122,201 research outputs found
Combining Semi-Analytic Models of Galaxy Formation with Simulations of Galaxy Clusters: the Need for AGN Heating
We present hydrodynamical N-body simulations of clusters of galaxies with
feedback taken from semi-analytic models of galaxy formation. The advantage of
this technique is that the source of feedback in our simulations is a
population of galaxies that closely resembles that found in the real universe.
We demonstrate that, to achieve the high entropy levels found in clusters,
active galactic nuclei must inject a large fraction of their energy into the
intergalactic/intracluster media throughout the growth period of the central
black hole. These simulations reinforce the argument of Bower et al. (2008),
who arrived at the same conclusion on the basis of purely semi-analytic
reasoning.Comment: 4 pages, 1 figure. To appear in the proceedings of "The Monster's
Fiery Breath", Eds. Sebastian Heinz and Eric Wilcots (AIP conference series
Making Claims for Migrant Workers: Human Rights and Citizenship
Migrant workers claims for greater protection in a globalized world are typically expressed either in the idiom of international human rights or citizenship. Instead of contrasting these two normative frames, the paper explores the extent to which human rights and citizenship discourses intersect when it comes to claims by migrant workers. An analysis of the international human and labour rights instruments that are specifically designed for migrant workers reveals how neither discourse questions the assumption of territorial state sovereignty. Drawing upon sociological and political approaches to human rights claims, I evaluate the Arendtian-inspired critique of international human rights, which is that they ignore the very basis âright to have rightsâ. In doing so, I discuss the different dimensions of citizenship and conclude that international rights can be used by migrant workers to assert right claims that reinforce a conception of citizenship that, although different from national citizenship, has the potential to address their distinctive social location
Evidence for Substantial Charge Symmetry Violation in Parton Distributions
In principle one can test the validity of charge symmetry for parton
distributions by comparing structure functions measured in neutrino and charged
lepton deep inelastic scattering. New experiments make such tests possible;
they provide rather tight upper limits on parton charge symmetry violation
[CSV] for intermediate Bjorken x, but appear to show evidence for CSV effects
at small x. We examine two effects which might account for this experimental
discrepancy: nuclear shadowing corrections for neutrinos, and strange quark
contributions s(x) unequal to sbar(x). We show that neither of these two
corrections removes the experimental discrepancy between the structure
functions. We are therefore forced to consider the possibility of a
surprisingly large CSV effect in the nucleon sea quark distributions.Comment: 10 pages, 3 postscript figure
Probing QCD approach to thermal equilibrium with ultrahigh energy cosmic rays
The Pierre Auger Collaboration has reported an excess in the number of muons
of a few tens of percent over expectations computed using extrapolation of
hadronic interaction models tuned to accommodate LHC data. Very recently, we
proposed an explanation for the muon excess assuming the formation of a
deconfined quark matter (fireball) state in central collisions of
ultrarelativistic cosmic rays with air nuclei. At the first stage of its
evolution the fireball contains gluons as well as and quarks. The very
high baryochemical potential inhibits gluons from fragmenting into
and , and so they fragment predominantly into pairs. In
the hadronization which follows this leads to the strong suppression of pions
and hence photons, but allows heavy hadrons to be emitted carrying away
strangeness. In this manner, the extreme imbalance of hadron to photon content
provides a way to enhance the muon content of the air shower. In this
communication we study theoretical systematics from hadronic interaction models
used to describe the cascades of secondary particles produced in the fireball
explosion. We study the predictions of one of the leading LHC-tuned models
QGSJET II-04 considered in the Auger analysis.Comment: 7 pages LaTeX, 6 .pdf figure
Issues Relevant to C-H Activation at Platinum(II): Comparative Studies between Cationic, Zwitterionic, and Neutral Platinum(II) Compounds in Benzene Solution
Cationic late metal systems are being highly scrutinized due to
their propensity to mediate so-called electrophilic C-H
activation reactions. This contribution compares the reactivity
of highly reactive cationic platinum(II) systems with
structurally related but neutral species. Our experimental
design exploits isostructural neutral and cationic complexes
supported by bis(phosphine) ligands amenable to mechanistic
examination in benzene solution. The data presented herein
collectively suggests that neutral platinum complexes can be
equally if not more reactive towards benzene than their
cationic counter-parts. Moreover, a number of unexpected
mechanistic distinctions between the two systems arise that
help to explain their respective reactivity
Talbot quadratures and rational approximations
Many computational problems can be solved with the aid of contour integrals containing in the the integrand: examples include inverse Laplace transforms, special functions, functions of matrices and operators, parabolic PDEs, and reaction-diffusion equations. One approach to the numerical quadrature of such integrals is to apply the trapezoid rule on a Hankel contour defined by a suitable change of variables. Optimal parameters for three classes of such contours have recently been derived: (a) parabolas, (b) hyperbolas, and (c) cotangent contours, following Talbot in 1979. The convergence rates for these optimized quadrature formulas are very fast: roughly , where is the number of sample points or function evaluations. On the other hand, convergence at a rate apparently about twice as fast, , can be achieved by using a different approach: best supremum-norm rational approximants to for , following Cody, Meinardus and Varga in 1969. (All these rates are doubled in the case of self-adjoint operators or real integrands.) It is shown that the quadrature formulas can be interpreted as rational approximations and the rational approximations as quadrature formulas, and the strengths and weaknesses of the different approaches are discussed in the light of these connections. A MATLAB function is provided for computing Cody--Meinardus--Varga approximants by the method of CarathĂšodory-FejĂšr approximation
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