7,134 research outputs found
The relative concentration of visible and dark matter in clusters of galaxies
[Abridged] We consider two clusters (A496 and Coma) that are representative
of the two classes of cool-core and non-cool-core clusters. We first refer to a
two-component dynamical model that ignores the contribution from the galaxy
density distribution and study the condition of hydrostatic equilibrium for the
hot intracluster medium (ICM) under the assumption of spherical symmetry, in
the presence of dark matter. We model the ICM density distribution in terms of
a standard -model with , i.e. with a distribution similar to
that of a regular isothermal sphere (RIS), and fit the observed X-ray
brightness profiles. With the explicit purpose of ignoring cosmological
arguments, we na\"ively assume that dark matter, if present, has an analogous
density distribution, with the freedom of two different density and length
scales. The relative distribution of visible and dark matter is then derived by
fitting the temperature data for the ICM under conditions of hydrostatic
equilibrium. For both clusters, we find that dark matter is more concentrated
with respect to visible matter. We then test whether the conclusion changes
significantly when dark matter is taken to be distributed according to
cosmologically favored density profiles and when the contribution of the mass
contained in galaxies is taken into account. Although the qualitative
conclusions remain unchanged, we find that the contribution of galaxies to the
mass budget is more important than generally assumed. We also show that,
without resorting to additional information on the small scale, it is not
possible to tell whether a density cusp is present or absent in these systems.
[Abridged]Comment: 13 pages, 3 figures, accepted for publication in Il Nuovo Cimento
Involutive constrained systems and Hamilton-Jacobi formalism
In this paper, we study singular systems with complete sets of involutive
constraints. The aim is to establish, within the Hamilton-Jacobi theory, the
relationship between the Frobenius' theorem, the infinitesimal canonical
transformations generated by constraints in involution with the Poisson
brackets, and the lagrangian point (gauge) transformations of physical systems
An experimental investigation of the flow field for double-wedge configurations in a Mach 4.97 stream
The viscous-inviscid interactions which perturb the flow around the wing leading edge are discussed. The flow field perturbation results when the fuselage-generated shock wave interacts with the wing-generated shock wave. Three types of shock interference patterns are possible for the wing leading edge of the orbiter
Hamilton-Jacobi formalism for Linearized Gravity
In this work we study the theory of linearized gravity via the
Hamilton-Jacobi formalism. We make a brief review of this theory and its
Lagrangian description, as well as a review of the Hamilton-Jacobi approach for
singular systems. Then we apply this formalism to analyze the constraint
structure of the linearized gravity in instant and front-form dynamics.Comment: To be published in Classical and Quantum Gravit
The Kovacs effect in model glasses
We discuss the `memory effect' discovered in the 60's by Kovacs in
temperature shift experiments on glassy polymers, where the volume (or energy)
displays a non monotonous time behaviour. This effect is generic and is
observed on a variety of different glassy systems (including granular
materials). The aim of this paper is to discuss whether some microscopic
information can be extracted from a quantitative analysis of the `Kovacs hump'.
We study analytically two families of theoretical models: domain growth and
traps, for which detailed predictions of the shape of the hump can be obtained.
Qualitatively, the Kovacs effect reflects the heterogeneity of the system: its
description requires to deal not only with averages but with a full probability
distribution (of domain sizes or of relaxation times). We end by some
suggestions for a quantitative analysis of experimental results.Comment: 17 pages, 6 figures; revised versio
Trap models with slowly decorrelating observables
We study the correlation and response dynamics of trap models of glassy
dynamics, considering observables that only partially decorrelate with every
jump. This is inspired by recent work on a microscopic realization of such
models, which found strikingly simple linear out-of-equilibrium
fluctuation-dissipation relations in the limit of slow decorrelation. For the
Barrat-Mezard model with its entropic barriers we obtain exact results at zero
temperature for arbitrary decorrelation factor . These are then
extended to nonzero , where the qualitative scaling behaviour and all
scaling exponents can still be found analytically. Unexpectedly, the choice of
transition rates (Glauber versus Metropolis) affects not just prefactors but
also some exponents. In the limit of slow decorrelation even complete scaling
functions are accessible in closed form. The results show that slowly
decorrelating observables detect persistently slow out-of-equilibrium dynamics,
as opposed to intermittent behaviour punctuated by excursions into fast,
effectively equilibrated states.Comment: 29 pages, IOP styl
Evidence for unidimensional low-energy excitations as the origin of persistent spin dynamics in geometrically frustrated magnets
We report specific heat, magnetic, and muon spin relaxation measurements
performed on a polycrystalline sample of the normal spinel CdHo2S4. The
rare-earth ions sit on a lattice of corner-sharing regular tetrahedra as in
pyrochlore compounds. Magnetic ordering is detected at Tc ~ 0.87 K. From
spin-lattice relaxation rate measurements on both sides of Tc we uncover
similar magnetic excitation modes driving the so-called persistent spin
dynamics at T < Tc. Unidimensional excitations are argued to be at its origin.
Often observed spin loop structures are suggested to support these excitations.
The possibility of a generic mechanism for their existence is discussed.Comment: 7 pages, 8 figure
Self-consistent nonspherical isothermal halos embedding zero-thickness disks
Disk-halo decompositions of galaxy rotation curves are generally performed in
a parametric way. We construct self-consistent models of nonspherical
isothermal halos embedding a zero-thickness disk, by assuming that the halo
distribution function is a Maxwellian. The method developed here can be used to
study other physically-based choices for the halo distribution function and the
case of a disk accompanied by a bulge. In a preliminary investigation we note
the existence of a fine tuning between the scalelengths R_{\Omega} and h,
respectively characterizing the rise of the rotation curve and the luminosity
profile of the disk, which surprisingly applies to both high surface brightness
and low surface brightness galaxies. This empirical correlation identifies a
much stronger conspiracy than the one required by the smoothness and flatness
of the rotation curve (disk-halo conspiracy). The self-consistent models are
characterized by smooth and flat rotation curves for very different
disk-to-halo mass ratios, hence suggesting that conspiracy is not as dramatic
as often imagined. For a typical rotation curve, with asymptotically flat
rotation curve at V_{\infty} (the precise value of which can also be treated as
a free parameter), and a typical density profile of the disk, self-consistent
models are characterized by two dimensionless parameters, which correspond to
the dimensional scales (the disk mass-to-light ratio M/L and the halo central
density) of standard disk-halo decompositions. We show that if the rotation
curve is decomposed by means of our self-consistent models, the disk-halo
degeneracy is removed and typical rotation curves are fitted by models that are
below the maximum-disk prescription. Similar results are obtained from a study
of NGC 3198. Finally, we quantify the flattening of the spheroidal halo, which
is significant, especially on the scale of the visible disk.Comment: accepted for publication in A&
Non-linear Response of the trap model in the aging regime : Exact results in the strong disorder limit
We study the dynamics of the one dimensional disordered trap model presenting
a broad distribution of trapping times , when an
external force is applied from the very beginning at , or only after a
waiting time , in the linear as well as in the non-linear response regime.
Using a real-space renormalization procedure that becomes exact in the limit of
strong disorder , we obtain explicit results for many observables,
such as the diffusion front, the mean position, the thermal width, the
localization parameters and the two-particle correlation function. In
particular, the scaling functions for these observables give access to the
complete interpolation between the unbiased case and the directed case.
Finally, we discuss in details the various regimes that exist for the averaged
position in terms of the two times and the external field.Comment: 27 pages, 1 eps figur
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