29,758 research outputs found
Measuring cosmic magnetic fields by rotation measure-galaxy cross-correlations in cosmological simulations
Using cosmological MHD simulations of the magnetic field in galaxy clusters
and filaments we evaluate the possibility to infer the magnetic field strength
in filaments by measuring cross-correlation functions between Faraday Rotation
Measures (RM) and the galaxy density field. We also test the reliability of
recent estimates considering the problem of data quality and Galactic
foreground (GF) removal in current datasets. Besides the two self-consistent
simulations of cosmological magnetic fields based on primordial seed fields and
galactic outflows analyzed here, we also explore a larger range of models
scaling up the resulting magnetic fields of one of the simulations. We find
that, if an unnormalized estimator for the cross-correlation functions and a GF
removal procedure is used, the detectability of the cosmological signal is only
possible for future instruments (e.g. SKA and ASKAP). However, mapping of the
observed RM signal to the underlying magnetization of the Universe (both in
space and time) is an extremely challenging task which is limited by the
ambiguities of our model parameters, as well as to the weak response of the RM
signal in low density environments. Therefore, we conclude that current data
cannot constrain the amplitude and distribution of magnetic fields within the
large scale structure and a detailed theoretical understanding of the build up
and distribution of magnetic fields within the Universe will be needed for the
interpretation of future observations.Comment: 11 pages, 11 figures, comparation between RM data and simulations in
fig. 8, submited to MNRAS
Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations
Numerical 3D radiative (M)HD simulations of solar convection are used to
understand the physical properties of the solar photosphere. To validate this
approach, it is important to check that no excessive thermodynamic fluctuations
arise as a consequence of the partially incomplete treatment of radiative
transfer. We investigate the realism of 3D convection simulations carried out
with the Stagger code. We compared the characteristic properties of several
spectral lines in solar disc centre observations with spectra synthesized from
the simulations. We degraded the synthetic spectra to the spatial resolution of
the observations using the continuum intensity distribution. We estimated the
necessary spectral degradation by comparing atlas spectra with averaged
observed spectra. In addition to deriving a set of line parameters directly, we
used the SIR code to invert the spectra. Most of the line parameters from the
observational data are matched well by the degraded simulation spectra. The
inversions predict a macroturbulent velocity below 10 m/s for the simulation at
full spatial resolution, whereas they yield ~< 1000 m/s at a spatial resolution
of 0.3". The temperature fluctuations in the inversion of the degraded
simulation do not exceed those from the observational data (of the order of
100-200 K rms for -2<log tau<-0.5). The comparison of line parameters in
spatially averaged profiles with the averaged values of line parameters in
spatially resolved profiles indicates a significant change of (average) line
properties at a spatial scale between 0.13" and 0.3". Up to a spatial
resolution of 0.3", we find no indications of the presence of excessive
thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that
simulations without spatial degradation contain fully realistic thermodynamic
fluctuations requires observations at even better spatial resolution.Comment: 21 pages, 15 figures + 2 pages Appendix, accepted for publication in
A&A; v2 version: corrected for an error in the calculation of stray-light
estimates, for details see the Corrigendum to A&A, 2013, 557, 109 (DOI:
10.1051/0004-6361/201321596). Corrected text and numbers are in bold font.
Apart from the stray-light estimates, nothing in the rest of the paper was
affected by the erro
Simulations of the Galaxy Cluster CIZA J2242.8+5301 I: Thermal Model and Shock Properties
The giant radio relic in CIZA J2242.8+5301 is likely evidence of a Mpc sized
shock in a massive merging galaxy cluster. However, the exact shock properties
are still not clearly determined. In particular, the Mach number derived from
the integrated radio spectrum exceeds the Mach number derived from the X-ray
temperature jump by a factor of two. We present here a numerical study, aiming
for a model that is consistent with the majority of observations of this galaxy
cluster. We first show that in the northern shock upstream X-ray temperature
and radio data are consistent with each other. We then derive progenitor masses
for the system using standard density profiles, X-ray properties and the
assumption of hydrostatic equilibrium. We find a class of models that is
roughly consistent with weak lensing data, radio data and some of the X-ray
data. Assuming a cool-core versus non-cool-core merger, we find a fiducial
model with a total mass of , a mass ratio of 1.76
and a Mach number that is consistent with estimates from the radio spectrum. We
are not able to match X-ray derived Mach numbers, because even low mass models
over-predict the X-ray derived shock speeds. We argue that deep X-ray
observations of CIZA J2242.8+5301 will be able to test our model and
potentially reconcile X-ray and radio derived Mach numbers in relics.Comment: 19 pages, 19 figure
The KATRIN Experiment
The KArlsruhe TRitium Neutrino mass experiment, KATRIN, aims to search for
the mass of the electron neutrino with a sensitivity of 0.2 eV/c^2 (90% C.L.)
and a detection limit of 0.35 eV/c^2 (5 sigma). Both a positive or a negative
result will have far reaching implications for cosmology and the standard model
of particle physics and will give new input for astroparticle physics and
cosmology. The major components of KATRIN are being set up at the Karlsruhe
Institut of Technology in Karlsruhe, Germany, and test measurements of the
individual components have started. Data taking with tritium is scheduled to
start in 2012.Comment: 3 pages, 1 figure, proceedings of the TAUP 2009 International
Conference on Topics in Astroparticle and Underground Physics, to be
published in Journal of Physics, Conference Serie
Connecting Angular Momentum and Galactic Dynamics: The complex Interplay between Spin, Mass, and Morphology
The evolution and distribution of the angular momentum of dark matter (DM)
halos have been discussed in several studies over the past decades. In
particular, the idea arose that angular momentum conservation should allow to
infer the total angular momentum of the entire DM halo from measuring the
angular momentum of the baryonic component, which is populating the center of
the halo, especially for disk galaxies. To test this idea and to understand the
connection between the angular momentum of the DM halo and its galaxy, we use
the Magneticum simulations. We successfully produce populations of spheroidal
and disk galaxies self-consistently. Thus, we are able to study the dependence
of galactic properties on their morphology. We find that (1) the specific
angular momentum of stars in disk and spheroidal galaxies as a function of
their stellar mass compares well with observational results; (2) the specific
angular momentum of the stars in disk galaxies is slightly smaller compared to
the specific angular momentum of the cold gas, in good agreement with
observations; (3) simulations including the baryonic component show a dichotomy
in the specific stellar angular momentum distribution when splitting the
galaxies according to their morphological type (this dichotomy can also be seen
in the spin parameter, where disk galaxies populate halos with slightly larger
spin compared to spheroidal galaxies); (4) disk galaxies preferentially
populate halos in which the angular momentum vector of the DM component in the
central part shows a better alignment to the angular momentum vector of the
entire halo; and (5) the specific angular momentum of the cold gas in disk
galaxies is approximately 40 percent smaller than the specific angular momentum
of the total DM halo and shows a significant scatter.Comment: 25 pages, accepted by ApJ, www.magneticum.or
Folding of Hitchin systems and crepant resolutions
Folding of ADE-Dynkin diagrams according to graph automorphisms yields irreducible Dynkin diagrams of ABCDEFG-types. This folding procedure allows to trace back the properties of the corresponding simple Lie algebras or groups to those of ADE-type. In this article, we implement the techniques of folding by graph automorphisms for Hitchin integrable systems. We show that the fixed point loci of these automorphisms are isomorphic as algebraic integrable systems to the Hitchin systems of the folded groups away from singular fibers. The latter Hitchin systems are isomorphic to the intermediate Jacobian fibrations of Calabi--Yau orbifold stacks constructed by the first author. We construct simultaneous crepant resolutions of the associated singular quasi-projective Calabi--Yau threefolds and compare the resulting intermediate Jacobian fibrations to the corresponding Hitchin systems
Prevalent morphometric vertebral fractures in professional male rugby players
There is an ongoing concern about the risk of injury to the spine in professional rugby players. The objective of this study was to investigate the prevalence of vertebral fracture using vertebral fracture assessment (VFA) dual energy X-ray absorptiometry (DXA) imaging in professional male rugby players. Ninety five professional rugby league (n = 52) and union (n = 43) players (n = 95; age 25.9 (SD 4.3) years; BMI: 29.5 (SD 2.9) kg.m2) participated in the research. Each participant received one VFA, and one total body and lumbar spine DXA scan (GE Lunar iDXA). One hundred and twenty vertebral fractures were identified in over half of the sample by VFA. Seventy four were graded mild (grade 1), 40 moderate (grade 2) and 6 severe (grade 3). Multiple vertebral fractures (≥2) were found in 37 players (39%). There were no differences in prevalence between codes, or between forwards and backs (both 1.2 v 1.4; p>0.05). The most common sites of fracture were T8 (n = 23), T9 (n = 18) and T10 (n = 21). The mean (SD) lumbar spine bone mineral density Z-score was 2.7 (1.3) indicating high player bone mass in comparison with age- and sex-matched norms. We observed a high number of vertebral fractures using DXA VFA in professional rugby players of both codes. The incidence, aetiology and consequences of vertebral fractures in professional rugby players are unclear, and warrant timely, prospective investigation
Technologische Alternativen zum herkömmlichen Einsatz von Pökelstoffen in Öko-Fleischwaren
On the basis of literature research and opinions of meat processors and other experts, this paper discusses alternatives to the currently permitted use of curing agents in the processing of organic meat. These alternatives include (i) the reduction of addition of nitrite to levels sufficient for the desired sensory properties, (ii) the in situ bacterial formation of nitrite from nitrate naturally present in added vegetable preparations, and (iii) not making use of the beneficial effects of nitrite on the colour and aroma of the product at all. Measures to be taken to compensate for the effects of nitrite, as well as problems in implementation of technological alternatives are discussed
An improved SPH scheme for cosmological simulations
We present an implementation of smoothed particle hydrodynamics (SPH) with
improved accuracy for simulations of galaxies and the large-scale structure. In
particular, we combine, implement, modify and test a vast majority of SPH
improvement techniques in the latest instalment of the GADGET code. We use the
Wendland kernel functions, a particle wake-up time-step limiting mechanism and
a time-dependent scheme for artificial viscosity, which includes a high-order
gradient computation and shear flow limiter. Additionally, we include a novel
prescription for time-dependent artificial conduction, which corrects for
gravitationally induced pressure gradients and largely improves the SPH
performance in capturing the development of gas-dynamical instabilities. We
extensively test our new implementation in a wide range of hydrodynamical
standard tests including weak and strong shocks as well as shear flows,
turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas
clouds. We jointly employ all modifications; however, when necessary we study
the performance of individual code modules. We approximate hydrodynamical
states more accurately and with significantly less noise than standard SPH.
Furthermore, the new implementation promotes the mixing of entropy between
different fluid phases, also within cosmological simulations. Finally, we study
the performance of the hydrodynamical solver in the context of radiative galaxy
formation and non-radiative galaxy cluster formation. We find galactic disks to
be colder, thinner and more extended and our results on galaxy clusters show
entropy cores instead of steadily declining entropy profiles. In summary, we
demonstrate that our improved SPH implementation overcomes most of the
undesirable limitations of standard SPH, thus becoming the core of an efficient
code for large cosmological simulations.Comment: 21 figures, 2 tables, accepted to MNRA
Distributed-Pair Programming can work well and is not just Distributed Pair-Programming
Background: Distributed Pair Programming can be performed via screensharing
or via a distributed IDE. The latter offers the freedom of concurrent editing
(which may be helpful or damaging) and has even more awareness deficits than
screen sharing. Objective: Characterize how competent distributed pair
programmers may handle this additional freedom and these additional awareness
deficits and characterize the impacts on the pair programming process. Method:
A revelatory case study, based on direct observation of a single, highly
competent distributed pair of industrial software developers during a 3-day
collaboration. We use recordings of these sessions and conceptualize the
phenomena seen. Results: 1. Skilled pairs may bridge the awareness deficits
without visible obstruction of the overall process. 2. Skilled pairs may use
the additional editing freedom in a useful limited fashion, resulting in
potentially better fluency of the process than local pair programming.
Conclusion: When applied skillfully in an appropriate context, distributed-pair
programming can (not will!) work at least as well as local pair programming
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