19 research outputs found
The mean density of the Universe from cluster evolution
The determination of the mean density of the Universe is a long standing
problem of modern cosmology. The number density evolution of x-ray clusters at
a fixed temperature is a powerful cosmological test, new in nature (Oukbir and
Blanchard, 1992), somewhat different from standard analyses based on the
dynamical measurement of individual objects. However, the absence of any
available sample of x-ray selected clusters with measured temperatures at high
redshift has prevented this test from being applied earlier. Recently,
temperature measurements of ten EMSS clusters at have
allowed the application of this test (Henry, 1997). In this work, we present
the first results of a new analysis we have performed of this data set as well
as a new estimation of the local temperature distribution function of clusters:
a likelihood analysis of the temperature distribution functions gives a
preferred value for the mean density of the universe which corresponds to 75%
of the critical density. An open model with a density smaller than 30% of the
critical density is rejected with a level of significance of 95%.Comment: 4 pages, shortened. To be published in Les Comptes Rendus de
l'Academie des Science
The evolution of clusters in the CLEF cosmological simulation: X-ray structural and scaling properties
We present results from a study of the X-ray cluster population that forms
within the CLEF cosmological hydrodynamics simulation, a large N-body/SPH
simulation of the Lambda CDM cosmology with radiative cooling, star formation
and feedback. The scaled projected temperature and entropy profiles at z=0 are
in good agreement with recent high-quality observations of cool core clusters,
suggesting that the simulation grossly follows the processes that structure the
intracluster medium (ICM) in these objects. Cool cores are a ubiquitous
phenomenon in the simulation at low and high redshift, regardless of a
cluster's dynamical state. This is at odds with the observations and so
suggests there is still a heating mechanism missing from the simulation. Using
a simple, observable measure of the concentration of the ICM, which correlates
with the apparent mass deposition rate in the cluster core, we find a large
dispersion within regular clusters at low redshift, but this diminishes at
higher redshift, where strong "cooling-flow" systems are absent in our
simulation. Consequently, our results predict that the normalisation and
scatter of the luminosity-temperature relation should decrease with redshift;
if such behaviour turns out to be a correct representation of X-ray cluster
evolution, it will have significant consequences for the number of clusters
found at high redshift in X-ray flux-limited surveys.Comment: 20 pages, 21 figures, MNRAS, accepted with minor modifications to
original manuscrip
COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study
Background:
The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms.
Methods:
International, prospective observational study of 60â109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms.
Results:
âTypicalâ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (â€â18 years: 69, 48, 23; 85%), older adults (â„â70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each Pâ<â0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country.
Interpretation:
This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men
Etude des proprietes structurelles et dynamiques des amas de galaxies : contribution au probleme de la masse cachee
SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc