13,065 research outputs found
Detection of a population gradient in the Sagittarius Stream
We present a quantitative comparison between the Horizontal Branch morphology
in the core of the Sagittarius dwarf spheroidal galaxy (Sgr) and in a wide
field sampling a portion of its tidal stream (Sgr Stream), located tens of kpc
away from the center of the parent galaxy. We find that the Blue Horizontal
Branch (BHB) stars in that part of the Stream are five times more abundant than
in the Sgr core, relative to Red Clump stars. The difference in the ratio of
BHB to RC stars between the two fields is significant at the 4.8 sigma level.
This indicates that the old and metal-poor population of Sgr was preferentially
stripped from the galaxy in past peri-Galactic passages with respect to the
intermediate-age metal rich population that presently dominates the bound core
of Sgr, probably due to a strong radial gradient that was settled within the
galaxy before its disruption. The technique adopted in the present study allows
to trace population gradients along the whole extension of the Stream.Comment: 4 pages, 3 .ps figures (fig. 1 at low resolution); Accepted for
publication by A&A Letter
Integer Factorization with a Neuromorphic Sieve
The bound to factor large integers is dominated by the computational effort
to discover numbers that are smooth, typically performed by sieving a
polynomial sequence. On a von Neumann architecture, sieving has log-log
amortized time complexity to check each value for smoothness. This work
presents a neuromorphic sieve that achieves a constant time check for
smoothness by exploiting two characteristic properties of neuromorphic
architectures: constant time synaptic integration and massively parallel
computation. The approach is validated by modifying msieve, one of the fastest
publicly available integer factorization implementations, to use the IBM
Neurosynaptic System (NS1e) as a coprocessor for the sieving stage.Comment: Fixed typos in equation for modular roots (Section II, par. 6;
Section III, par. 2) and phase calculation (Section IV, par 2
The Local Galaxy Density and the Arm Class of Spiral Galaxies
We have examined the effect of the environmental density on the arm
classification of an extensive sample of spiral galaxies included in the Nearby
Galaxy Catalog (Tully, 1988a). We have also explored the dependence of the arm
class of a galaxy on other factors, such as its blue absolute magnitude and its
disk-to-total mass ratio, inferred in the literature either from the gradient
of a good galaxy rotation curve or from a photometric mass decomposition
method. We have found that the arm class is strongly related to the absolute
magnitude in the mid-type spirals (in the sense that grand design galaxies are,
on average, more luminous than flocculent objects), whilst this relation is
considerably weaker in the early and late types. In general the influence of
the local density on the arm structure appears to be much weaker than that of
the absolute magnitude. The local density acts essentially in strengthening the
arm class--absolute magnitude relation for the mid types, whereas no
environmental density effects are observed in the early and late types. Using
the most recent estimates of the disk-to-total mass ratio, we do not confirm
this ratio to be a significant factor which affects the arm class;
nevertheless, owing to poor statistics and large uncertanties, the issue
remains open. Neither a local density effect nor an unambiguous bar effect on
the disk-to-total mass ratio is detectable; the latter finding may challenge
some theoretical viewpoints on the formation of bar structures.Comment: 15 pages, Latex, SISSA 102/93/A openbib.sty and 4 POSTSCRIPT figures
appende
Discrete port-controlled Hamiltonian dynamics and average passivation
The paper discusses the modeling and control of port-controlled Hamiltonian dynamics in a pure discrete-time domain. The main result stands in a novel differential-difference representation of discrete port-controlled Hamiltonian systems using the discrete gradient. In these terms, a passive output map is exhibited as well as a passivity based damping controller underlying the natural involvement of discrete-time average passivity
A quantitative investigation of the effect of a close-fitting superconducting shield on the coil-factor of a solenoid
Superconducting shields are commonly used to suppress external magnetic
interference. We show, that an error of almost an order of magnitude can occur
in the coil-factor in realistic configurations of the solenoid and the shield.
The reason is that the coil-factor is determined by not only the geometry of
the solenoid, but also the nearby magnetic environment. This has important
consequences for many cryogenic experiments involving magnetic fields such as
the determination of the parameters of Josephson junctions, as well as other
superconducting devices. It is proposed to solve the problem by inserting a
thin sheet of high-permeability material, and the result numerically tested.Comment: 3 pages, 4 figures, submitted to AP
Planar Josephson Tunnel Junctions in an Asymmetric Magnetic Field
We analyze the consequences resulting from the asymmetric boundary conditions
imposed by a non-uniform external magnetic field at the extremities of a planar
Josephson tunnel junction and predict a number of testable signatures. When the
junction length is smaller than its Josephson penetration depth
, static analytical calculations lead to a Fresnel-like magnetic
diffraction pattern, rather than a Fraunhofer-like one typical of a uniform
field. Numerical simulations allow to investigate intermediate length
() and long () junctions. We consider both
uniform and -shaped bias distributions. We also speculate on the
possibility of exploiting the unique static properties of this system for basic
experiments and devices.Comment: 9 pages, 3 figure
Fish Habitat Utilization Patterns and Evaluation of the Efficacy of Marine Protected Areas in Hawaii: Integration of NOAA Digital Benthic Habitat Mapping and Coral Reef Ecological Studies
Over the past four decades, the state of Hawaii has developed a system of eleven Marine Life Conservation Districts (MLCDs) to conserve and replenish marine resources around the state. Initially established to provide opportunities for public interaction with the marine environment, these MLCDs vary in size, habitat quality, and management regimes, providing an excellent opportunity to test hypotheses concerning marine protected area (MPA) design and function using multiple discreet sampling units. NOAA/NOS/NCCOS/Center for Coastal Monitoring and Assessment’s Biogeography Team developed digital benthic habitat maps for all MLCD and adjacent habitats. These maps were used to evaluate the efficacy of existing MLCDs for biodiversity conservation and fisheries replenishment, using a spatially explicit stratified random sampling design. Coupling the distribution of habitats and species habitat affinities using GIS technology elucidates species habitat utilization patterns at scales that are commensurate with ecosystem processes and is useful in defining essential fish habitat and biologically relevant boundaries for MPAs.
Analysis of benthic cover validated the a priori classification of habitat types and provided justification for using these habitat strata to conduct stratified random sampling and analyses of fish habitat utilization patterns. Results showed that the abundance and distribution of species and assemblages exhibited strong correlations with habitat types. Fish assemblages in the colonized and uncolonized hardbottom habitats were found to be most similar among all of the habitat types. Much of the macroalgae habitat sampled was macroalgae growing on hard substrate, and as a result showed similarities with the other hardbottom assemblages. The fish assemblages in the sand habitats were highly variable but distinct from the other habitat types.
Management regime also played an important role in the abundance and distribution of fish assemblages. MLCDs had higher values for most fish assemblage characteristics (e.g. biomass, size, diversity) compared with adjacent fished areas and Fisheries Management Areas (FMAs) across all habitat types. In addition, apex predators and other targeted resources species were more abundant and larger in the MLCDs, illustrating the effectiveness of these closures in conserving fish populations. Habitat complexity, quality, size and level of protection from fishing were important determinates of MLCD effectiveness with respect to their associated fish assemblages. (PDF contains 217 pages
Plasmons in Sodium under Pressure: Increasing Departure from Nearly-Free-Electron Behavior
We have measured plasmon energies in Na under high pressure up to 43 GPa
using inelastic x-ray scattering (IXS). The momentum-resolved results show
clear deviations, growing with increasing pressure, from the predictions for a
nearly-free electron metal. Plasmon energy calculations based on
first-principles electronic band structures and a quasi-classical plasmon model
allow us to identify a pressure-induced increase in the electron-ion
interaction and associated changes in the electronic band structure as the
origin of these deviations, rather than effects of exchange and correlation.
Additional IXS results obtained for K and Rb are addressed briefly.Comment: 5 pages, 4 figure
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