3,443 research outputs found
A universal density slope - velocity anisotropy relation for relaxed structures
We identify a universal relation between the radial density slope \alpha (r)
and the velocity anisotropy \beta (r) for equilibrated structures. This
relation holds for a variety of systems, including disk galaxy mergers,
spherical collapses, cold dark matter (CDM) halos both with and without
cooling. We argue that the shape of the relation is reasonable from fundamental
principles when the dark matter or stars are assumed to obey Tsallis
statistics, and in that case we fit the \alpha - \beta relation with just one
free parameter. One can use this result to close the Jeans equations, for
example to construct mass models of elliptical galaxies from observational data
or to tune dark matter direct detection experiments. We also predict the
asymptotic central slope and anisotropy of CDM halos to be approximately -1 and
0.Comment: 11 pages, 2 figures. Extended discussion. Conclusions unchanged.
Matches accepted versio
A universal velocity distribution of relaxed collisionless structures
Several general trends have been identified for equilibrated,
self-gravitating collisionless systems, such as density or anisotropy profiles.
These are integrated quantities which naturally depend on the underlying
velocity distribution function (VDF) of the system. We study this VDF through a
set of numerical simulations, which allow us to extract both the radial and the
tangential VDF. We find that the shape of the VDF is universal, in the sense
that it depends only on two things namely the dispersion (radial or tangential)
and the local slope of the density. Both the radial and the tangential VDF's
are universal for a collection of simulations, including controlled collisions
with very different initial conditions, radial infall simulation, and
structures formed in cosmological simulations.Comment: 13 pages, 6 figures; oversimplified analysis corrected; changed
abstract and conclusions; significantly extended discussio
What it takes to measure a fundamental difference between dark matter and baryons: the halo velocity anisotropy
Numerous ongoing experiments aim at detecting WIMP dark matter particles from
the galactic halo directly through WIMP-nucleon interactions. Once such a
detection is established a confirmation of the galactic origin of the signal is
needed. This requires a direction-sensitive detector. We show that such a
detector can measure the velocity anisotropy beta of the galactic halo.
Cosmological N-body simulations predict the dark matter anisotropy to be
nonzero, beta~0.2. Baryonic matter has beta=0 and therefore a detection of a
nonzero beta would be strong proof of the fundamental difference between dark
and baryonic matter. We estimate the sensitivity for various detector
configurations using Monte Carlo methods and we show that the strongest signal
is found in the relatively few high recoil energy events. Measuring beta to the
precision of ~0.03 will require detecting more than 10^4 WIMP events with
nuclear recoil energies greater than 100 keV for a WIMP mass of 100 GeV and a
32S target. This number corresponds to ~10^6 events at all energies. We discuss
variations with respect to input parameters and we show that our method is
robust to the presence of backgrounds and discuss the possible improved
sensitivity for an energy-sensitive detector.Comment: 15 pages, 8 figures, accepted by JCAP. Matches accepted versio
Measurement of the dark matter velocity anisotropy in galaxy clusters
The internal dynamics of a dark matter structure may have the remarkable
property that the local temperature in the structure depends on direction. This
is parametrized by the velocity anisotropy beta which must be zero for relaxed
collisional structures, but has been shown to be non-zero in numerical
simulations of dark matter structures. Here we present a method to infer the
radial profile of the velocity anisotropy of the dark matter halo in a galaxy
cluster from X-ray observables of the intracluster gas. This non-parametric
method is based on a universal relation between the dark matter temperature and
the gas temperature which is confirmed through numerical simulations. We apply
this method to observational data and we find that beta is significantly
different from zero at intermediate radii. Thus we find a strong indication
that dark matter is effectively collisionless on the dynamical time-scale of
clusters, which implies an upper limit on the self-interaction cross-section
per unit mass sigma/m < 1 cm2/g. Our results may provide an independent way to
determine the stellar mass density in the central regions of a relaxed cluster,
as well as a test of whether a cluster is in fact relaxed.Comment: 10 pages, 8 figures, submitted to Ap
Might we eventually understand the origin of the dark matter velocity anisotropy?
The density profile of simulated dark matter structures is fairly
well-established, and several explanations for its characteristics have been
put forward. In contrast, the radial variation of the velocity anisotropy has
still not been explained. We suggest a very simple origin, based on the shapes
of the velocity distributions functions, which are shown to differ between the
radial and tangential directions. This allows us to derive a radial variation
of the anisotropy profile which is in good agreement with both simulations and
observations. One of the consequences of this suggestion is that the velocity
anisotropy is entirely determined once the density profile is known. We
demonstrate how this explains the origin of the \gamma-\beta relation, which is
the connection between the slope of the density profile and the velocity
anisotropy. These findings provide us with a powerful tool, which allows us to
close the Jeans equations.Comment: 10 pages, 7 figures, subm to ap
Nurses' knowledge and practices in cases of acute and chronic confusion: a questionnaire survey
PURPOSE:
This study aimed to describe nurses' knowledge and practices toward patients with acute or chronic confusion.
DESIGN AND METHODS:
A cross-sectional design was used, and 249 nurses engaged in clinical practice fulfilled an online self-report questionnaire.
FINDINGS:
Tools for diagnosing acute confusion/delirium are never used by 57.80% of the nurses. Between 80% and 81% of nursing interventions involve managing patients' physical environment and between 62% and 71% deal with managing communication. Theoretical training in the use of tools for assessing and intervening in cases of confusion was significantly associated with nurses' knowledge and practices.
PRACTICE IMPLICATIONS:
These results suggest the need for increased investment in nurses' training
Reduction in BACE1 decreases body weight, protects against diet-induced obesity and enhances insulin sensitivity in mice
Insulin resistance and impaired glucose homoeostasis are important indicators of Type 2 diabetes and are early risk factors of AD (Alzheimer's disease). An essential feature of AD pathology is the presence of BACE1 (β-site amyloid precursor protein-cleaving enzyme 1), which regulates production of toxic amyloid peptides. However, whether BACE1 also plays a role in glucose homoeostasis is presently unknown. We have used transgenic mice to analyse the effects of loss of BACE1 on body weight, and lipid and glucose homoeostasis. BACE1−/− mice are lean, with decreased adiposity, higher energy expenditure, and improved glucose disposal and peripheral insulin sensitivity than wild-type littermates. BACE1−/− mice are also protected from diet-induced obesity. BACE1-deficient skeletal muscle and liver exhibit improved insulin sensitivity. In a skeletal muscle cell line, BACE1 inhibition increased glucose uptake and enhanced insulin sensitivity. The loss of BACE1 is associated with increased levels of UCP1 (uncoupling protein 1) in BAT (brown adipose tissue) and UCP2 and UCP3 mRNA in skeletal muscle, indicative of increased uncoupled respiration and metabolic inefficiency. Thus BACE1 levels may play a critical role in glucose and lipid homoeostasis in conditions of chronic nutrient excess. Therefore strategies that ameliorate BACE1 activity may be important novel approaches for the treatment of diabetes
Correlates and Responsiveness to Change of Measures of Skin and Musculoskeletal Disease in Early Diffuse Systemic Sclerosis
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109290/1/acr22339.pd
The velocity anisotropy - density slope relation
One can solve the Jeans equation analytically for equilibrated dark matter
structures, once given two pieces of input from numerical simulations. These
inputs are 1) a connection between phase-space density and radius, and 2) a
connection between velocity anisotropy and density slope, the \alpha-\beta
relation. The first (phase-space density v.s. radius) has already been analysed
through several different simulations, however the second (\alpha-\beta
relation) has not been quantified yet. We perform a large set of numerical
experiments in order to quantify the slope and zero-point of the \alpha-\beta
relation. We find strong indication that the relation is indeed an attractor.
When combined with the assumption of phase-space being a power-law in radius,
this allows us to conclude that equilibrated dark matter structures indeed have
zero central velocity anisotropy \beta_0 = 0, central density slope of \alpha_0
= -0.8, and outer anisotropy of \beta_\infty = 0.5.Comment: 15 pages, 7 figure
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