS DR6
QSOs), as well as by their cross-correlation functions (CCFs) with the cosmic
microwave background (CMB) temperature map (Integrated Sachs-Wolfe effect).
Several systematic effects that may affect the observational estimates of the
ACFs and of the CCFs are investigated and conservatively accounted for. Our
approach exploits the large-scale scale-dependence of the non-Gaussian halo
bias. The derived constraints on {$f_{\rm NL}$} coming from the NVSS CCF and
from the QSO ACF and CCF are weaker than those previously obtained from the
NVSS ACF, but still consistent with them. Finally, we obtain the constraints on
$f_{\rm NL}=53\pm25$ ($1\,\sigma$) and $f_{\rm NL}=58\pm24$ ($1\,\sigma$) from
NVSS data and SDSS DR6 QSO data, respectively.Comment: 16 pages, 8 figures, 1 table, Accepted for publication on JCA

### CMBR Weak Lensing and HI 21-cm Cross-correlation Angular Power Spectrum

Weak gravitational lensing of the CMBR manifests as a secondary anisotropy in
the temperature maps. The effect, quantified through the shear and convergence
fields imprint the underlying large scale structure (LSS), geometry and
evolution history of the Universe. It is hence perceived to be an important
observational probe of cosmology. De-lensing the CMBR temperature maps is also
crucial for detecting the gravitational wave generated B-modes. Future
observations of redshifted 21-cm radiation from the cosmological neutral
hydrogen (HI) distribution hold the potential of probing the LSS over a large
redshift range. We have investigated the correlation between post-reionization
HI signal and weak lensing convergence field. Assuming that the HI follows the
dark matter distribution, the cross-correlation angular power spectrum at a
multipole \ell is found to be proportional to the cold dark matter power
spectrum evaluated at \ell/r, where r denotes the comoving distance to the
redshift where the HI is located. The amplitude of the ross-correlation depends
on quantities specific to the HI distribution, growth of perturbations and also
the underlying cosmological model. In an ideal ituation, we found that a
statistically significant detection of the cross-correlation signal is
possible. If detected, the cross-correlation signal hold the possibility of a
joint estimation of cosmological parameters and also test various CMBR
de-lensing estimators.Comment: 14 pages, 4 figures, publishe

### Consistency test of general relativity from large scale structure of the Universe

We construct a consistency test of General Relativity (GR) on cosmological
scales. This test enables us to distinguish between the two alternatives to
explain the late-time accelerated expansion of the universe, that is, dark
energy models based on GR and modified gravity models without dark energy. We
derive the consistency relation in GR which is written only in terms of
observables - the Hubble parameter, the density perturbations, the peculiar
velocities and the lensing potential. The breakdown of this consistency
relation implies that the Newton constant which governs large-scale structure
is different from that in the background cosmology, which is a typical feature
in modified gravity models. We propose a method to perform this test by
reconstructing the weak lensing spectrum from measured density perturbations
and peculiar velocities. This reconstruction relies on Poisson's equation in GR
to convert the density perturbations to the lensing potential. Hence any
inconsistency between the reconstructed lensing spectrum and the measured
lensing spectrum indicates the failure of GR on cosmological scales. The
difficulties in performing this test using actual observations are discussed.Comment: 7 pages, 1 figur

### Multiple sources of infection and potential endemic characteristics of the large outbreak of dengue in Guangdong in 2014

A large outbreak of dengue, with the most documented cases, occurred in Guangdong China in 2014. Epidemiological studies and phylogenetic analysis of the isolated dengue virus (DENV) showed this outbreak was attributed to multiple sources and caused by at least two genotypes of DENV-1 (Genotypes I and III) and two genotypes of DENV-2 (Cosmopolitan and Asian I Genotypes). A retrospective review and phylogenetic analysis of DENV isolated in Guangdong showed that DENV-1 Genotype I strains were reported continuously during 2004-2014, Genotype III strains were reported during 2009-2014 ; DENV-2 Cosmopolitan and Asian I Genotype strains were reported continuously during 2012-2014. At least 45,171 cases were reported in this outbreak, with 65.9% of the patients in the 21-55-year-old group. A trend toward a decrease in the daily newly emerged cases lagged by approximately 20 days compared with the mosquito density curve. Several epidemiological characteristics of this outbreak and the stably sustained serotypes and genotypes of DENV isolated in Guangdong suggest that Guangdong has been facing a threat of transforming from a dengue epidemic area to an endemic area. The high temperature, drenching rain, rapid urbanization, and pandemic of dengue in Southeast Asia may have contributed to this large outbreak of dengue

- âŠ