8,437 research outputs found
Cooling and Clusters: When Is Heating Needed?
There are (at least) two unsolved problems concerning the current state of
the thermal gas in clusters of galaxies. The first is identifying the source of
the heating which offsets cooling in the centers of clusters with short cooling
times (the ``cooling flow'' problem). The second is understanding the mechanism
which boosts the entropy in cluster and group gas. Since both of these problems
involve an unknown source of heating it is tempting to identify them with the
same process, particular since AGN heating is observed to be operating at some
level in a sample of well-observed ``cooling flow'' clusters. Here we show,
using numerical simulations of cluster formation, that much of the gas ending
up in clusters cools at high redshift and so the heating is also needed at
high-redshift, well before the cluster forms. This indicates that the same
process operating to solve the cooling flow problem may not also resolve the
cluster entropy problem.Comment: 10 pages, 5 figures, published in Philosophical Transactions A (Royal
Society
HOW FAR CAN WE FORECAST? FORECAST CONTENT HORIZONS FOR SOME IMPORTANT MACROECONOMIC TIME SERIES
For stationary transformations of variables, there exists a maximum horizon beyond which forecasts can provide no more information about the variable than is present in the unconditional mean. Meteorological forecasts, typically excepting only experimental or exploratory situations, are not reported beyond this horizon; by contrast, little generally-accepted information about such maximum horizons is available for economic variables. In this paper we estimate such content horizons for a variety of economic variables, and compare these with the maximum horizons which we observe reported in a large sample of empirical economic forecasting studies. We find that there are many instances of published studies which provide forecasts exceeding, often by substantial margins, our estimates of the content horizon for the particular variable and frequency. We suggest some simple reporting practices for forecasts that could potentially bring greater transparency to the process of making the interpreting economic forecasts.
Asymptotic analysis of multiscale approximations to reaction networks
A reaction network is a chemical system involving multiple reactions and
chemical species. Stochastic models of such networks treat the system as a
continuous time Markov chain on the number of molecules of each species with
reactions as possible transitions of the chain. In many cases of biological
interest some of the chemical species in the network are present in much
greater abundance than others and reaction rate constants can vary over several
orders of magnitude. We consider approaches to approximation of such models
that take the multiscale nature of the system into account. Our primary example
is a model of a cell's viral infection for which we apply a combination of
averaging and law of large number arguments to show that the ``slow'' component
of the model can be approximated by a deterministic equation and to
characterize the asymptotic distribution of the ``fast'' components. The main
goal is to illustrate techniques that can be used to reduce the dimensionality
of much more complex models.Comment: Published at http://dx.doi.org/10.1214/105051606000000420 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
Confusion of Diffuse Objects in the X-ray Sky
Most of the baryons in the present-day universe are thought to reside in
intergalactic space at temperatures of 10^5-10^7 K. X-ray emission from these
baryons contributes a modest (~10%) fraction of the ~ 1 keV background whose
prominence within the large-scale cosmic web depends on the amount of
non-gravitational energy injected into intergalactic space by supernovae and
AGNs. Here we show that the virialized regions of groups and clusters cover
over a third of the sky, creating a source-confusion problem that may hinder
X-ray searches for individual intercluster filaments and contaminate
observations of distant groups.Comment: accepted to ApJ Letters, 7 pages, 3 figure
The baseline intracluster entropy profile from gravitational structure formation
The radial entropy profile of the hot gas in clusters of galaxies tends to
follow a power law in radius outside of the cluster core. Here we present a
simple formula giving both the normalization and slope for the power-law
entropy profiles of clusters that form in the absence of non-gravitational
processes such as radiative cooling and subsequent feedback. It is based on
seventy-one clusters drawn from four separate cosmological simulations, two
using smoothed-particle hydrodynamics (SPH) and two using adaptive-mesh
refinement (AMR), and can be used as a baseline for assessing the impact of
non-gravitational processes on the intracluster medium outside of cluster
cores. All the simulations produce clusters with self-similar structure in
which the normalization of the entropy profile scales linearly with cluster
temperature, and these profiles are in excellent agreement outside of 0.2
r_200. Because the observed entropy profiles of clusters do not scale linearly
with temperature, our models confirm that non-gravitational processes are
necessary to break the self-similarity seen in the simulations. However, the
core entropy levels found by the two codes used here significantly differ, with
the AMR code producing nearly twice as much entropy at the centre of a cluster.Comment: Accepted to MNRAS, 8 pages, 9 figure
Are Tomato-Spotted Wilt Virus Management Tactics Good Enough?
Management of tomato-spotted wilt virus is complex and requires more than one treatment for near optimum results. We investigated tomato and pepper growers’ perception on the effectiveness of tactics using Bayesian Logistic regression. The perceived chance that each tactics will control the disease was about a coin toss.Tomato-spotted wilt virus, pest and disease management, Bayesian logistic regression, Agricultural and Food Policy, Crop Production/Industries, Farm Management, Food Security and Poverty, Production Economics, Research and Development/Tech Change/Emerging Technologies, Risk and Uncertainty,
Perichoretic Worship: Cultivating Relationships with the Triune God, with One Another, and with the World
A Participatory Action Research (PAR) study focusing on perichoretic worship as a way of cultivating holy relationship with the Triune God, with others, and with the world. Theoretical lenses include ritual studies, Faith Development Theory, and personhood in social relationships. Theological lenses include perichoresis, Lutheran worship, and faith practices. Using both quantitative and qualitative research, the study explores the practice of Christian worship in a large Lutheran congregation and seeks to grow active participation in the missional work of the Triune God through worship grounded in Word and Sacrament. Presents the Triune God as the active subject of Christian worship
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