1,791 research outputs found
Cluster Lenses
Clusters of galaxies are the most recently assembled, massive, bound
structures in the Universe. As predicted by General Relativity, given their
masses, clusters strongly deform space-time in their vicinity. Clusters act as
some of the most powerful gravitational lenses in the Universe. Light rays
traversing through clusters from distant sources are hence deflected, and the
resulting images of these distant objects therefore appear distorted and
magnified. Lensing by clusters occurs in two regimes, each with unique
observational signatures. The strong lensing regime is characterized by effects
readily seen by eye, namely, the production of giant arcs, multiple-images, and
arclets. The weak lensing regime is characterized by small deformations in the
shapes of background galaxies only detectable statistically. Cluster lenses
have been exploited successfully to address several important current questions
in cosmology: (i) the study of the lens(es) - understanding cluster mass
distributions and issues pertaining to cluster formation and evolution, as well
as constraining the nature of dark matter; (ii) the study of the lensed objects
- probing the properties of the background lensed galaxy population - which is
statistically at higher redshifts and of lower intrinsic luminosity thus
enabling the probing of galaxy formation at the earliest times right up to the
Dark Ages; and (iii) the study of the geometry of the Universe - as the
strength of lensing depends on the ratios of angular diameter distances between
the lens, source and observer, lens deflections are sensitive to the value of
cosmological parameters and offer a powerful geometric tool to probe Dark
Energy. In this review, we present the basics of cluster lensing and provide a
current status report of the field.Comment: About 120 pages - Published in Open Access at:
http://www.springerlink.com/content/j183018170485723/ . arXiv admin note:
text overlap with arXiv:astro-ph/0504478 and arXiv:1003.3674 by other author
How Zwicky already ruled out modified gravity theories without dark matter
Various theories, such as MOND, MOG, Emergent Gravity and theories
avoid dark matter by assuming a change in General Relativity and/or in Newton's
law. Galactic rotation curves are typically described well. Here the
application to galaxy clusters is considered, focussed on the good lensing and
X-ray data for A1689. As a start, the no-dark-matter case is confirmed to work
badly: the need for dark matter starts near the cluster centre, where Newton's
law is still supposed to be valid. This leads to the conundrum discovered by
Zwicky, which is likely only solvable in his way, namely by assuming additional
(dark) matter. Neutrinos with eV masses serve well without altering the
successes in (dwarf) galaxies.Comment: 10 pages, 2 figures. Matches published versio
3D Modeling of the Magnetization of Superconducting Rectangular-Based Bulks and Tape Stacks
In recent years, numerical models have become popular and powerful tools to
investigate the electromagnetic behavior of superconductors. One domain where
this advances are most necessary is the 3D modeling of the electromagnetic
behavior of superconductors. For this purpose, a benchmark problem consisting
of superconducting cube subjected to an AC magnetic field perpendicular to one
of its faces has been recently defined and successfully solved. In this work, a
situation more relevant for applications is investigated: a superconducting
parallelepiped bulk with the magnetic field parallel to two of its faces and
making an angle with the other one without and with a further constraint on the
possible directions of the current. The latter constraint can be used to model
the magnetization of a stack of high-temperature superconductor tapes, which
are electrically insulated in one direction. For the present study three
different numerical approaches are used: the Minimum Electro-Magnetic Entropy
Production (MEMEP) method, the -formulation of Maxwell's equations and the
Volume Integral Method (VIM) for 3D eddy currents computation. The results in
terms of current density profiles and energy dissipation are compared, and the
differences in the two situations of unconstrained and constrained current flow
are pointed out. In addition, various technical issues related to the 3D
modeling of superconductors are discussed and information about the
computational effort required by each model is provided. This works constitutes
a concrete result of the collaborative effort taking place within the HTS
numerical modeling community and will hopefully serve as a stepping stone for
future joint investigations
X-ray and Sunyaev-Zel'dovich scaling relations in galaxy clusters
[Abridged] We present an analysis of the scaling relations between X-ray
properties and Sunyaev-Zel'dovich (SZ) parameters for a sample of 24 X-ray
luminous galaxy clusters observed with Chandra and with measured SZ effect.
These objects are in the redshift range 0.14--0.82 and have X-ray bolometric
luminosity L>10^45 erg/s. We perform a spatially resolved spectral analysis and
recover the density, temperature and pressure profiles of the ICM, just relying
on the spherical symmetry of the cluster and the hydrostatic equilibrium
hypothesis. We observe that the correlations among X-ray quantities only are in
agreement with previous results obtained for samples of high-z X-ray luminous
galaxy clusters. On the relations involving SZ quantities, we obtain that they
correlate with the gas temperature with a logarithmic slope significantly
larger than the predicted value from the self-similar model. The measured
scatter indicates, however, that the central Compton parameter y_0 is a proxy
of the gas temperature at the same level of other X-ray quantities like
luminosity. Our results on the X-ray and SZ scaling relations show a tension
between the quantities more related to the global energy of the system (e.g.
gas temperature, gravitating mass) and the indicators of the structure of the
ICM (e.g. gas density profile, central Compton parameter y_0), showing the most
significant deviations from the values of the slope predicted from the
self-similar model in the L-T, L-M_{tot}, M_{gas}-T, y_0-T relations. When the
slope is fixed to the self-similar value, these relations consistently show a
negative evolution suggesting a scenario in which the ICM at higher redshift
has lower both X-ray luminosity and pressure in the central regions than the
expectations from self-similar model.Comment: MNRAS in press - Minor revision to match published versio
Entropy profiles in X-ray luminous galaxy clusters at z>0.1
[Abridged] The entropy distribution of the intracluster gas reflects both
accretion history of the gas and processes of feedback which provide a further
non-gravitational energy besides the potential one. In this work, we study the
profiles and the scaling properties of the gas entropy in 24 hot (kT_{gas} > 6
keV) galaxy clusters observed with Chandra in the redshift range 0.14-0.82 and
showing different states of relaxation. We recover the gas density, temperature
and entropy profiles in a non-parametric way. Adding the hydrostatic
equilibrium hypothesis, radial profiles are also obtained from the deprojection
of the surface brightness, allowing to verify whether the hydrostatic
equilibrium is a tenable hypothesis by comparison with the spectral
measurements. We confirm that this is the case on scales larger than 100 kpc
and discuss the deviations observed in few non-cooling core clusters in the
inner regions. We show that the entropy profiles are remarkably similar outside
the core and can be described by simple power-laws with slope of 1.0-1.2. We
measure an entropy level at 0.1 R_{200} of 100-500 keV cm^2 and a central
plateau which spans a wide range of value (~ a few-200 keV cm^2) depending on
the state of relaxation of the source. To characterize the energetic of the
central regions, we compare the radial behaviour of the temperature of the gas
with the temperature of the dark matter T_{DM} by estimating the excess of
energy Delta E = 3/2 k(T_{gas}- T_{DM}). We point out that Delta E ranges from
~ 0 in typical cooling-core clusters to few keV within 100 kpc in non-cooling
core systems. We also measure a significant correlation between the total iron
mass and the entropy outside the cooling region,whereas in the inner regions
they anti-correlate strongly.Comment: MNRAS in press - Minor revision to match the accepted versio
The Three-Dimensional Shapes of Galaxy Clusters
While clusters of galaxies are considered one of the most important
cosmological probes, the standard spherical modelling of the dark matter and
the intracluster medium is only a rough approximation. Indeed, it is well
established both theoretically and observationally that galaxy clusters are
much better approximated as triaxial objects. However, investigating the
asphericity of galaxy clusters is still in its infancy. We review here this
topic which is currently gathering a growing interest from the cluster
community. We begin by introducing the triaxial geometry. Then we discuss the
topic of deprojection and demonstrate the need for combining different probes
of the cluster's potential. We discuss the different works that have been
addressing these issues. We present a general parametric framework intended to
simultaneously fit complementary data sets (X-ray, Sunyaev Zel'dovich and
lensing data). We discuss in details the case of Abell 1689 to show how
different models/data sets lead to different haloe parameters. We present the
results obtained from fitting a 3D NFW model to X-ray, SZ, and lensing data for
4 strong lensing clusters. We argue that a triaxial model generally allows to
lower the inferred value of the concentration parameter compared to a spherical
analysis. This may alleviate tensions regarding, e.g. the over-concentration
problem. However, we stress that predictions from numerical simulations rely on
a spherical analysis of triaxial halos. Given that triaxial analysis will have
a growing importance in the observational side, we advocate the need for
simulations to be analysed in the very same way, allowing reliable and
meaningful comparisons. Besides, methods intended to derive the three
dimensional shape of galaxy clusters should be extensively tested on simulated
multi-wavelength observations.Comment: (Biased) Review paper. Comments welcome. Accepted for publication in
Space Science Reviews. This is a product of the work done by an international
team at the International Space Science Institute (ISSI) in Bern on
"Astrophysics and Cosmology with Galaxy Clusters: the X-ray and lensing view
Under-reporting bicycle accidents to police in the COST TU1101 international survey: Cross-country comparisons and associated factors
Police crash reports are often the main source for official data in many countries. However, with the exception of fatal crashes, crashes are often underreported in a biased manner. Consequently, the countermeasures adopted according to them may be inefficient. In the case of bicycle crashes, this bias is most acute and it probably varies across countries, with some of them being more prone to reporting accidents to police than others. Assessing if this bias occurs and the size of it can be of great importance for evaluating the risks associated with bicycling. This study utilized data collected in the COST TU1101 action “Towards safer bicycling through optimization of bicycle helmets and usage”. The data came from an online survey that included questions related to bicyclists' attitudes, behaviour, cycling habits, accidents, and patterns of use of helmets. The survey was filled by 8655 bicyclists from 30 different countries. After applying various exclusion factors, 7015 questionnaires filled by adult cyclists from 17 countries, each with at least 100 valid responses, remained in our sample. The results showed that across all countries, an average of only 10% of all crashes were reported to the police, with a wide range among countries: from a minimum of 0.0% (Israel) and 2.6% (Croatia) to a maximum of a 35.0% (Germany). Some factors associated with the reporting levels were type of crash, type of vehicle involved, and injury severity. No relation was found between the likelihood of reporting and the cyclist's gender, age, educational level, marital status, being a parent, use of helmet, and type of bicycle. The significant under-reporting – including injury crashes that do not lead to hospitalization – justifies the use of self-report survey data for assessment of bicycling crash patterns as they relate to (1) crash risk issues such as location, infrastructure, cyclists' characteristics, and use of helmet and (2) strategic approaches to bicycle crash prevention and injury reduction.Fil: Shinar, D.. Ben Gurion University of the Negev; IsraelFil: Valero Mora, Pedro. Universidad de Valencia; EspañaFil: van Strijp Houtenbos, M.. Institute For Road Safety Research; PaĂses BajosFil: Haworth, N.. Queensland University of Technology; AustraliaFil: Schramm, A.. Queensland University of Technology; AustraliaFil: de Bruyne, G.. Universiteit Antwerp; BĂ©lgicaFil: Cavallo, V.. No especifĂca;Fil: Chliaoutakis, J.. No especifĂca;Fil: Pereira Dias, Joao. Instituto Superior Tecnico; PortugalFil: Ferraro, Ottavia Eleonora. Universita Degli Studi Di Pavia; ItaliaFil: Fyhri, Aslak. No especifĂca;Fil: Sajatovic, Anika Hursa. No especifĂca;Fil: Kuklane, Kalev. Lund University; SueciaFil: Ledesma, Ruben Daniel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Mar del Plata. Instituto de PsicologĂa Básica, Aplicada y TecnologĂa. Universidad Nacional de Mar del Plata. Facultad de PsicologĂa. Instituto de PsicologĂa Básica, Aplicada y TecnologĂa.; ArgentinaFil: CalvĂ© Mascarell, Oscar. Ben Gurion University of the Negev; IsraelFil: Morandi, A.. Universita Degli Studi Di Pavia; ItaliaFil: Muser, Markus. No especifĂca;Fil: Otte, Diettmar. No especifĂca;Fil: Papadakaki, M.. No especifĂca;Fil: SanmartĂn, J.. Universidad de Valencia; EspañaFil: Dulf, D.. No especifĂca;Fil: Saplioglu, M.. No especifĂca;Fil: Tzamalouka, Georgia. No especifĂca
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