16,702 research outputs found
The cluster of galaxies Abell 376
We present a dynamical analysis of the galaxy cluster Abell 376 based on a
set of 73 velocities, most of them measured at Pic du Midi and Haute-Provence
observatories and completed with data from the literature. Data on individual
galaxies are presented and the accuracy of the determined velocities is
discussed as well as some properties of the cluster. We obtained an improved
mean redshift value z=0.0478^{+0.005}_{-0.006} and velocity dispersion
sigma=852^{+120}_{-76}km/s. Our analysis indicates that inside a radius of
900h_{70}^{-1}kpc (15 arcmin) the cluster is well relaxed without any
remarkable feature and the X-ray emission traces fairly well the galaxy
distribution. A possible substructure is seen at 20 arcmin from the centre
towards the Southwest direction, but is not confirmed by the velocity field.
This SW clump is, however, kinematically bound to the main structure of Abell
376. A dense condensation of galaxies is detected at 46 arcmin (projected
distance 2.6h_{70}^{-1}Mpc) from the centre towards the Northwest and analysis
of the apparent luminosity distribution of its galaxies suggests that this
clump is part of the large scale structure of Abell 376. X-ray spectroscopic
analysis of ASCA data resulted in a temperature kT = 4.3+/-0.4 keV and metal
abundance Z = 0.32+/-0.08 Z_solar. The velocity dispersion corresponding to
this temperature using the T_X-sigma scaling relation is in agreement with the
measured galaxies velocities.Comment: 11 pages, 10 figures, accepted for publication in A&
Irreversible magnetization under rotating fields and lock-in effect on ErBa_2Cu_3O_7 single crystal with columnar defects
We have measured the irreversible magnetization M_i of an ErBa_2Cu_3O_7
single crystal with columnar defects (CD), using a technique based on sample
rotation under a fixed magnetic field H. This method is valid for samples whose
magnetization vector remains perpendicular to the sample surface over a wide
angle range - which is the case for platelets and thin films - and presents
several advantages over measurements of M_L(H) loops at fixed angles. The
resulting M_i(\Theta) curves for several temperatures show a peak in the CD
direction at high fields. At lower fields, a very well defined plateau
indicative of the vortex lock-in to the CD develops. The H dependence of the
lock-in angle \phi_L follows the H^{-1} theoretical prediction, while the
temperature dependence is in agreement with entropic smearing effects
corresponding to short range vortex-defects interactions.Comment: 7 pages, 6 figures, to be published in Phys. Rev.
Physical approximations for the nonlinear evolution of perturbations in dark energy scenarios
The abundance and distribution of collapsed objects such as galaxy clusters
will become an important tool to investigate the nature of dark energy and dark
matter. Number counts of very massive objects are sensitive not only to the
equation of state of dark energy, which parametrizes the smooth component of
its pressure, but also to the sound speed of dark energy as well, which
determines the amount of pressure in inhomogeneous and collapsed structures.
Since the evolution of these structures must be followed well into the
nonlinear regime, and a fully relativistic framework for this regime does not
exist yet, we compare two approximate schemes: the widely used spherical
collapse model, and the pseudo-Newtonian approach. We show that both
approximation schemes convey identical equations for the density contrast, when
the pressure perturbation of dark energy is parametrized in terms of an
effective sound speed. We also make a comparison of these approximate
approaches to general relativity in the linearized regime, which lends some
support to the approximations.Comment: 15 pages, 2 figure
Exact Lyapunov Exponent for Infinite Products of Random Matrices
In this work, we give a rigorous explicit formula for the Lyapunov exponent
for some binary infinite products of random real matrices. All
these products are constructed using only two types of matrices, and ,
which are chosen according to a stochastic process. The matrix is singular,
namely its determinant is zero. This formula is derived by using a particular
decomposition for the matrix , which allows us to write the Lyapunov
exponent as a sum of convergent series. Finally, we show with an example that
the Lyapunov exponent is a discontinuous function of the given parameter.Comment: 1 pages, CPT-93/P.2974,late
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