17,204 research outputs found
The relation between gas density and velocity power spectra in galaxy clusters: high-resolution hydrodynamic simulations and the role of conduction
Exploring the ICM power spectrum can help us to probe the physics of galaxy
clusters. Using high-resolution 3D plasma simulations, we study the statistics
of the velocity field and its relation with the thermodynamic perturbations.
The normalization of the ICM spectrum (density, entropy, or pressure) is
linearly tied to the level of large-scale motions, which excite both gravity
and sound waves due to stratification. For low 3D Mach number M~0.25, gravity
waves mainly drive entropy perturbations, traced by preferentially tangential
turbulence. For M>0.5, sound waves start to significantly contribute, passing
the leading role to compressive pressure fluctuations, associated with
isotropic (or slightly radial) turbulence. Density and temperature fluctuations
are then characterized by the dominant process: isobaric (low M), adiabatic
(high M), or isothermal (strong conduction). Most clusters reside in the
intermediate regime, showing a mixture of gravity and sound waves, hence
drifting towards isotropic velocities. Remarkably, regardless of the regime,
the variance of density perturbations is comparable to the 1D Mach number. This
linear relation allows to easily convert between gas motions and ICM
perturbations, which can be exploited by Chandra, XMM data and by the
forthcoming Astro-H. At intermediate and small scales (10-100 kpc), the
turbulent velocities develop a Kolmogorov cascade. The thermodynamic
perturbations act as effective tracers of the velocity field, broadly
consistent with the Kolmogorov-Obukhov-Corrsin advection theory. Thermal
conduction acts to damp the gas fluctuations, washing out the filamentary
structures and steepening the spectrum, while leaving unaltered the velocity
cascade. The ratio of the velocity and density spectrum thus inverts the
downtrend shown by the non-diffusive models, allowing to probe the presence of
significant conductivity in the ICM.Comment: Accepted by A&A; 15 pages, 10 figures; added insights and references
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Quantifying properties of ICM inhomogeneities
We present a new method to identify and characterize the structure of the
intracluster medium (ICM) in simulated galaxy clusters. The method uses the
median of gas properties, such as density and pressure, which we show to be
very robust to the presence of gas inhomogeneities. In particular, we show that
the radial profiles of median gas properties are smooth and do not exhibit
fluctuations at locations of massive clumps in contrast to mean and mode
properties. It is shown that distribution of gas properties in a given radial
shell can be well described by a log-normal PDF and a tail. The former
corresponds to a nearly hydrostatic bulk component, accounting for ~99% of the
volume, while the tail corresponds to high density inhomogeneities. We show
that this results in a simple and robust separation of the diffuse and clumpy
components of the ICM. The FWHM of the density distribution grows with radius
and varies from ~0.15 dex in cluster centre to ~0.5 dex at 2r_500 in relaxed
clusters. The small scatter in the width between relaxed clusters suggests that
the degree of inhomogeneity is a robust characteristic of the ICM. It broadly
agrees with the amplitude of density perturbations in the Coma cluster. We
discuss the origin of ICM density variations in spherical shells and show that
less than 20% of the width can be attributed to the triaxiality of the cluster
gravitational potential. As a link to X-ray observations of real clusters we
evaluated the ICM clumping factor with and without high density
inhomogeneities. We argue that these two cases represent upper and lower limits
on the departure of the observed X-ray emissivity from the median value. We
find that the typical value of the clumping factor in the bulk component of
relaxed clusters varies from ~1.1-1.2 at r_500 up to ~1.3-1.4 at r_200, in
broad agreement with recent observations.Comment: 16 pages, 12 figure, accepted to MNRA
Sixth Sense Transport : Challenges in Supporting Flexible Time Travel
In this paper, we consider the challenges associated with providing a mobile computing system that helps users enjoy a
more flexible relationship between time and travel. Current
travel plans, especially in Western cultures, are dominated
by a strict notion of time. The need to conform to schedules
leads to increased pressures for travellers and inefficiencies when these schedules cannot be met. We are interested in exploring the extent to which mobile computing can be used to help travellers relax these schedules and adopt a more opportunistic approach to travel – potentially helping to reduce the environmental, financial and societal costs of modern travel
The relation between gas density and velocity power spectra in galaxy clusters: qualitative treatment and cosmological simulations
We address the problem of evaluating the power spectrum of the velocity field
of the ICM using only information on the plasma density fluctuations, which can
be measured today by Chandra and XMM-Newton observatories. We argue that for
relaxed clusters there is a linear relation between the rms density and
velocity fluctuations across a range of scales, from the largest ones, where
motions are dominated by buoyancy, down to small, turbulent scales:
, where
is the spectral amplitude of the density perturbations at wave number ,
is the mean square component of the velocity field,
is the sound speed, and is a dimensionless constant of order unity.
Using cosmological simulations of relaxed galaxy clusters, we calibrate this
relation and find . We argue that this value is set at
large scales by buoyancy physics, while at small scales the density and
velocity power spectra are proportional because the former are a passive scalar
advected by the latter. This opens an interesting possibility to use gas
density power spectra as a proxy for the velocity power spectra in relaxed
clusters, across a wide range of scales.Comment: 6 pages, 3 figures, submitted to ApJ Letter
New variables, the gravitational action, and boosted quasilocal stress-energy-momentum
This paper presents a complete set of quasilocal densities which describe the
stress-energy-momentum content of the gravitational field and which are built
with Ashtekar variables. The densities are defined on a two-surface which
bounds a generic spacelike hypersurface of spacetime. The method used
to derive the set of quasilocal densities is a Hamilton-Jacobi analysis of a
suitable covariant action principle for the Ashtekar variables. As such, the
theory presented here is an Ashtekar-variable reformulation of the metric
theory of quasilocal stress-energy-momentum originally due to Brown and York.
This work also investigates how the quasilocal densities behave under
generalized boosts, i. e. switches of the slice spanning . It is
shown that under such boosts the densities behave in a manner which is similar
to the simple boost law for energy-momentum four-vectors in special relativity.
The developed formalism is used to obtain a collection of two-surface or boost
invariants. With these invariants, one may ``build" several different mass
definitions in general relativity, such as the Hawking expression. Also
discussed in detail in this paper is the canonical action principle as applied
to bounded spacetime regions with ``sharp corners."Comment: Revtex, 41 Pages, 4 figures added. Final version has been revised and
improved quite a bit. To appear in Classical and Quantum Gravit
The Mean and Scatter of the Velocity Dispersion-Optical Richness Relation for maxBCG Galaxy Clusters
The distribution of galaxies in position and velocity around the centers of
galaxy clusters encodes important information about cluster mass and structure.
Using the maxBCG galaxy cluster catalog identified from imaging data obtained
in the Sloan Digital Sky Survey, we study the BCG-galaxy velocity correlation
function. By modeling its non-Gaussianity, we measure the mean and scatter in
velocity dispersion at fixed richness. The mean velocity dispersion increases
from 202+/-10 km/s for small groups to more than 854+/-102 km/s for large
clusters. We show the scatter to be at most 40.5+/-3.5%, declining to
14.9+/-9.4% in the richest bins. We test our methods in the C4 cluster catalog,
a spectroscopic cluster catalog produced from the Sloan Digital Sky Survey DR2
spectroscopic sample, and in mock galaxy catalogs constructed from N-body
simulations. Our methods are robust, measuring the scatter to well within
one-sigma of the true value, and the mean to within 10%, in the mock catalogs.
By convolving the scatter in velocity dispersion at fixed richness with the
observed richness space density function, we measure the velocity dispersion
function of the maxBCG galaxy clusters. Although velocity dispersion and
richness do not form a true mass-observable relation, the relationship between
velocity dispersion and mass is theoretically well characterized and has low
scatter. Thus our results provide a key link between theory and observations up
to the velocity bias between dark matter and galaxies.Comment: 25 pages, 15 figures, 2 tables, published in Ap
A millimeter-wave antireflection coating for cryogenic silicon lenses
We have developed and tested an antireflection (AR) coating method for
silicon lenses at cryogenic temperatures and millimeter wavelengths. Our
particular application is a measurement of the cosmic microwave background. The
coating consists of machined pieces of Cirlex glued to the silicon. The
measured reflection from an AR coated flat piece is less than 1.5% at the
design wavelength. The coating has been applied to flats and lenses and has
survived multiple thermal cycles from 300 to 4 K. We present the manufacturing
method, the material properties, the tests performed, and estimates of the loss
that can be achieved in practical lenses
Outcomes of interferon alpha and ribavirin treatment for chronic hepatitis C in patients with normal serum aminotransaminases
INTRODUCTION: Information on treatment outcomes with interferon plus ribavirin combination therapy in chronic hepatitis C patients with normal alanine aminotransaminase (ALT) levels is limited. AIM: The aims of this study were to assess outcomes of treatment with interferon plus ribavirin in patients with normal ALT levels (normal ALT group, n=52) compared with those with elevated ALT levels (raised ALT group, n=53), and to document the rate at which patients with normal ALT levels have an apparent worsening of disease, as shown by increases in ALT levels. RESULTS: At the end of treatment (week 48), 31 patients (59.6%) in the normal ALT group and 30 patients (56.6%) in the raised ALT group had undetectable hepatitis C virus (HCV) RNA (p=0.75). A sustained virological response (SVR) was achieved in 20 patients (38.5%) in the normal ALT group and in 21 patients (39.6%) in the raised ALT group (p=0.90). Patients were subsequently followed up for a median of 29.8 (interquartile range 25th-75th percentile (IQR) 20.8-36.2) months in the normal ALT group and for a median of 26.1 (IQR 17.7-36.3) months in the raised group (p=0.20) after week 72 of treatment. Among patients without SVR in the normal ALT group, only three patients (9.4%) developed persistently raised ALT levels following therapy. CONCLUSIONS: Combination therapy with interferon plus ribavirin is associated with a similar SVR in patients with normal ALT levels compared with those with elevated ALT levels. In patients with normal ALT levels, virological non-response to therapy results in new elevations in serum ALT levels in a small minority only.published_or_final_versio
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