4,703 research outputs found
Measurement of the Mass Profile of Abell 1689
In this letter we present calibrated mass and light profiles of the rich
cluster of galaxies Abell 1689 out to 1 Mpc from the center. The high
surface density of faint blue galaxies at high redshift, selected by their low
surface brightness, are unique tools for mapping the projected mass
distribution of foreground mass concentrations. The systematic gravitational
lens distortions of of these background galaxies in 15\arcmin\ fields
reveal detailed mass profiles for intervening clusters of galaxies, and are a
direct measure of the growth of mass inhomogeneity. The mass is measured
directly, avoiding uncertainties encountered in velocity or X-ray derived mass
estimates.
Mass in the rich cluster Abell 1689 follows smoothed light, outside 100
h kpc, with a rest-frame V band mass-to-light ratio of
. Near the cluster center, mass appears to be more
smoothly distributed than light. Out to a radius of 1 Mpc the total
mass follows a steeper than isothermal profile. Comparing with preliminary high
resolution N-body clustering simulations for various cosmogonies on these
scales, these data are incompatible with hot dark matter, a poor fit to most
mixed dark matter models, and favor open or cold dark matter.
Substructure is seen in both the mass and the light, but detailed
correspondence is erased on scales less than 100 kpc.Comment: 13 pages, uuencoded, compressed postscript file, 2 figures included
additional 1Mbyte figure available on request. Only change is that in
original errorbars on Fig. 5 were a factor of 2 too big
Spiral Waves in Media with Complex Excitable Dynamics
The structure of spiral waves is investigated in super-excitable
reaction-diffusion systems where the local dynamics exhibits multi-looped phase
space trajectories. It is shown that such systems support stable spiral waves
with broken symmetry and complex temporal dynamics. The main structural
features of such waves, synchronization defect lines, are demonstrated to be
similar to those of spiral waves in systems with complex-oscillatory dynamics.Comment: to appear in International Journal of Bifurcation and Chao
The LSST Data Mining Research Agenda
We describe features of the LSST science database that are amenable to
scientific data mining, object classification, outlier identification, anomaly
detection, image quality assurance, and survey science validation. The data
mining research agenda includes: scalability (at petabytes scales) of existing
machine learning and data mining algorithms; development of grid-enabled
parallel data mining algorithms; designing a robust system for brokering
classifications from the LSST event pipeline (which may produce 10,000 or more
event alerts per night); multi-resolution methods for exploration of petascale
databases; indexing of multi-attribute multi-dimensional astronomical databases
(beyond spatial indexing) for rapid querying of petabyte databases; and more.Comment: 5 pages, Presented at the "Classification and Discovery in Large
Astronomical Surveys" meeting, Ringberg Castle, 14-17 October, 200
X-ray absorption study of Ti-activated sodium aluminum hydride
Ti K-edge x-ray absorption near edge spectroscopy (XANES) was used to explore
the Ti valence and coordination in Ti-activated sodium alanate. An empirical
relationship was established between the Ti valence and the Ti K-edge onset
based on a set of standards. This relationship was used to estimate oxidation
states of the titanium catalyst in 2 mol% and 4 mol% Ti-doped NaAlH4. These
results demonstrate that the formal titanium valence is zero in doped sodium
alanate and nearly invariant during hydrogen cycling. A qualitative comparison
of the edge fine structure suggests that the Ti is present on the surface in
the form of amorphous TiAl3.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
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