88,628 research outputs found
Studying the properties of galaxy cluster morphology estimators
X-ray observations of galaxy clusters reveal a large range of morphologies
with various degrees of disturbance, showing that the assumptions of
hydrostatic equilibrium and spherical shape which are used to determine the
cluster mass from X-ray data are not always satisfied. It is therefore
important for the understanding of cluster properties as well as for
cosmological applications to detect and quantify substructure in X-ray images
of galaxy clusters. Two promising methods to do so are power ratios and center
shifts. Since these estimators can be heavily affected by Poisson noise and
X-ray background, we performed an extensive analysis of their statistical
properties using a large sample of simulated X-ray observations of clusters
from hydrodynamical simulations. We quantify the measurement bias and error in
detail and give ranges where morphological analysis is feasible. A new,
computationally fast method to correct for the Poisson bias and the X-ray
background contribution in power ratio and center shift measurements is
presented and tested for typical XMM-Newton observational data sets. We studied
the morphology of 121 simulated cluster images and establish structure
boundaries to divide samples into relaxed, mildly disturbed and disturbed
clusters. In addition, we present a new morphology estimator - the peak of the
0.3-1 r500 P3/P0 profile to better identify merging clusters. The analysis
methods were applied to a sample of 80 galaxy clusters observed with
XMM-Newton. We give structure parameters (P3/P0 in r500, w and P3/P0_max) for
all 80 observed clusters. Using our definition of the P3/P0 (w) substructure
boundary, we find 41% (47%) of our observed clusters to be disturbed.Comment: Replaced to match version published in A&A, Eq. 1 correcte
Deep Over-sampling Framework for Classifying Imbalanced Data
Class imbalance is a challenging issue in practical classification problems
for deep learning models as well as traditional models. Traditionally
successful countermeasures such as synthetic over-sampling have had limited
success with complex, structured data handled by deep learning models. In this
paper, we propose Deep Over-sampling (DOS), a framework for extending the
synthetic over-sampling method to exploit the deep feature space acquired by a
convolutional neural network (CNN). Its key feature is an explicit, supervised
representation learning, for which the training data presents each raw input
sample with a synthetic embedding target in the deep feature space, which is
sampled from the linear subspace of in-class neighbors. We implement an
iterative process of training the CNN and updating the targets, which induces
smaller in-class variance among the embeddings, to increase the discriminative
power of the deep representation. We present an empirical study using public
benchmarks, which shows that the DOS framework not only counteracts class
imbalance better than the existing method, but also improves the performance of
the CNN in the standard, balanced settings
Local structure of Liquid-Vapour Interfaces
The structure of a simple liquid may be characterised in terms of ground
state clusters of small numbers of atoms of that same liquid. Here we use this
sensitive structural probe to consider the effect of a liquid-vapour interface
upon the liquid structure. At higher temperatures (above around half the
critical temperature) we find that the predominant effect of the interface is
to reduce the local density, which significantly suppresses the local cluster
populations. At lower temperatures, however, pronounced interfacial layering is
found. This appears to be connected with significant orientational ordering of
clusters based on 3- and 5-membered rings, with the rings aligning
perpendicular and parallel to the interface respectively. At all temperatures,
we find that the population of five-fold symmetric structures is suppressed,
rather than enhanced, close to the interface.Comment: 10 pages, 8 figures, accepted for publication by Molecular Physic
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