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 $f(R)$ 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 $H$-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; Países BajosFil: Haworth, N.. Queensland University of Technology; AustraliaFil: Schramm, A.. Queensland University of Technology; AustraliaFil: de Bruyne, G.. Universiteit Antwerp; BélgicaFil: Cavallo, V.. No especifíca;Fil: Chliaoutakis, J.. No especifíca;Fil: Pereira Dias, Joao. Instituto Superior Tecnico; PortugalFil: Ferraro, Ottavia Eleonora. Universita Degli Studi Di Pavia; ItaliaFil: Fyhri, Aslak. No especifíca;Fil: Sajatovic, Anika Hursa. No especifíca;Fil: Kuklane, Kalev. Lund University; SueciaFil: Ledesma, Ruben Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Psicología Básica, Aplicada y Tecnología. Universidad Nacional de Mar del Plata. Facultad de Psicología. Instituto de Psicología Básica, Aplicada y Tecnología.; ArgentinaFil: Calvé Mascarell, Oscar. Ben Gurion University of the Negev; IsraelFil: Morandi, A.. Universita Degli Studi Di Pavia; ItaliaFil: Muser, Markus. No especifíca;Fil: Otte, Diettmar. No especifíca;Fil: Papadakaki, M.. No especifíca;Fil: Sanmartín, J.. Universidad de Valencia; EspañaFil: Dulf, D.. No especifíca;Fil: Saplioglu, M.. No especifíca;Fil: Tzamalouka, Georgia. No especifíca