82 research outputs found
Cosmic ray confinement in fossil cluster bubbles
Most cool core clusters of galaxies possess active galactic nuclei (AGN) in
their centers. These AGN inflate buoyant bubbles containing non-thermal radio
emitting particles. If such bubbles efficiently confine cosmic rays (CR) then
this could explain ``radio ghosts'' seen far from cluster centers. We simulate
the diffusion of cosmic rays from buoyant bubbles inflated by AGN. Our
simulations include the effects of the anisotropic particle diffusion
introduced by magnetic fields. Our models are consistent with the X-ray
morphology of AGN bubbles, with disruption being suppressed by the magnetic
draping effect. We conclude that for such magnetic field topologies, a
substantial fraction of cosmic rays can be confined inside the bubbles on
buoyant rise timescales even when the parallel diffusivity coefficient is very
large. For isotropic diffusion at a comparable level, cosmic rays would leak
out of the bubbles too rapidly to be consistent with radio observations. Thus,
the long confinement times associated with the magnetic suppression of CR
diffusion can explain the presence of radio ghosts. We show that the partial
escape of cosmic rays is mostly confined to the wake of the rising bubbles, and
speculate that this effect could: (1) account for the excitation of the
H filaments trailing behind the bubbles in the Perseus cluster, (2)
inject entropy into the metal enriched material being lifted by the bubbles
and, thus, help to displace it permanently from the cluster center and (3)
produce observable -rays via the interaction of the diffusing cosmic
rays with the thermal intracluster medium (ICM).Comment: submitte
Magnetic fields in barred galaxies. IV. NGC 1097 and NGC 1365
We present 3.5cm and 6.2cm radio continuum maps in total and polarized
intensity of the barred galaxies NGC 1097 and NGC 1365. Both galaxies exhibit
radio ridges roughly overlapping with the massive dust lanes in the bar region.
The contrast in total intensity across the radio ridges is compatible with
compression and shear of an isotropic random magnetic field. The contrast in
polarized intensity is significantly smaller than that expected from
compression and shearing of the regular magnetic field; this could be the
result of decoupling of the regular field from the dense molecular clouds. The
regular field in the ridge is probably strong enough to reduce significantly
shear in the diffuse gas (to which it is coupled) and hence to reduce magnetic
field amplification by shearing. This contributes to the misalignment of the
observed field orientation with respect to the velocity vectors of the dense
gas. Our observations, for the first time, indicate that magnetic forces can
control the flow of the diffuse interstellar gas at kiloparsec scales. The
total radio intensity reaches its maximum in the circumnuclear starburst
regions, where the equipartition field strength is about 60\mu G, amongst the
strongest fields detected in spiral galaxies so far. The regular field in the
inner region has a spiral shape with large pitch angle, indicating the action
of a dynamo. Magnetic stress leads to mass inflow towards the centre,
sufficient to feed the active nucleus in NGC 1097. We detected diffuse X-ray
emission, possibly forming a halo of hot gas around NGC 1097.Comment: 32 pages with 45 PostScript figures. Accepted for publication in A&A;
Typos corrected 12/10/200
Mask wearing in community settings reduces SARS-CoV-2 transmission
The effectiveness of mask wearing at controlling severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission has been unclear. While masks are known to substantially reduce disease transmission in healthcare settings [D. K. Chu et al., Lancet 395, 1973–1987 (2020); J. Howard et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2014564118 (2021); Y. Cheng et al., Science eabg6296 (2021)], studies in community settings report inconsistent results [H. M. Ollila et al., medRxiv (2020); J. Brainard et al., Eurosurveillance 25, 2000725 (2020); T. Jefferson et al., Cochrane Database Syst. Rev. 11, CD006207 (2020)]. Most such studies focus on how masks impact transmission, by analyzing how effective government mask mandates are. However, we find that widespread voluntary mask wearing, and other data limitations, make mandate effectiveness a poor proxy for mask-wearing effectiveness. We directly analyze the effect of mask wearing on SARS-CoV-2 transmission, drawing on several datasets covering 92 regions on six continents, including the largest survey of wearing behavior ([Formula: see text] 20 million) [F. Kreuter et al., https://gisumd.github.io/COVID-19-API-Documentation (2020)]. Using a Bayesian hierarchical model, we estimate the effect of mask wearing on transmission, by linking reported wearing levels to reported cases in each region, while adjusting for mobility and nonpharmaceutical interventions (NPIs), such as bans on large gatherings. Our estimates imply that the mean observed level of mask wearing corresponds to a 19% decrease in the reproduction number R. We also assess the robustness of our results in 60 tests spanning 20 sensitivity analyses. In light of these results, policy makers can effectively reduce transmission by intervening to increase mask wearing
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
An untargeted multi-technique metabolomics approach to studying intracellular metabolites of HepG2 cells exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin
<p>Abstract</p> <p>Background</p> <p><it>In vitro </it>cell systems together with omics methods represent promising alternatives to conventional animal models for toxicity testing. Transcriptomic and proteomic approaches have been widely applied <it>in vitro </it>but relatively few studies have used metabolomics. Therefore, the goal of the present study was to develop an untargeted methodology for performing reproducible metabolomics on <it>in vitro </it>systems. The human liver cell line HepG2, and the well-known hepatotoxic and non-genotoxic carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), were used as the <it>in vitro </it>model system and model toxicant, respectively.</p> <p>Results</p> <p>The study focused on the analysis of intracellular metabolites using NMR, LC-MS and GC-MS, with emphasis on the reproducibility and repeatability of the data. State of the art pre-processing and alignment tools and multivariate statistics were used to detect significantly altered levels of metabolites after exposing HepG2 cells to TCDD. Several metabolites identified using databases, literature and LC-nanomate-Orbitrap analysis were affected by the treatment. The observed changes in metabolite levels are discussed in relation to the reported effects of TCDD.</p> <p>Conclusions</p> <p>Untargeted profiling of the polar and apolar metabolites of <it>in vitro </it>cultured HepG2 cells is a valid approach to studying the effects of TCDD on the cell metabolome. The approach described in this research demonstrates that highly reproducible experiments and correct normalization of the datasets are essential for obtaining reliable results. The effects of TCDD on HepG2 cells reported herein are in agreement with previous studies and serve to validate the procedures used in the present work.</p
Why design history? : A multi-national perspective on the state and purpose of the field
This article asks: what is the significance of design history within higher education? It reviews the practice and purpose of design history, in terms of its role in the education of historically aware and critically engaged designers, as an emerging independent discipline, and in terms of what the subject has to offer allied fields such as history, sociology, cultural studies, area studies and anthropology. It considers the development and current state of design history as it is taught in the UK and wider Europe, the US, Australia, and Japan, China and Korea. The argument that follows is grounded in recent design historical scholarship in order to provide both a contemporary perspective on current practice and suggestions about possible futures.Peer reviewe
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