17,489 research outputs found
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Engineering adiabaticity at an avoided crossing with optimal control
We investigate ways to optimize adiabaticity and diabaticity in the
Landau-Zener model with non-uniform sweeps. We show how diabaticity can be
engineered with a pulse consisting of a linear sweep augmented by an
oscillating term. We show that the oscillation leads to jumps in populations
whose value can be accurately modeled using a model of multiple,
photon-assisted Landau-Zener transitions, which generalizes work by Wubs et al.
[New J. Phys. 7, 218 (2005)]. We extend the study on diabaticity using methods
derived from optimal control. We also show how to preserve adiabaticity with
optimal pulses at limited time, finding a non-uniform quantum speed limit
Properties of galaxy dark matter halos from weak lensing
We present the results of a study of weak lensing by galaxies based on 45.5
deg of band imaging data from the Red-Sequence Cluster Survey (RCS).
We present the first weak lensing detection of the flattening of galaxy dark
matter halos. We use a simple model in which the ellipticity of the halo is
times the observed ellipticity of the lens. We find a best fit value of
, suggesting that the dark matter halos are somewhat
rounder than the light distribution. The fact that we detect a significant
flattening implies that the halos are well aligned with the light distribution.
Given the average ellipticity of the lenses, this implies a halo ellipticity of
, in fair agreement with results from
numerical simulations of CDM. This result provides strong support for the
existence of dark matter, as an isotropic lensing signal is excluded with 99.5%
confidence. We also study the average mass profile around the lenses, using a
maximum likelihood analysis. We consider two models for the halo mass profile:
a truncated isothermal sphere (TIS) and an NFW profile. We adopt
observationally motivated scaling relations between the lens luminosity and the
velocity dispersion and the extent of the halo. The best fit NFW model yields a
mass and a scale
radius kpc. This value for the scale radius is
in excellent agreement with predictions from numerical simulations for a halo
of this mass.Comment: Significantly revised version, accepted for publication in ApJ 11
pages, 6 figure
Bayes-X: a Bayesian inference tool for the analysis of X-ray observations of galaxy clusters
We present the first public release of our Bayesian inference tool, Bayes-X,
for the analysis of X-ray observations of galaxy clusters. We illustrate the
use of Bayes-X by analysing a set of four simulated clusters at z=0.2-0.9 as
they would be observed by a Chandra-like X-ray observatory. In both the
simulations and the analysis pipeline we assume that the dark matter density
follows a spherically-symmetric Navarro, Frenk and White (NFW) profile and that
the gas pressure is described by a generalised NFW (GNFW) profile. We then
perform four sets of analyses. By numerically exploring the joint probability
distribution of the cluster parameters given simulated Chandra-like data, we
show that the model and analysis technique can robustly return the simulated
cluster input quantities, constrain the cluster physical parameters and reveal
the degeneracies among the model parameters and cluster physical parameters. We
then analyse Chandra data on the nearby cluster, A262, and derive the cluster
physical profiles. To illustrate the performance of the Bayesian model
selection, we also carried out analyses assuming an Einasto profile for the
matter density and calculated the Bayes factor. The results of the model
selection analyses for the simulated data favour the NFW model as expected.
However, we find that the Einasto profile is preferred in the analysis of A262.
The Bayes-X software, which is implemented in Fortran 90, is available at
http://www.mrao.cam.ac.uk/facilities/software/bayesx/.Comment: 22 pages, 11 figure
Can Galactic Observations Be Explained by a Relativistic Gravity Theory?
We consider the possibility of an alternative gravity theory explaining the
dynamics of galactic systems without dark matter. From very general assumptions
about the structure of a relativistic gravity theory we derive a general
expression for the metric to order . This allows us to compare the
predictions of the theory with various experimental data: the Newtonian limit,
light deflection and retardation, rotation of galaxies and gravitational
lensing. Our general conclusion is that the possibility for any gravity theory
to explain the behaviour of galaxies without dark matter is rather improbable.Comment: 12p, REVTeX 3.
Low Mach number effect in simulation of high Mach number flow
In this note, we relate the two well-known difficulties of Godunov schemes:
the carbuncle phenomena in simulating high Mach number flow, and the inaccurate
pressure profile in simulating low Mach number flow. We introduced two simple
low-Mach-number modifications for the classical Roe flux to decrease the
difference between the acoustic and advection contributions of the numerical
dissipation. While the first modification increases the local numerical
dissipation, the second decreases it. The numerical tests on the double-Mach
reflection problem show that both modifications eliminate the kinked Mach stem
suffered by the original flux. These results suggest that, other than
insufficient numerical dissipation near the shock front, the carbuncle
phenomena is strongly relevant to the non-comparable acoustic and advection
contributions of the numerical dissipation produced by Godunov schemes due to
the low Mach number effect.Comment: 9 pages, 1 figur
Dynamical properties of Ultraluminous Infrared Galaxies I: Mass ratio conditions for ULIRG activity in interacting pairs
We present first results from our Very Large Telescope large program to study
the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs), which are
the products of mergers of gas-rich galaxies. The full data set consists of
high resolution, long-slit, H- and K-band spectra of 38 ULIRGs and 12 QSOs
(between 0.042<z<0.268). In this paper, we present the sources that have not
fully coalesced, and therefore have two distinct nuclei. This sub-sample
consists of 21 ULIRGs, the nuclear separation of which varies between 1.6 and
23.3 kpc. From the CO bandheads that appear in our spectra, we extract the
stellar velocity dispersion, sigma, and the rotational velocity, V_rot. The
stellar dispersion equals 142 km/s on average, while V_rot is often of the same
order. We combine our spectroscopic results with high-resolution infrared (IR)
imaging data to study the conditions for ULIRG activity in interacting pairs.
We find that the majority of ULIRGs are triggered by almost equal-mass major
mergers of 1.5:1 average ratio. Less frequently, 3:1 encounters are also
observed in our sample. However, less violent mergers of mass ratio >3:1
typically do not force enough gas into the center to generate ULIRG
luminosities.Comment: Accepted for publication in "The Astrophysical Journal
On the Nonlocal Equations and Nonlocal Charges Associated with the Harry Dym Hierarchy
A large class of nonlocal equations and nonlocal charges for the Harry Dym
hierarchy is exhibited. They are obtained from nonlocal Casimirs associated
with its bi-Hamiltonian structure. The Lax representation for some of these
equations is also given.Comment: to appear in Journal of Mathematical Physics, 17 pages, Late
Relating chaos to deterministic diffusion of a molecule adsorbed on a surface
Chaotic internal degrees of freedom of a molecule can act as noise and affect
the diffusion of the molecule on a substrate. A separation of time scales
between the fast internal dynamics and the slow motion of the centre of mass on
the substrate makes it possible to directly link chaos to diffusion. We discuss
the conditions under which this is possible, and show that in simple atomistic
models with pair-wise harmonic potentials, strong chaos can arise through the
geometry. Using molecular-dynamics simulations, we demonstrate that a realistic
model of benzene is indeed chaotic, and that the internal chaos affects the
diffusion on a graphite substrate
Reaction Time of a Group of Physics Students
The reaction time of a group of students majoring in Physics is reported
here. Strong co-relation between fatigue, reaction time and performance have
been seen and may be useful for academicians and administrators responsible of
working out time-tables, course structures, students counsellings etc.Comment: 10 pages, 4 figure
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