1,609 research outputs found

### The Effects of Clumping and Substructure on ICM Mass Measurements

We examine an ensemble of 48 simulated clusters to determine the effects of
small-scale density fluctuations and large-scale substructure on X-ray
measurements of the intracluster medium (ICM) mass. We measure RMS density
fluctuations in the ICM which can be characterized by a mean mass-weighted
clumping factor C = /^2 between 1.3 and 1.4 within a density
contrast of 500 times the critical density. These fluctuations arise from the
cluster history of accretion shocks and major mergers, and their presence
enhances the cluster's luminosity relative to the smooth case. We expect,
therefore, that ICM mass measurements utilizing models which assume uniform
density at a given radius carry a bias of order sqrt(C) = 1.16. We verify this
result by performing ICM mass measurements on X-ray images of the simulations
and finding the expected level of bias.
The varied cluster morphologies in our ensemble also allow us to investigate
the effects of departures from spherical symmetry on our measurements. We find
that the presence of large-scale substructure does not further bias the
resulting gas mass unless it is pronounced enough to produce a second peak in
the image of at least 1% the maximum surface brightness. We analyze the subset
of images with no secondary peaks and find a bias of 9% and a Gaussian random
error of 4% in the derived mass.Comment: To appear in ApJ

### Artificial viscosity model to mitigate numerical artefacts at fluid interfaces with surface tension

The numerical onset of parasitic and spurious artefacts in the vicinity of uid interfaces with surface tension is an important and well-recognised problem with respect to the accuracy and numerical stability of interfacial ow simulations. Issues of particular interest are spurious capillary waves, which are spatially underresolved by the computational mesh yet impose very restrictive time-step requirements, as well as parasitic currents, typically the result of a numerically unbalanced curvature evaluation. We present an arti cial viscosity model to mitigate numerical artefacts at surface-tension-dominated interfaces without adversely a ecting the accuracy of the physical solution. The proposed methodology computes an additional interfacial shear stress term, including an interface viscosity, based on the local ow data and uid properties that reduces the impact of numerical artefacts and dissipates underresolved small scale interface movements. Furthermore, the presented methodology can be readily applied to model surface shear viscosity, for instance to simulate the dissipative e ect of surface-active substances adsorbed at the interface. The presented analysis of numerical test cases demonstrates the e cacy of the proposed methodology in diminishing the adverse impact of parasitic and spurious interfacial artefacts on the convergence and stability of the numerical solution algorithm as well as on the overall accuracy of the simulation results

### Constraints on \Omega_0 and Cluster Evolution Using the ROSAT LogN-LogS

We examine the likelihoods of different cosmological models and cluster
evolutionary histories by comparing semi-analytical predictions of X-ray
cluster number counts to observational data from the ROSAT satellite. We model
cluster abundance as a function of mass and redshift using a Press-Schechter
distribution, and assume the temperature T(M,z) and bolometric luminosity
L_X(M,z) scale as power laws in mass and epoch, in order to construct expected
counts as a function of X-ray flux. The L_X-M scaling is fixed using the local
luminosity function while the degree of evolution in the X-ray luminosity with
redshift L_X \propto (1+z)^s is left open, with s an interesting free parameter
which we investigate. We examine open and flat cosmologies with initial,
scale-free fluctuation spectra having indices n = 0, -1 and -2. An independent
constraint arising from the slope of the luminosity-temperature relation
strongly favors the n = -2 spectrum.
The expected counts demonstrate a strong dependence on \Omega_0 and s, with
lesser dependence on \lambda_0 and n. Comparison with the observed counts
reveals a "ridge" of acceptable models in the \Omega_0 - s plane, roughly
following the relation s = 6 \Omega_0 and spanning low-density models with a
small degree of evolution to \Omega = 1 models with strong evolution. Models
with moderate evolution are revealed to have a strong lower limit of \Omega_0
\gtrsim 0.3, and low-evolution models imply that \Omega_0 < 1 at a very high
confidence level. We suggest observational tests for breaking the degeneracy
along this ridge, and discuss implications for evolutionary histories of the
intracluster medium.Comment: MNRAS LaTeX style format, submitted to MNRAS 3/26/97. Thirteen pages,
eleven postscript figures. Uses epsf macros to include figure

### Effects of Selection and Covariance on X-ray Scaling Relations of Galaxy Clusters

We explore how the behavior of galaxy cluster scaling relations are affected
by flux-limited selection biases and intrinsic covariance among observable
properties. Our models presume log-normal covariance between luminosity (L) and
temperature (T) at fixed mass (M), centered on evolving, power-law mean
relations as a function of host halo mass. Selection can mimic evolution; the
\lm and \lt relations from shallow X-ray flux-limited samples will deviate from
mass-limited expectations at nearly all scales while the relations from deep
surveys (10^{-14} \cgsflux) become complete, and therefore unbiased, at
masses above \sims 2 \times 10^{14} \hinv \msol. We derive expressions for
low-order moments of the luminosity distribution at fixed temperature, and show
that the slope and scatter of the \lt relation observed in flux-limited samples
is sensitive to the assumed \lt correlation coefficient. In addition, \lt
covariance affects the redshift behavior of halo counts and mean luminosity in
a manner that is nearly degenerate with intrinsic population evolution.Comment: 5pages, 4 Figures, Submitted to MNRA

### Estimation of curvature from volume fractions using parabolic reconstruction on two-dimensional unstructured meshes

This paper proposes a method to estimate the curvature of an interface represented implicitly by discrete volume fractions on an unstructured two-dimensional mesh. The method relies on the computation of local parabolic reconstructions of the interface. The parabolic reconstruction of the interface in a given computational cell is obtained by solving a local non-linear minimisation problem, and only requires additional information from two neighbouring cells. This compactness ensures a robust behaviour on poorly-resolved interfaces. The proposed method is proven to be analogous to the height-function method for Cartesian configurations with consistent heights, and can be interpreted as a generalisation of the height-function method to meshes of any type. Tests are conducted on a range of interfaces with known curvature. The method is shown to converge with mesh refinement with the same order of accuracy as the height-function method for all three types of meshes tested, i.e. Cartesian, triangular, and polygonal

### Cosmological Parameters from Observations of Galaxy Clusters

Studies of galaxy clusters have proved crucial in helping to establish the
standard model of cosmology, with a universe dominated by dark matter and dark
energy. A theoretical basis that describes clusters as massive,
multi-component, quasi-equilibrium systems is growing in its capability to
interpret multi-wavelength observations of expanding scope and sensitivity. We
review current cosmological results, including contributions to fundamental
physics, obtained from observations of galaxy clusters. These results are
consistent with and complementary to those from other methods. We highlight
several areas of opportunity for the next few years, and emphasize the need for
accurate modeling of survey selection and sources of systematic error.
Capitalizing on these opportunities will require a multi-wavelength approach
and the application of rigorous statistical frameworks, utilizing the combined
strengths of observers, simulators and theorists.Comment: 53 pages, 21 figures. To appear in Annual Review of Astronomy &
Astrophysic

### TraĂ§abilitĂ© dans la filiĂ¨re viande. I. La traĂ§abilitĂ© administrative.

peer reviewedLe secteur de la viande a Ă©tĂ© secouĂ© ces derniĂ¨res annĂ©es par quelques scandales, tels ceux
des hormones et de la dioxine, avec pour consĂ©quences une perte de confiance de la part du consommateur
et une perturbation du marchĂ© de la viande. Pour redresser lâ€™image des produits carnĂ©s belges, il
est important de pouvoir en dĂ©terminer et en garantir lâ€™origine. En Belgique, il existe divers systĂ¨mes de
traĂ§abilitĂ© administrative dont le principal est le systĂ¨me SANITEL qui comprend un systĂ¨me automatisĂ©
de traitement de donnĂ©es relatives Ă lâ€™identification et lâ€™enregistrement des animaux. Au-delĂ de lâ€™aspect
lĂ©gal et rĂ©glementaire, diffĂ©rentes initiatives, visant une amĂ©lioration de la qualitĂ©, fleurissent : "les labels".
Ceux-ci intĂ¨grent frĂ©quemment la traĂ§abilitĂ© dans leur cahier des charges.
La traĂ§abilitĂ© administrative nâ€™est pas infaillible, la perte de documents et les fraudes peuvent ternir lâ€™image
de celle-ci. Câ€™est pourquoi le systĂ¨me documentaire a Ă©tĂ© associĂ© aux empreintes gĂ©nĂ©tiques des animaux.

### Four Measures of the Intracluster Medium Temperature and Their Relation to a Cluster's Dynamical State

We employ an ensemble of hydrodynamic cluster simulations to create spatially
and spectrally resolved images of quality comparable to Chandra's expected
performance. Emission from simulation mass elements is represented using the
XSPEC mekal program assuming 0.3 solar metallicity, and the resulting spectra
are fit with a single-temperature model. Despite significant departures from
isothermality in the cluster gas, single-temperature models produce acceptable
fits to 20,000 source photon spectra. The spectral fit temperature T_s is
generally lower than the mass weighted average temperature T_m due to the
influence of soft line emission from cooler gas being accreted as part of the
hierarchical clustering process. In a Chandra-like bandpass of 0.5 to 9.5 keV
we find a nearly uniform fractional bias of (T_m-T_s)/T_s = 20% with occasional
large deviations in smaller clusters. In the more traditional 2.0 to 9.5 keV
bandpass, the fractional deviation is scale-dependent and on average follows
the relation (T_m-T_s)/T_s = 0.2 log(T_m). This bias results in a spectral
mass-temperature relationship with slope about 1.6, intermediate between the
virial relation M ~ T_m^{3/2} and the observed relation M_{ICM} ~ T^2. Imaging
each cluster in the ensemble at 16 epochs in its evolutionary history, we
catalogue merger events with mass ratios exceeding 10% in order to investigate
the relationship between spectral temperature and proximity to a major merger
event. Clusters that are very cool relative to the mean mass-temperature
relationship lie preferentially close to a merger, suggesting a viable
observational method to cull a subset of dynamically young clusters from the
general population.Comment: 34 pages, including 2 tables and 14 figures (one in color). Compiled
using LaTeX 2.09 with graphics package and aaspp4 style. The simulated
spectral data files used in this paper are available for public consumption
at http://redshift.stanford.edu/bfm

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