5,353 research outputs found
Galaxy Distances in the Nearby Universe: Corrections For Peculiar Motions
By correcting the redshift--dependent distances for peculiar motions through
a number of peculiar velocity field models, we recover the true distances of a
wide, all-sky sample of nearby galaxies (~ 6400 galaxies with velocities
cz<5500 km/s), which is complete up to the blue magnitude B=14 mag. Relying on
catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups
and the remaining objects as field galaxies.
We model the peculiar velocity field using: i) a cluster dipole
reconstruction scheme; ii) a multi--attractor model fitted to the Mark II and
Mark III catalogs of galaxy peculiar velocities. According to Mark III data the
Great Attractor has a smaller influence on local dynamics than previously
believed, whereas the Perseus-Pisces and Shapley superclusters acquire a
specific dynamical role. Remarkably, the Shapley structure, which is found to
account for nearly half the peculiar motion of the Local Group, is placed by
Mark III data closer to the zone of avoidance with respect to its optical
position.
Our multi--attractor model based on Mark III data favors a cosmological
density parameter Omega ~ 0.5 (irrespective of a biasing factor of order
unity). Differences among distance estimates are less pronounced in the ~ 2000
- 4000 km/s distance range than at larger or smaller distances. In the last
regions these differences have a serious impact on the 3D maps of the galaxy
distribution and on the local galaxy density --- on small scales.Comment: 24 pages including (9 eps figures and 7 tables). Figures 1,2,3,4 are
available only upon request. Accepted by Ap
The X-ray Cluster Dipole
We estimate the dipole of the whole sky X-ray flux-limited sample of
Abell/ACO clusters (XBACs) and compare it to the optical Abell/ACO cluster
dipole. The X-ray cluster dipole is well aligned () with the
CMB dipole, while it follows closely the radial profile of its optical cluster
counterpart although its amplitude is per cent lower. In view of
the fact that the the XBACs sample is not affected by the volume incompleteness
and the projection effects that are known to exist at some level in the optical
parent Abell/ACO cluster catalogue, our present results confirm the previous
optical cluster dipole analysis that there are significant contributions to the
Local Group motion from large distances (Mpc). In order to
assess the expected contribution to the X-ray cluster dipole from a purely
X-ray selected sample we compare the dipoles of the XBACs and the Brightest
Cluster Sample (Ebeling et al. 1997a) in their overlap region. The resulting
dipoles are in mutual good aggreement with an indication that the XBACs sample
slightly underestimates the full X-ray dipole (by per cent) while the
Virgo cluster contributes about 10 - 15 per cent to the overall X-ray cluster
dipole. Using linear perturbation theory to relate the X-ray cluster dipole to
the Local group peculiar velocity we estimate the density parameter to be
.Comment: 16 pages, latex, + 4 ps figures, submitted to Ap
Wear and Friction Modeling on Lifeboat Launch Systems
The RNLI provides search and rescue cover along the UK and RoI coast using a variety of lifeboats
and launch techniques. In locations where there is no natural harbour it is necessary to use a slipway
to launch the lifeboat into the sea. Lifeboat slipway stations consist of an initial section where the
boat is held on rollers followed by an inclined keelway lined with low friction composite materials,
the lifeboat is released from the top of the slipway and proceeds under its own weight into the water.
The lifeboat is later recovered using a winch line. It is common to manually apply grease to the
composite slipway lining before each launch and recovery in order to ensure sufficiently low friction
for successful operation. With the introduction of the Tamar class lifeboat it is necessary to upgrade
existing boathouses and standardise slipway operational procedures to ensure consistent operation.
The higher contact pressures associated with the new lifeboat have led to issues of high friction and
wear on the composite slipway linings and the manual application of grease to reduce friction is to be
restricted due to environmental impact and cost factors. This paper presents a multidisciplinary
approach to modelling slipway panel wear and friction using tribometer testing in conjunction with
finite element analysis and slipway condition surveys to incorporate common real-world effects such
as panel misalignments. Finally, it is shown that a freshwater lubrication system is effective,
reducing cost and environmental impacts while maintaining good friction and wear performance
The SBF Survey of Galaxy Distances. II. Local and Large-Scale Flows
We present analysis of local large scale flows using the Surface Brightness
Fluctuation (SBF) Survey for the distances to 300 early-type galaxies. Our
models of the distribution function of mean velocity and velocity dispersion at
each point in space include a uniform thermal velocity dispersion and spherical
attractors whose position, amplitude, and radial shape are free to vary. Our
fitting procedure performs a maximum likelihood fit of the model to the
observations. We obtain a Hubble constant of Ho = 77 +/- 4 +/- 7 km/s/Mpc, but
a uniform Hubble flow is not acceptable fit to the data. Inclusion of two
attractors, one of whose fit location coincides with the Virgo cluster and the
other whose fit location is slightly beyond the Centaurus clusters nearly
explain the peculiar velocities, but the quality of the fit can be further
improved by the addition of a quadrupole correction to the Hubble flow.
Although the dipole and quadrupole may be genuine manifestations of more
distant density fluctuations, we find evidence that they are more likely due to
non-spherical attractors. We find no evidence for bulk flows which include our
entire survey volume (R < 3000 km/s); our volume is at rest with respect to the
CMB. The fits to the attractors both have isothermal radial profiles (v ~ 1/r)
over a range of overdensity between about 10 and 1, but fall off more steeply
at larger radius. The best fit value for the small scale, cosmic thermal
velocity is 180 +/- 14 km/s.Comment: 37 pages, AASTeX Latex, including 30 Postscript figures, submitted to
Astrophysical Journal, July 2, 199
Dye staining and excavation of a lateral preferential flow network
International audiencePreferential flow features have been found to be important for runoff generation, solute transport, and slope stability in many areas around the world. Although many studies have identified the particular characteristics of individual features and measured the runoff generation and solute transport within hillslopes, no studies have determined how individual features are hydraulically connected at a hillslope scale. In this study, we used dye staining and excavation to determine the morphology and spatial pattern of a preferential flow network over a large scale (30 m). We explore the feasibility of extending small-scale dye staining techniques to the hillslope scale. We determine the lateral preferential flow features that are active during the steady state flow conditions and their interaction with the surrounding soil matrix. We also calculate the velocities of the flow through each cross-section of the hillslope and compare them to hillslope scale applied tracer measurements. Finally, we investigate the relationship between the contributing area and the characteristics of the preferential features. The experiment revealed that larger contributing areas coincided with highly developed and hydraulically connected preferential features that had flow with little interaction with the surrounding soil matrix. We found evidence of subsurface erosion and deposition of soil and organic material laterally and vertically within the soil. These results are important because they add to the understanding of the runoff generation, solute transport, and slope stability of these types of hillslopes
Chloroplast microsatellites: measures of genetic diversity and the effect of homoplasy
Chloroplast microsatellites have been widely used in population genetic
studies of conifers in recent years. However, their haplotype configurations
suggest that they could have high levels of homoplasy, thus limiting the power
of these molecular markers. A coalescent-based computer simulation was used to
explore the influence of homoplasy on measures of genetic diversity based on
chloroplast microsatellites. The conditions of the simulation were defined to
fit isolated populations originating from the colonization of one single
haplotype into an area left available after a glacial retreat. Simulated data
were compared with empirical data available from the literature for a species
of Pinus that has expanded north after the Last Glacial Maximum. In the
evaluation of genetic diversity, homoplasy was found to have little influence
on Nei's unbiased haplotype diversity (H(E)) while Goldstein's genetic distance
estimates (D2sh) were much more affected. The effect of the number of
chloroplast microsatellite loci for evaluation of genetic diversity is also
discussed
On the Cosmology of Massive Vector Fields with SO(3) Global Symmetry
A relevant reference ([14]) has been added.Comment: 19 pages, plain tex, DF/IST-3/92 and DFFCUL 03-5/199
The Dipole Anisotropy of the First All-Sky X-ray Cluster Sample
We combine the recently published CIZA galaxy cluster catalogue with the
XBACs cluster sample to produce the first all-sky catalogue of X-ray clusters
in order to examine the origins of the Local Group's peculiar velocity without
the use of reconstruction methods to fill the traditional Zone of Avoidance.
The advantages of this approach are (i) X-ray emitting clusters tend to trace
the deepest potential wells and therefore have the greatest effect on the
dynamics of the Local Group and (ii) our all-sky sample provides data for
nearly a quarter of the sky that is largely incomplete in optical cluster
catalogues. We find that the direction of the Local Group's peculiar velocity
is well aligned with the CMB as early as the Great Attractor region 40 h^-1 Mpc
away, but that the amplitude of its dipole motion is largely set between 140
and 160 h^-1 Mpc. Unlike previous studies using galaxy samples, we find that
without Virgo included, roughly ~70% of our dipole signal comes from mass
concentrations at large distances (>60 h^-1 Mpc) and does not flatten,
indicating isotropy in the cluster distribution, until at least 160 h^-1 Mpc.
We also present a detailed discussion of our dipole profile, linking observed
features to the structures and superclusters that produce them. We find that
most of the dipole signal can be attributed to the Shapley supercluster
centered at about 150 h^-1 Mpc and a handful of very massive individual
clusters, some of which are newly discovered and lie well in the Zone of
Avoidance.Comment: 15 Pages, 9 Figures. Accepted by Ap
High-precision performance testing of the LHC power converters
The magnet power converters for LHC were procured in three parts, power part, current transducers and control electronics, to enable a maximum of industrial participation in the manufacturing and still guarantee the very high precision (a few parts in 10-6) required by LHC. One consequence of this approach was several stages of system tests: factory reception tests, CERN reception tests, integration tests , short-circuit tests and commissioning on the final load in the LHC tunnel. The majority of the power converters for LHC have now been delivered, integrated into complete converter and high-precision performance testing is well advanced. This paper presents the techniques used for high-precision testing and the results obtained
- âŠ