1,567 research outputs found
On the Dynamical Origin of Bias in Clusters of Galaxies
We study the effect of the dynamical friction induced by the presence of
substructure on the statistics of the collapse of density peaks. Applying the
results of a former paper we show that within high density environments, like
rich clusters of galaxies, the collapse of smaller peaks is strongly delayed
until very late epochs. A bias of dynamical nature thus naturally arises
because high density peaks preferentially collapse For a standard CDM model we
find that this dynamical bias can account for a substantial part of the total
bias required by observations on cluster scales.Comment: 14 pages, postscript, 3 postscript figures available by anonymous ftp
on convex.ct.astro.it in /nbody/fig#.ps. Submitted to Astrophysical Journal
Letter
An automatic system to discriminate malignant from benign massive lesions in mammograms
Evaluating the degree of malignancy of a massive lesion on the basis of the
mere visual analysis of the mammogram is a non-trivial task. We developed a
semi-automated system for massive-lesion characterization with the aim to
support the radiological diagnosis. A dataset of 226 masses has been used in
the present analysis. The system performances have been evaluated in terms of
the area under the ROC curve, obtaining A_z=0.80+-0.04.Comment: 4 pages, 2 figure; Proceedings of the Frontier Science 2005, 4th
International Conference on Frontier Science, 12-17 September, 2005, Milano,
Ital
Substructure recovery by 3D Discrete Wavelet Transforms
We present and discuss a method to identify substructures in combined
angular-redshift samples of galaxies within Clusters. The method relies on the
use of Discrete Wavelet Transform (hereafter DWT) and has already been applied
to the analysis of the Coma cluster (Gambera et al. 1997). The main new
ingredient of our method with respect to previous studies lies in the fact that
we make use of a 3D data set rather than a 2D. We test the method on mock
cluster catalogs with spatially localized substructures and on a N-body
simulation. Our main conclusion is that our method is able to identify the
existing substructures provided that: a) the subclumps are detached in part or
all of the phase space, b) one has a statistically significant number of
redshifts, increasing as the distance decreases due to redshift distortions; c)
one knows {\it a priori} the scale on which substructures are to be expected.
We have found that to allow an accurate recovery we must have both a
significant number of galaxies ( for clusters at z or
about 800 at z 0.4) and a limiting magnitude for completeness .
The only true limitation to our method seems to be the necessity of knowing
{\it a priori} the scale on which the substructure is to be found. This is an
intrinsic drawback of the method and no improvement in numerical codes based on
this technique could make up for it.Comment: Accepted for publication in MNRAS. 7 pages, 2 figure
Computer-aided detection of pulmonary nodules in low-dose CT
A computer-aided detection (CAD) system for the identification of pulmonary
nodules in low-dose multi-detector helical CT images with 1.25 mm slice
thickness is being developed in the framework of the INFN-supported MAGIC-5
Italian project. The basic modules of our lung-CAD system, a dot enhancement
filter for nodule candidate selection and a voxel-based neural classifier for
false-positive finding reduction, are described. Preliminary results obtained
on the so-far collected database of lung CT scans are discussed.Comment: 3 pages, 4 figures; Proceedings of the CompIMAGE - International
Symposium on Computational Modelling of Objects Represented in Images:
Fundamentals, Methods and Applications, 20-21 Oct. 2006, Coimbra, Portuga
A scalable system for microcalcification cluster automated detection in a distributed mammographic database
A computer-aided detection (CADe) system for microcalcification cluster
identification in mammograms has been developed in the framework of the
EU-founded MammoGrid project. The CADe software is mainly based on wavelet
transforms and artificial neural networks. It is able to identify
microcalcifications in different datasets of mammograms (i.e. acquired with
different machines and settings, digitized with different pitch and bit depth
or direct digital ones). The CADe can be remotely run from GRID-connected
acquisition and annotation stations, supporting clinicians from geographically
distant locations in the interpretation of mammographic data. We report and
discuss the system performances on different datasets of mammograms and the
status of the GRID-enabled CADe analysis.Comment: 6 pages, 4 figures; Proceedings of the IEEE NNS and MIC Conference,
October 23-29, 2005, Puerto Ric
3D simulations of the early stages of AGN jets: geometry, thermodynamics and backflow
We investigate the interplay between jets from Active Galactic Nuclei (AGNs)
and the surrounding InterStellar Medium (ISM) through full 3D, high resolution,
Adaptive Mesh Refinement simulations performed with the FLASH code. We follow
the jet- ISM system for several Myr in its transition from an early, compact
source to an extended one including a large cocoon. During the jet evolution,
we identify three major evolutionary stages and we find that, contrary to the
prediction of popular theoretical models, none of the simulations shows a
self-similar behavior. We also follow the evolution of the energy budget, and
find that the fraction of input power deposited into the ISM (the AGN coupling
constant) is of order of a few percent during the first few Myr. This is in
broad agreement with galaxy formation models employing AGN feedback. However,
we find that in these early stages, this energy is deposited only in a small
fraction (< 1%) of the total ISM volume. Finally we demonstrate the relevance
of backflows arising within the extended cocoon generated by a relativistic AGN
jet within the ISM of its host galaxy, previously proposed as a mechanism for
self-regulating the gas accretion onto the central object. These backflows tend
later to be destabilized by the 3D dynamics, rather than by hydrodynamic
(Kelvin- Helmholtz) instabilities. Yet, in the first few hundred thousand
years, backflows may create a central accretion region of significant extent,
and convey there as much as a few millions of solar masses.Comment: Accepted in MNRAS - 16 pages, 12 figures - Multimedia available on
the author's webpage: http://www.mpia.de/~ciel
Properties of galaxy halos in Clusters and Voids
We use the results of a high resolution N-body simulation to investigate the
role of the environment on the formation and evolution of galaxy-sized halos.
Starting from a set of constrained initial conditions, we have produced a final
configuration hosting a double cluster in one octant and a large void extending
over two octants of the simulation box. We present results for two statistics:
the relationship between 1-D velocity dispersion and mass and the probability
distribution of the spin parameter . The \svm relationship is well
reproduced by the Truncated Isothermal Sphere (TIS) model introduced by Shapiro
et al. (1999), although the slope is different from the original prediction. A
series of \svm relationships for different values of the anisotropy parameter
, obtained using the theoretical predictions by Lokas and Mamon (2001)
for NFW density profiles are found to be only marginally consistent with the
data. Using some properties of the equilibrium TIS models, we construct
subsamples of {\em fiducial} equilibrium TIS halos from each of the three
subregions, and we study their properties. For these halos, we do find an
environmental dependence of their properties, in particular of the spin
parameter distribution . We study in more detail the TIS model, and
we find new relationships between the truncation radius and other structural
parameters. No gravitationally bound halo is found having a radius larger than
the critical value for gravithermal instability for TIS halos (\rt , where is the core radius of the TIS solution). We do however
find a dependence of this relationship on the environment, like for the
statistics. These facts hint at a possible r\^{o}le of tidal
fields at determining the statistical properties of halos.Comment: 12 pages, 14 figures. Accepted by MNRAS. Adopted an improved
algorithm for halo finding and added a comparison with NFW model
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