564 research outputs found
Open cell fire-resistant foam
Candidate polyphosphazene polymers were investigated to develop a fire-resistant, thermally stable and flexible open cell foam. The copolymers were prepared in several mole ratios of the substituent side chains and a (nominal) 40:60 derivative was selected for formulation studies. Synthesis of the polymers involved solution by polymerization of hexachlorophosphazene to soluble high molecular weight poly(dichlorophosphazene), followed by derivatization of the resultant polymer in a normal fashion to give polymers in high yield and high molecular weight. Small amounts of a cure site were incorporated into the polymer for vulcanization purposes. The poly(aryloxyphosphazenes) exhibited good thermal stability and the first polymer mentioned above exhibited the best thermal behavior of all the candidate polymers studied
The Dark Matter Telescope
Weak gravitational lensing enables direct reconstruction of dark matter maps
over cosmologically significant volumes. This research is currently
telescope-limited. The Dark Matter Telescope (DMT) is a proposed 8.4 m
telescope with a 3 degree field of view, with an etendue of 260 , ten times greater than any other current or planned telescope. With
its large etendue and dedicated observational mode, the DMT fills a nearly
unexplored region of parameter space and enables projects that would take
decades on current facilities. The DMT will be able to reach 10-sigma limiting
magnitudes of 27-28 magnitude in the wavelength range .3 - 1 um over a 7 square
degree field in 3 nights of dark time. Here we review its unique weak lensing
cosmology capabilities and the design that enables those capabilities.Comment: in-press version with additions; to appear in proceedings of the Dark
Matter 2000 conference (Santa Monica, February 2000) to be published by
Springe
Detection of weak gravitational lensing distortions of distant galaxies by cosmic dark matter at large scales
Most of the matter in the universe is not luminous and can be observed
directly only through its gravitational effect. An emerging technique called
weak gravitational lensing uses background galaxies to reveal the foreground
dark matter distribution on large scales. Light from very distant galaxies
travels to us through many intervening overdensities which gravitationally
distort their apparent shapes. The observed ellipticity pattern of these
distant galaxies thus encodes information about the large-scale structure of
the universe, but attempts to measure this effect have been inconclusive due to
systematic errors. We report the first detection of this ``cosmic shear'' using
145,000 background galaxies to reveal the dark matter distribution on angular
scales up to half a degree in three separate lines of sight. The observed
angular dependence of this effect is consistent with that predicted by two
leading cosmological models, providing new and independent support for these
models.Comment: 18 pages, 5 figures: To appear in Nature. (This replacement fixes tex
errors and typos.
Multi-wavelength Analysis of the Merging Galaxy Cluster A115
A115 is a merging galaxy cluster at with a number of remarkable
features including a giant ( Mpc) radio relic, two asymmetric X-ray
peaks with trailing tails, and a peculiar line-of-sight velocity structure. We
present a multi-wavelength study of A115 using optical imaging data from
Subaru, X-ray data from , and spectroscopic data from the Keck/DEIMOS
and MMT/Hectospec instruments. Our weak-lensing analysis shows that the cluster
is comprised of two subclusters whose mass centroids are in excellent agreement
with the two BCG positions ("). By modeling A115 with a
superposition of two Navarro-Frenk-White halos, we determine the masses of the
northern and southern subclusters to be and , respectively. Combining the two halos, we estimate the total
cluster mass to be
at Mpc. These weak-lensing masses are
significantly (a factor of 3-10) lower than what is implied by the X-ray and
optical spectroscopic data. We attribute the difference to the gravitational
and hydrodynamic disruption caused by the collision between the two
subclusters.Comment: 20 pages, 17 figures, Accepted for publication in Ap
Measuring the Reduced Shear
Neglecting the second order corrections in weak lensing measurements can lead
to a few percent uncertainties on cosmic shears, and becomes more important for
cluster lensing mass reconstructions. Existing methods which claim to measure
the reduced shears are not necessarily accurate to the second order when a
point spread function (PSF) is present. We show that the method of Zhang (2008)
exactly measures the reduced shears at the second order level in the presence
of PSF. A simple theorem is provided for further confirming our calculation,
and for judging the accuracy of any shear measurement method at the second
order based on its properties at the first order. The method of Zhang (2008) is
well defined mathematically. It does not require assumptions on the
morphologies of galaxies and the PSF. To reach a sub-percent level accuracy,
the CCD pixel size is required to be not larger than 1/3 of the Full Width at
Half Maximum (FWHM) of the PSF. Using a large ensemble (> 10^7) of mock
galaxies of unrestricted morphologies, we find that contaminations to the shear
signals from the noise of background photons can be removed in a well defined
way because they are not correlated with the source shapes. The residual shear
measurement errors due to background noise are consistent with zero at the
sub-percent level even when the amplitude of such noise reaches about 1/10 of
the source flux within the half-light radius of the source. This limit can in
principle be extended further with a larger galaxy ensemble in our simulations.
On the other hand, the source Poisson noise remains to be a cause of systematic
errors. For a sub-percent level accuracy, our method requires the amplitude of
the source Poisson noise to be less than 1/80 ~ 1/100 of the source flux within
the half-light radius of the source, corresponding to collecting roughly 10^4
source photons.Comment: 18 pages, 3 figures, 4 tables, minor changes from the previous
versio
Decuplet Baryon Structure from Lattice QCD
The electromagnetic properties of the SU(3)-flavor baryon decuplet are
examined within a lattice simulation of quenched QCD. Electric charge radii,
magnetic moments, and magnetic radii are extracted from the E0 and M1 form
factors. Preliminary results for the E2 and M3 moments are presented giving the
first model independent insight to the shape of the quark distribution in the
baryon ground state. As in our octet baryon analysis, the lattice results give
evidence of spin-dependent forces and mass effects in the electromagnetic
properties. The quark charge distribution radii indicate these effects act in
opposing directions. Some baryon dependence of the effective quark magnetic
moments is seen. However, this dependence in decuplet baryons is more subtle
than that for octet baryons. Of particular interest are the lattice predictions
for the magnetic moments of and for which new recent
experimental measurements are available. The lattice prediction of the
ratio appears larger than the experimental ratio, while the
lattice prediction for the magnetic moment ratio is in good
agreement with the experimental ratio.Comment: RevTeX manuscript, 34 pages plus 21 figures (available upon request
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio
First Results On Shear-Selected Clusters From the Deep Lens Survey: Optical Imaging, Spectroscopy, and X-ray Followup
We present the first sample of galaxy clusters selected on the basis of their
weak gravitational lensing shear. The shear induced by a cluster is a function
of its mass profile and its redshift relative to the background galaxies being
sheared; in contrast to more traditional methods of selecting clusters, shear
selection does not depend on the cluster's star formation history, baryon
content, or dynamical state. Because mass is the property of clusters which
provides constraints on cosmological parameters, the dependence on these other
parameters could induce potentially important biases in traditionally-selected
samples. Comparison of a shear-selected sample with optically and X-ray
selected samples is therefore of great importance. Here we present the first
step toward a new shear-selected sample: the selection of cluster candidates
from the first 8.6 deg of the 20 deg Deep Lens Survey (DLS), and
tabulation of their basic properties such as redshifts and optical and X-ray
counterparts.Comment: v2 as published in ApJ, 45 pages, 18 figures. Biggest change from v1
is addition of a figure showing the 1-point distribution of pixels in the
mass maps and control "mass maps" using the non-tangential component of
shear, demonstrating that all candidate peaks are higher than any peak seen
in the control map
Cosmic Shear from STIS Pure Parallels: Analysis
The measurement of cosmic shear requires deep imaging with high image quality
on many lines of sight to sample the statistics of large-scale structure. The
expected distortion of galaxy images by cosmic shear on the STIS angular scale
is a few percent, therefore the PSF anisotropy has to be understood and
controlled to an accuracy better than 1%. In this poster we present the
analysis of the PSF of STIS and a preliminary cosmic shear measurement using
archival data from the STIS pure parallel program to show that the STIS camera
on-board HST is well suited for our project. The data reduction and catalog
production are described in an accompanying paper (astro-ph/0102330).Comment: 4 pages, 5 figures, to appear in the proceedings of the STScI 2001
spring symposium "The Dark Universe: Matter, Energy and Gravity" Baltimore
April 2-5 2001, acknowledgements adde
Data Deluge in Astrophysics: Photometric Redshifts as a Template Use Case
Astronomy has entered the big data era and Machine Learning based methods
have found widespread use in a large variety of astronomical applications. This
is demonstrated by the recent huge increase in the number of publications
making use of this new approach. The usage of machine learning methods, however
is still far from trivial and many problems still need to be solved. Using the
evaluation of photometric redshifts as a case study, we outline the main
problems and some ongoing efforts to solve them.Comment: 13 pages, 3 figures, Springer's Communications in Computer and
Information Science (CCIS), Vol. 82
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