908 research outputs found
Dark Matter Halos within Clusters
We examine the properties of dark matter halos within a rich galaxy cluster
using a high resolution simulation that captures the cosmological context of a
cold dark matter universe. The mass and force resolution permit the resolution
of 150 halos with circular velocities larger than 80 kms within the cluster's
virial radius of 2 Mpc. This enables an unprecedented study of the statistical
properties of a large sample of dark matter halos evolving in a dense
environment. The cumulative fraction of mass attached to these halos varies
from 0% at 200 kpc, to 13% at the virial radius. Even at this resolution the
overmerging problem persists; halos that pass within 200 kpc of the cluster
center are tidally disrupted. Additional substructure is lost at earlier epochs
within the massive progenitor halos. The median ratio of apocentric to
pericentric radii is 6:1; the orbital distribution is close to isotropic,
circular orbits are rare, radial orbits are common. The orbits of halos are
unbiased with respect to both position within the cluster and with the orbits
of the smooth dark matter background and no velocity bias is detected. The
tidal radii of surviving halos are generally well-fit using the simple analytic
prediction applied to their orbital pericenters. Halos within clusters have
higher concentrations than those in the field. Within the cluster, halo density
profiles can be modified by tidal forces and individual encounters with other
halos that cause significant mass loss - ``galaxy harassment''. Mergers between
halos do not occur inside the clusters virial radius.Comment: LaTeX MN style, 20 pages, 30 figures included + 1 colour plo
Dynamical Dzyaloshinsky-Moriya interaction in KCuF3: Raman evidence for an antiferrodistortive lattice instability
In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet
KCuF3 the low-energy Eg and B1g phonon modes show an anomalous softening (25%
and 13%) between room temperature and the characteristic temperature T_S = 50
K. In this temperature range a freezing-in of F ion dynamic displacements is
proposed to occur. In addition, the Eg mode at about 260 cm-1 clearly splits
below T_S. The width of the phonon lines above T_S follows an activated
behavior with an activation energy of about 50 K. Our observations clearly
evidence a reduction of the structural symmetry below T_S and indicate a strong
coupling of lattice and spin fluctuations for T>T_S.Comment: 7 pages, 9 figure
Density Profile Asymptotes at the Centre of Dark Matter Halos
For the spherical symmetric case, all quantities describing the relaxed dark
matter halo can be expressed as functions of the gravitational potential
. Decomposing the radial velocity dispersion with respect to
at very large and very small radial distances the possible asymptotic
behavior for the density and velocity profiles can be obtained. If reasonable
boundary conditions are posed such as a finite halo mass and force-free halo
centre the asymptotic density profiles at the centre should be much less steep
than the profiles obtained within numerical simulations. In particular cases
profiles like Plummer's model are obtained. The reasons of that seeming
discrepancy with respect to the results of N-body simulations are discussed.Comment: Accepted for publication in Astronomy & Astrophysics, LaTeX, 7 pages,
2 figure
Strong electronic correlations in LiZnPc organic metals
Nuclear magnetic resonance, electron paramagnetic resonance and magnetization
measurements show that bulk LiZnPc are strongly correlated one-dimensional
metals. The temperature dependence of the nuclear spin-lattice relaxation rate
and of the static uniform susceptibility on approaching room
temperature are characteristic of a Fermi liquid. Moreover, while for the electrons are delocalized down to low temperature, for a
tendency towards localization is noticed upon cooling, yielding an increase
both in and . The -dependence of the effective density of
states at the Fermi level displays a sharp enhancement for , at the half filling of the ZnPc lowest unoccupied molecular orbitals. This
suggests that LiZnPc is on the edge of a metal-insulator transition where
enhanced superconducting fluctuations could develop.Comment: 5 pages, 4 figure
Alternative Splicing and Tumor Progression
Alternative splicing is a key molecular mechanism for increasing the functional diversity of the eukaryotic proteomes. A large body of experimental data implicates aberrant splicing in various human diseases, including cancer. Both mutations in cis-acting splicing elements and alterations in the expression and/or activity of splicing regulatory factors drastically affect the splicing profile of many cancer-associated genes. In addition, the splicing profile of several cancer-associated genes is altered in particular types of cancer arguing for a direct role of specific splicing isoforms in tumor progression. Deciphering the mechanisms underlying aberrant splicing in cancer may prove crucial to understand how splicing machinery is controlled and integrated with other cellular processes, in particular transcription and signaling pathways. Moreover, the characterization of splicing deregulation in cancer will lead to a better comprehension of malignant transformation. Cancer-associated alternative splicing variants may be new tools for the diagnosis and classification of cancers and could be the targets for innovative therapeutical interventions based on highly selective splicing correction approaches
Cosmological Constraints on an Invisibly Decaying Higgs Boson
Working in the context of a proposal for collisional dark matter, we derive
bounds on the Higgs boson coupling to a stable light scalar
particle, which we refer to as phion (), required to solve problems with
small scale structure formation which arise in collisionless dark matter
models. We discuss the behaviour of the phion in the early universe for
different ranges of its mass. We find that a phion in the mass range of 100 MeV
is excluded and that a phion in the mass range of 1 GeV requires a large
coupling constant, g^{\prime} \gsim 2, and m_h \lsim 130 GeV in order to
avoid overabundance, in which case the invisible decay mode of the Higgs boson
would be dominant.Comment: 5 pages, 2 figures, Revtex style, changed conten
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