1,630 research outputs found
On Bootstrap Percolation in Living Neural Networks
Recent experimental studies of living neural networks reveal that their
global activation induced by electrical stimulation can be explained using the
concept of bootstrap percolation on a directed random network. The experiment
consists in activating externally an initial random fraction of the neurons and
observe the process of firing until its equilibrium. The final portion of
neurons that are active depends in a non linear way on the initial fraction.
The main result of this paper is a theorem which enables us to find the
asymptotic of final proportion of the fired neurons in the case of random
directed graphs with given node degrees as the model for interacting network.
This gives a rigorous mathematical proof of a phenomena observed by physicists
in neural networks
The Environmental Dependence of the Infrared Luminosity and Stellar Mass Functions
We investigate the dependence of the galaxy infrared luminosity function (LF)
and the associated stellar mass function (SMF) on environment and spectral type
using photometry from the Two Micron All Sky Survey and redshifts from the Las
Campanas Redshift Survey for galaxies brighter than M_J<-19+5 log h. In the
field environment, galaxies with emission lines have LFs with much steeper
faint end slopes (alpha_J=-1.39) than galaxies without emission lines
(alpha_J=-0.59). In the cluster environment, however, even the non-emission
line galaxies have a steep faint-end LF (alpha_J=-1.22). There is also a
significant (95%) difference between the overall cluster and field LFs, . All of these variations are more
pronounced in the SMFs, which we compute by relating the strength of the 4000 A
break in the optical spectra to a mass-to-light ratio.Comment: 14 pages, 13 figures, emulateapj style ApJ, post-referee. Very minor
changes, mostly typographical in natur
Models of the ICM with Heating and Cooling: Explaining the Global and Structural X-ray Properties of Clusters
(Abridged) Theoretical models that include only gravitationally-driven
processes fail to match the observed mean X-ray properties of clusters. As a
result, there has recently been increased interest in models in which either
radiative cooling or entropy injection play a central role in mediating the
properties of the intracluster medium. Both sets of models give reasonable fits
to the mean properties of clusters, but cooling only models result in fractions
of cold baryons in excess of observationally established limits and the
simplest entropy injection models do not treat the "cooling core" structure
present in many clusters and cannot account for entropy profiles revealed by
recent X-ray observations. We consider models that marry radiative cooling with
entropy injection, and confront model predictions for the global and structural
properties of massive clusters with the latest X-ray data. The models
successfully and simultaneously reproduce the observed L-T and L-M relations,
yield detailed entropy, surface brightness, and temperature profiles in
excellent agreement with observations, and predict a cooled gas fraction that
is consistent with observational constraints. The model also provides a
possible explanation for the significant intrinsic scatter present in the L-T
and L-M relations and provides a natural way of distinguishing between clusters
classically identified as "cooling flow" clusters and dynamically relaxed
"non-cooling flow" clusters. The former correspond to systems that had only
mild levels (< 300 keV cm^2) of entropy injection, while the latter are
identified as systems that had much higher entropy injection. This is borne out
by the entropy profiles derived from Chandra and XMM-Newton.Comment: 20 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Evolution of Group Galaxies from the First Red-Sequence Cluster Survey
We study the evolution of the red galaxy fraction (f_red) in 905 galaxy
groups with 0.15 < z < 0.52. The galaxy groups are identified by the
`probability Friends-of-Friends' algorithm from the first Red-Sequence Cluster
Survey (RCS1) photometric-redshift sample. There is a high degree of uniformity
in the properties of the red-sequence of the group galaxies, indicating that
the luminous red-sequence galaxies in the groups are already in place by z~0.5
and that they have a formation epoch of z>2. In general, groups at lower
redshifts exhibit larger f_red than those at higher redshifts, showing a group
Butcher-Oemler effect. We investigate the evolution of f_red by examining its
dependence on four parameters, which can be classified as one intrinsic and
three environmental: galaxy stellar mass (M_*), total group stellar
mass(M_{*,grp}, a proxy for group halo mass), normalized group-centric radius
(r_grp), and local galaxy density (Sigma_5). We find that M_* is the dominant
parameter such that there is a strong correlation between f_red and galaxy
stellar mass. Furthermore, the dependence of f_red on the environmental
parameters is also a strong function of M_*. Massive galaxies (M_* > 10^11
M_sun) show little dependence of f_red on r_grp, M_{*,grp}, and Sigma_5 over
the redshift range. The dependence of f_red on these parameters is primarily
seen for galaxies with lower masses, especially for M_* < 10^{10.6} M_{sun}. We
observe an apparent `group down-sizing' effect, in that galaxies in lower-mass
halos, after controlling for galaxy stellar mass, have lower f_red. We find a
dependence of \fred on both \rgrp and \SigmaF after the other parameters are
controlled. At a fixed \rgrp, there is a significant dependence of f_red on
Sigma_5, while r_grp gradients of f_red are seen for galaxies in similar
Sigma_5 regions. This indicates .....Comment: ApJ accepte
WINGS: a WIde-field Nearby Galaxy-cluster Survey. I - Optical imaging
This is the first paper of a series that will present data and scientific
results from the WINGS project, a wide-field, multiwavelength imaging and
spectroscopic survey of galaxies in 77 nearby clusters. The sample was
extracted from the ROSAT catalogs with constraints on the redshift
(0.0420). The global goal of
the WINGS project is the systematic study of the local cosmic variance of the
cluster population and of the properties of cluster galaxies as a function of
cluster properties and local environment. This data collection will allow to
define a local 'Zero-Point' reference against which to gauge the cosmic
evolution when compared to more distant clusters. The core of the project
consists of wide-field optical imaging of the selected clusters in the B and V
bands. We have also completed a multi-fiber, medium resolution spectroscopic
survey for 51 of the clusters in the master sample. In addition, a NIR (JK)
survey of ~50 clusters and an H_alpha + UV survey of some 10 clusters are
presently ongoing, while a very-wide-field optical survey has also been
programmed. In this paper we briefly outline the global objectives and the main
characteristics of the WINGS project. Moreover, the observing strategy and the
data reduction of the optical imaging survey (WINGS-OPT) are presented. We have
achieved a photometric accuracy of ~0.025mag, reaching completeness to V~23.5.
Field size and resolution (FWHM) span the absolute intervals (1.6-2.7)Mpc and
(0.7-1.7)kpc, respectively, depending on the redshift and on the seeing. This
allows the planned studies to get a valuable description of the local
properties of clusters and galaxies in clusters.Comment: 24 pages, 15 figures, Accepted by Astronomy and Astrophysic
An ASCA Study of the Heavy Element Distribution in Clusters of Galaxies
We perform a spatially resolved X-ray spectroscopic study of a set of 11
relaxed clusters of galaxies observed by the ROSAT/PSPC and ASCA/SIS. Using a
method which corrects for the energy dependent effects of the ASCA PSF based on
ROSAT images, we constrain the spatial distribution of Ne, Si, S and Fe in each
cluster. Theoretical prescriptions for the chemical yields of Type Ia and II
supernovae, then allow determination of the Fe enrichment from both types of
supernovae as a function of radius within each cluster. Using optical
measurements from the literature, we also determine the iron mass-to-light
ratio (IMLR) separately for Fe synthesized in both types of supernovae. For
clusters with the best photon statistics, we find that the total Fe abundance
decreases significantly with radius, while the Si abundance is either flat or
decreases less rapidly, resulting in an increasing Si/Fe ratio with radius.
This result indicates a greater predominance of Type II SNe enrichment at large
radii in clusters. We suggest that the high Si/Fe ratios in the outskirts of
rich clusters may arise from enrichment by Type II SNe released to ICM via
galactic star burst driven winds. Abridged.Comment: 17 pages, ApJ in press (Nov. 2000), a study of systematics is adde
Investigation of fiber/matrix adhesion: test speed and specimen shape effects in the cylinder test
The cylinder test, developed from the microdroplet test, was adapted to assess the interfacial adhesion strength between fiber and matrix. The sensitivity of cylinder test to pull-out speed and specimen geometry was measured. It was established that the effect of test speed can be described as a superposition of two opposite, simultaneous effects which have been modeled mathematically by fitting two parameter Weibull curves on the measured datas. Effects of the cylinder size and its geometrical relation on the measured strength values have been analyzed by finite element method. It was concluded that the geometry has a direct influence on the stress formation. Based on the results achieved, recommendations were given on how to perform the novel single fiber cylinder test
Clustering and the hyperbolic geometry of complex networks
Clustering is a fundamental property of complex networks and it is the
mathematical expression of a ubiquitous phenomenon that arises in various types
of self-organized networks such as biological networks, computer networks or
social networks. In this paper, we consider what is called the global
clustering coefficient of random graphs on the hyperbolic plane. This model of
random graphs was proposed recently by Krioukov et al. as a mathematical model
of complex networks, under the fundamental assumption that hyperbolic geometry
underlies the structure of these networks. We give a rigorous analysis of
clustering and characterize the global clustering coefficient in terms of the
parameters of the model. We show how the global clustering coefficient can be
tuned by these parameters and we give an explicit formula for this function.Comment: 51 pages, 1 figur
Control of quality and silo storage of sunflower seeds using near infrared technology
This work assesses the application of near infrared spectroscopy technology for the quality control of sunflower seeds direct from farmers and from a storage silo. The results show that the analytical method employing near infrared spectroscopy can be used as a rapid and non-destructive tool for the determination of moisture, fat and high/low oleic acid contents in samples of sunflower seeds. The ranges obtained were comparable to those reported for classic chemical methods, and were between 4.6-21.4% for moisture; 38.4-49.6% for fat, and 60.0-93.1% for oleic acid expressed as percentage of total fatty acids. A stepwise discriminant analysis was performed to determine the most useful wavelengths for classifying sunflower seeds in terms of their (high/low) oleic acid composition. The discriminant model allows the classification of sunflower seeds with high or low oleic acid contents, with a prediction rate of 90.5% for internal validation and of 89.4% for cross-validation.<br><br>En este trabajo se evalúa la espectroscopía de infrarrojo cercano para su uso en el control de calidad y almacenamiento de semillas de girasol. Los resultados indican que el método analítico empleado puede utilizarse como método de determinación rápida de humedad, grasa y contenidos altos/bajos de ácido oleico. Los rangos de aplicación son comparables con los valores que se han determinado mediante métodos clásicos de análisis, encontrándose entre 4.6-21.4% la humedad, 38.4-49.6% la grasa y 60.0- 93.1% de ácido oleico del total de los ácidos grasos. Además se ha utilizado un análisis discriminarte lineal por pasos determinando las longitudes de onda más adecuadas para la clasificación de semillas de girasol en los grupos alto/bajo oleico. El modelo generado permitió la clasificación de semillas de girasol en los grupos alto y bajo oleico con unos porcentajes de muestras correctamente clasificadas de un 90.5% en validación interna y de un 89.4% en validación cruzada
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