14,345 research outputs found
Sampling eigenmodes in colloidal solids
We study the properties of correlation matrices widely used in the
characterisation of vibrational modes in colloidal materials. We show that the
eigenvectors in the middle of the spectrum are strongly mixed, but that at both
the top and the bottom of the spectrum it is possible to extract a good
approximation to the true eigenmodes of an elastic system.Comment: 6 pages, 10 figure
Cavity averages for hard spheres in the presence of polydispersity and incomplete data
We develop a cavity-based method which allows to extract thermodynamic
properties from position information in hard-sphere/disk systems. So far, there
are 'available-volume' and 'free-volume' methods. We add a third one, which we
call 'available-volume-after-takeout', and which is shown to be mathematically
equivalent to the others. In applications, where data sets are finite, all
three methods show limitations, and they do this in different parameter ranges.
We illustrate the principal equivalence and the limitations on data from
molecular dynamics -- In particular, we test robustness against missing data.
We have in mind experimental limitations where there is a small polydispersity,
say 4% in the particle radii, but individual radii cannot be determined. We
observe that, depending on the used method, the errors in such a situation are
easily 100% for the pressure and 10kT for the chemical potentials. Our work is
meant as guideline to the experimentalist for choosing the right one of the
three methods, in order to keep the outcome of experimental data analysis
meaningful.Comment: 13 pages, 6 figures. The final publication is available at Springer
via http://dx.doi.org/10.1140/epje/i2015-15097-
The range and nature of effective interactions in hard-sphere solids
Colloidal systems observed in video microscopy are often analysed using the
displacements correlation matrix of particle positions. In non-thermal systems,
the inverse of this matrix can be interpreted as a pair-interaction potential
between particles. If the system is thermally agitated, however, only an
effective interaction is accessible from the correlation matrix. We show how
this effective interaction differs from the non-thermal case by comparing with
high-statistics numerical data from hard-sphere crystals.Comment: 12 pages, 10 figures, Soft Matter 201
The effect of ram pressure on the star formation, mass distribution and morphology of galaxies
We investigate the dependence of star formation and the distribution of the
components of galaxies on the strength of ram pressure. Several mock
observations in X-ray, H and HI wavelength for different ram-pressure
scenarios are presented. By applying a combined N-body/hydrodynamic description
(GADGET-2) with radiative cooling and a recipe for star formation and stellar
feedback 12 different ram-pressure stripping scenarios for disc galaxies were
calculated. Special emphasis was put on the gas within the disc and in the
surroundings. All gas particles within the computational domain having the same
mass resolution. The relative velocity was varied from 100 km/s to 1000 km/s in
different surrounding gas densities in the range from to
g/cm. The temperature of the surrounding gas was
initially K. The star formation of a galaxy is enhanced by more
than a magnitude in the simulation with a high ram-pressure (
dyn/cm) in comparison to the same system evolving in isolation. The
enhancement of the star formation depends more on the surrounding gas density
than on the relative velocity. Up to 95% of all newly formed stars can be found
in the wake of the galaxy out to distances of more than 350 kpc behind the
stellar disc. Continuously stars fall back to the old stellar disc, building up
a bulge-like structure. Young stars can be found throughout the stripped wake
with surface densities locally comparable to values in the inner stellar disc.
Ram-pressure stripping can shift the location of star formation from the disc
into the wake on very short timescales. (Abridged)Comment: 19 pages, 25 figures, A&A accepted, high resolution version can be
found at http://astro.uibk.ac.at/~wolfgang/kapferer_rps_galaxies.pd
Losing Weight: A KECK Spectroscopic Survey of the Massive Cluster of Galaxies RX J1347-1145
We present a sample of 47 spectroscopically confirmed members of RX
J1347-1145, the most luminous X-ray cluster of galaxies discovered to date.
With two exceptions, all the galaxies in this sample have red B-R colors and
red spectral indices, with spectra similar to old local ellipticals. Using all
47 cluster members, we derive a mean redshift of 0.4509\pm 0.003, and a
velocity dispersion of 910\pm130 km/sec, which corresponds to a virial mass of
4.4 x 10^{14} h^{-1} Solar masses with an harmonic radius of 380 h^{-1} kpc.
The derived total dynamical mass is marginally consistent with that deduced
from the cluster's X-ray emission based on the analysis of ROSAT/ASCA images
(Schindler et al. 1997), but not consistent with the more recent X-ray analyses
of Allen (2000), Ettori, Allen & Fabian (2001) and Allen, Schmidt & Fabian
(2002). Furthermore, the dynamical mass is significantly smaller than that
derived from weak lensing (Fischer & Tyson 1997) and from strong lensing (Sahu
et al. 1998). We propose that these various discrepant mass estimates may be
understood if RX J1347-1145 is the product of two clusters caught in the act of
merging in a direction perpendicular to the line of sight, although there is no
evidence from the galaxy redshift distribution supporting this hypothesis. Even
with this hypothesis, a significant part of the extremely high X-ray luminosity
must still arise from non-virialized, presumably shocked, gas. Finally, we
report the serendipitous discovery of a lensed background galaxy at z=4.083
which will put strong constraints on the lensing mass determination once its
counter-image is securely identified.Comment: Minor changes to conform to version accepted by Ap
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