1,317 research outputs found
A comparison of estimators for the two-point correlation function
Nine of the most important estimators known for the two-point correlation
function are compared using a predetermined, rigorous criterion. The indicators
were extracted from over 500 subsamples of the Virgo Hubble Volume simulation
cluster catalog. The ``real'' correlation function was determined from the full
survey in a 3000Mpc/h periodic cube. The estimators were ranked by the
cumulative probability of returning a value within a certain tolerance of the
real correlation function. This criterion takes into account bias and variance,
and it is independent of the possibly non-Gaussian nature of the error
statistics. As a result for astrophysical applications a clear recommendation
has emerged: the Landy & Szalay (1993) estimator, in its original or grid
version Szapudi & Szalay (1998), are preferred in comparison to the other
indicators examined, with a performance almost indistinguishable from the
Hamilton (1993) estimator.Comment: aastex, 10 pages, 1 table, 1 figure, revised version, accepted in
ApJ
A global descriptor of spatial pattern interaction in the galaxy distribution
We present the function J as a morphological descriptor for point patterns
formed by the distribution of galaxies in the Universe. This function was
recently introduced in the field of spatial statistics, and is based on the
nearest neighbor distribution and the void probability function. The J
descriptor allows to distinguish clustered (i.e. correlated) from ``regular''
(i.e. anti-correlated) point distributions. We outline the theoretical
foundations of the method, perform tests with a Matern cluster process as an
idealised model of galaxy clustering, and apply the descriptor to galaxies and
loose groups in the Perseus-Pisces Survey. A comparison with mock-samples
extracted from a mixed dark matter simulation shows that the J descriptor can
be profitably used to constrain (in this case reject) viable models of cosmic
structure formation.Comment: Significantly enhanced version, 14 pages, LaTeX using epsf, aaspp4, 7
eps-figures, accepted for publication in the Astrophysical Journa
Minkowski Functionals of Abell/ACO Clusters
We determine the Minkowski functionals for a sample of Abell/ACO clusters,
401 with measured and 16 with estimated redshifts. The four Minkowski
functionals (including the void probability function and the mean genus)
deliver a global description of the spatial distribution of clusters on scales
from to 60\hMpc with a clear geometric interpretation. Comparisons with
mock catalogues of N--body simulations using different variants of the CDM
model demonstrate the discriminative power of the description. The standard CDM
model and the model with tilted perturbation spectrum cannot generate the
Minkowski functionals of the cluster data, while a model with a cosmological
constant and a model with breaking of the scale invariance of perturbations
(BSI) yield compatible results.Comment: 10 pages, 13 Postscript figures, uses epsf.sty and mn.sty (included),
submitted to MNRA
A study of white etching crack formation by compression-torsion experiments
In this study, an attempt was made to recreate the bearing damage phenomenon âWhite Etching Cracksâ with a simplified testing setup. Rolling contact fatigue conditions were simulated with in-phase and out-ofphase cyclic compression-torsion experiments on 100Cr6 steel specimens. The results are compared in terms of microstructural change. Focused Ion Beam and metallographic analysis reveal that a fine-grained, white etching zone formed in the vicinity of the fatigue cracks of specimens tested with the in-phase load pattern. In contrast, no such structures were found after testing the out-of-phase load pattern. The properties of the white etching zone are characterised in more detail and compared with White Etching Cracks. KEYWORDS. White Etching Cracks; Multiaxial fatigue; Bearing steel
Numerical study of rainbows and glories in water-drop clouds
In this paper we study scattering phase functions in water-drop clouds for various distributions of droplet size and various conditions of glory, rainbow and corona formation, and discuss the hypothesis proposed by A. N. Nevzorov that a considerable amount of water in cold clouds can exist in a specific phase state with the refractive index â 1.8 (so called A-water). Polarization and angular distributions are studied by the Monte Carlo method for radiation reflected by cloud layers with drops of water or hypothetical A-water taking into account multiple scattering. Computational results make it possible to develop procedures for analysis of microphysical structure of clouds and confirmation or disproof of the existence of A-water
Biased-estimations of the Variance and Skewness
Nonlinear combinations of direct observables are often used to estimate
quantities of theoretical interest. Without sufficient caution, this could lead
to biased estimations. An example of great interest is the skewness of
the galaxy distribution, defined as the ratio of the third moment \xibar_3
and the variance squared \xibar_2^2. Suppose one is given unbiased estimators
for \xibar_3 and \xibar_2^2 respectively, taking a ratio of the two does
not necessarily result in an unbiased estimator of . Exactly such an
estimation-bias affects most existing measurements of . Furthermore,
common estimators for \xibar_3 and \xibar_2 suffer also from this kind of
estimation-bias themselves: for \xibar_2, it is equivalent to what is
commonly known as the integral constraint. We present a unifying treatment
allowing all these estimation-biases to be calculated analytically. They are in
general negative, and decrease in significance as the survey volume increases,
for a given smoothing scale. We present a re-analysis of some existing
measurements of the variance and skewness and show that most of the well-known
systematic discrepancies between surveys with similar selection criteria, but
different sizes, can be attributed to the volume-dependent estimation-biases.
This affects the inference of the galaxy-bias(es) from these surveys. Our
methodology can be adapted to measurements of analogous quantities in quasar
spectra and weak-lensing maps. We suggest methods to reduce the above
estimation-biases, and point out other examples in LSS studies which might
suffer from the same type of a nonlinear-estimation-bias.Comment: 28 pages of text, 9 ps figures, submitted to Ap
Luminosity- and morphology-dependent clustering of galaxies
How does the clustering of galaxies depend on their inner properties like
morphological type and luminosity? We address this question in the mathematical
framework of marked point processes and clarify the notion of luminosity and
morphological segregation. A number of test quantities such as conditional
mark-weighted two-point correlation functions are introduced. These descriptors
allow for a scale-dependent analysis of luminosity and morphology segregation.
Moreover, they break the degeneracy between an inhomogeneous fractal point set
and actual present luminosity segregation. Using the Southern Sky Redshift
Survey~2 (da Costa et al. 1998, SSRS2) we find both luminosity and
morphological segregation at a high level of significance, confirming claims by
previous works using these data (Benoist et al. 1996, Willmer et al. 1998).
Specifically, the average luminosity and the fluctuations in the luminosity of
pairs of galaxies are enhanced out to separations of 15Mpc/h. On scales smaller
than 3Mpc/h the luminosities on galaxy pairs show a tight correlation. A
comparison with the random-field model indicates that galaxy luminosities
depend on the spatial distribution and galaxy-galaxy interactions. Early-type
galaxies are also more strongly correlated, indicating morphological
segregation. The galaxies in the PSCz catalog (Saunders et al. 2000) do not
show significant luminosity segregation. This again illustrates that mainly
early-type galaxies contribute to luminosity segregation. However, based on
several independent investigations we show that the observed luminosity
segregation can not be explained by the morphology-density relation alone.Comment: aastex, emulateapj5, 20 pages, 13 figures, several clarifying
comments added, ApJ accepte
Spatial distribution of galactic halos and their merger histories
We use a novel statistical tool, the mark correlation functions (MCFs), to study clustering of galaxy-size halos as a function of their properties and environment in a high-resolution numerical simulation of the LambdaCDM cosmology. We applied MCFs using several types of continuous and discrete marks: maximum circular velocity of halos, merger mark indicating whether halos experienced or not a major merger in their evolution history, and a stripping mark indicating whether the halo underwent a tidal stripping. We find that halos which experienced a relatively early (z>1) major merger or mass loss (due to tidal stripping) in their evolution histories are over-abundant in halo pairs with separations < 3 Mpc/h. This result can be interpreted as spatial segregation of halos with different merger histories, qualitatively similar to the morphological segregation in the observed galaxy distribution. The analysis presented in this paper demonstrate that MCFs provide powerful, yet algorithmically simple, quantitative measures of segregation in the spatial distribution of objects with respect to their various properties (marks)
Shape Statistics of Sloan Digital Survey superclusters
We study the supercluster shape properties of the recently compiled SDSS
cluster catalog using an approach based on differential geometry. We detect
superclusters by applying the percolation algorithm to observed cluster
populations, extended out to in order to avoid selection
biases. We utilize a set of shapefinders in order to study the morphological
features of superclusters with cluster members and find that
filamentary morphology is the dominant supercluster shape feature, in agreement
with previous studies.Comment: 6 pages, 6 figures, accepted for publication in the MNRAS, (minor
changes
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