14,287 research outputs found
Alignment and morphology of elliptical galaxies: the influence of the cluster tidal field
We investigate two possible effects of the tidal field induced by a spherical
cluster on its elliptical galaxy members: the modification of the ellipticity
of a spherical galaxy and the isophotal alignment in the cluster radial
direction of a misaligned prolate galaxy. Numerical N-body simulations have
been performed for radial and circular galactic orbits. The properties of the
stars' zero--velocity surfaces in the perturbed galaxies are explored briefly,
and the adiabaticity of the galaxy to the external field is discussed. For a
choice of parameters characteristic of rich clusters we find that the induced
ellipticity on a spherical galaxy is below or close to the detectability level.
But we find that the tidal torque can result in significant isophotal alignment
of the galaxies' major axis with the cluster radial direction if the galaxy is
outside the cluster core radius. The time required for the alignment is very
short compared with the Hubble time. A significant increase in the ellipticity
of the outer isophotes of the prolate model is also found, but with no
observable isophotal twisting. Our main prediction is an alignment segregation
of the elliptical galaxy population according to whether their orbits lie
mostly outside or inside the cluster core radius. These results also suggest
that galactic alignment in rich clusters is not incompatible with a bottom-up
galaxy formation scenario.Comment: 20 pages, uuencoded compressed tarred postscrip
Cooperative information sharing to improve distributed learning in multi-agent systems
Effective coordination of agents' actions in partially-observable domains is a major challenge of multi-agent systems research. To address this, many researchers have developed techniques that allow the agents to make decisions based on estimates of the states and actions of other agents that are typically learnt using some form of machine learning algorithm. Nevertheless, many of these approaches fail to provide an actual means by which the necessary information is made available so that the estimates can be learnt. To this end, we argue that cooperative communication of state information between agents is one such mechanism. However, in a dynamically changing environment, the accuracy and timeliness of this communicated information determine the fidelity of the learned estimates and the usefulness of the actions taken based on these. Given this, we propose a novel information-sharing protocol, post-task-completion sharing, for the distribution of state information. We then show, through a formal analysis, the improvement in the quality of estimates produced using our strategy over the widely used protocol of sharing information between nearest neighbours. Moreover, communication heuristics designed around our information-sharing principle are subjected to empirical evaluation along with other benchmark strategies (including Littman's Q-routing and Stone's TPOT-RL) in a simulated call-routing application. These studies, conducted across a range of environmental settings, show that, compared to the different benchmarks used, our strategy generates an improvement of up to 60% in the call connection rate; of more than 1000% in the ability to connect long-distance calls; and incurs as low as 0.25 of the message overhead
The Capture and Escape of Stars
The shape of galaxies depends on their orbital populations. These populations
change through capture into and escape from resonance. Capture problems fall
into distinct cases depending upon the shape of the potential well. To
visualise the effective potential well for orbital capture, a diagrammatic
approach to the resonant perturbation theory of Born is presented. These
diagrams we call equiaction sections. To illustrate their use, we present
examples drawn from both galactic and Solar System dynamics. The probability of
capture for generic shapes of the potential well is calculated. A number of
predictions are made. First, there are barred galaxies that possess two outer
rings of gas and stars (type R1R2). We show how to relate changes in the
pattern speed and amplitude of the bar to the strength of the two rings.
Secondly, under certain conditions, small disturbances can lead to dramatic
changes in orbital shape. This can be exploited as a mechanism to pump
counter-rotating stars and gas into the nuclei of disk galaxies. Tidal resonant
forcing of highly inclined orbits around a central mass causes a substantial
increase in the likelihood of collision. Thirdly, the angular momentum of a
potential well is changed by the passage of stars across or capture into the
well. This can lead to the creation of holes, notches and high velocity tails
in the stellar distribution function, whose form we explicitly calculate.Comment: 22 pages, Tex file, 19 Postscript figures, uses psfig 2 further
figures are not Postscript, but are available on emailing the authors, MNRA
Generating Cosmological Gaussian Random Fields
We present a generic algorithm for generating Gaussian random initial
conditions for cosmological simulations on periodic rectangular lattices. We
show that imposing periodic boundary conditions on the real-space correlator
and choosing initial conditions by convolving a white noise random field
results in a significantly smaller error than the traditional procedure of
using the power spectrum. This convolution picture produces exact correlation
functions out to separations of L/2, where L is the box size, which is the
maximum theoretically allowed. This method also produces tophat sphere
fluctuations which are exact at radii . It is equivalent to
windowing the power spectrum with the simulation volume before discretizing,
thus bypassing sparse sampling problems. The mean density perturbation in the
volume is no longer constrained to be zero, allowing one to assemble a large
simulation using a series of smaller ones. This is especially important for
simulations of Lyman- systems where small boxes with steep power
spectra are routinely used.
We also present an extension of this procedure which generates exact initial
conditions for hierarchical grids at negligible cost.Comment: 12 pages incl 3 figures, accepted in ApJ Letter
Struggling to a monumental triumph : Re-assessing the final stages of the smallpox eradication program in India, 1960-1980
The global smallpox program is generally presented as the brainchild of a handful of actors from the WHO headquarters in Geneva and at the agency's regional offices. This article attempts to present a more complex description of the drive to eradicate smallpox. Based on the example of India, a major focus of the campaign, it is argued that historians and public health officials should recognize the varying roles played by a much wider range of participants. Highlighting the significance of both Indian and international field officials, the author shows how bureaucrats and politicians at different levels of administration and society managed to strengthen—yet sometimes weaken—important program components. Centrally dictated strategies developed at WHO offices in Geneva and New Delhi, often in association with Indian federal authorities, were reinterpreted by many actors and sometimes changed beyond recognition
Nucleon decay in gauge unified models with intersecting D6-branes
Baryon number violation is discussed in gauge unified orbifold models of type
II string theory with intersecting Dirichlet branes. We consider setups of
D6-branes which extend along the flat Minkowski space-time directions and wrap
around 3-cycles of the internal 6-d manifold. The discussion is motivated by
the enhancement effect of low energy amplitudes anticipated for M-theory and
type II string theory models with matter modes localized at points of the
internal manifold. The conformal field theory formalism is used to evaluate the
open string amplitudes at tree level. We study the single baryon number
violating processes of dimension 6 and 5, involving four quarks and leptons and
in supersymmetry models, two pairs of matter fermions and superpartner
sfermions. The higher order processes associated with the baryon number
violating operators of dimension 7 and 9 are also examined, but in a
qualitative way. We discuss the low energy representation of string theory
amplitudes in terms of infinite series of poles associated to exchange of
string Regge resonance and compactification modes. The comparison of string
amplitudes with the equivalent field theory amplitudes is first studied in the
large compactification radius limit. Proceeding next to the finite
compactification radius case, we present a numerical study of the ratio of
string to field theory amplitudes based on semi-realistic gauge unified
non-supersymmetric and supersymmetric models employing the Z3 and Z2xZ2
orbifolds. We find a moderate enhancement of string amplitudes which becomes
manifest in the regime where the gauge symmetry breaking mass parameter exceeds
the compactification mass parameter, corresponding to a gauge unification in a
seven dimensional space-time.Comment: 63 pages revtex4. 8 postscript figures. 4 tables. Subsection II.B
revised. Several new references added. To appear in Physical Review
Continuous Cold-atom Inertial Sensor with Rotation Stability
We report the operation of a cold-atom inertial sensor which continuously
captures the rotation signal. Using a joint interrogation scheme, where we
simultaneously prepare a cold-atom source and operate an atom interferometer
(AI) enables us to eliminate the dead times. We show that such continuous
operation improves the short-term sensitivity of AIs, and demonstrate a
rotation sensitivity of in a
cold-atom gyroscope of Sagnac area. We also demonstrate a
rotation stability of at s of integration time,
which establishes the record for atomic gyroscopes. The continuous operation of
cold-atom inertial sensors will enable to benefit from the full sensitivity
potential of large area AIs, determined by the quantum noise limit.Comment: 4 pages, 3 figure
Oscillations of solar atmosphere neutrinos
The Sun is a source of high energy neutrinos (E > 10 GeV) produced by cosmic
ray interactions in the solar atmosphere. We study the impact of three-flavor
oscillations (in vacuum and in matter) on solar atmosphere neutrinos, and
calculate their observable fluxes at Earth, as well as their event rates in a
kilometer-scale detector in water or ice. We find that peculiar three-flavor
oscillation effects in matter, which can occur in the energy range probed by
solar atmosphere neutrinos, are significantly suppressed by averaging over the
production region and over the neutrino and antineutrino components. In
particular, we find that the relation between the neutrino fluxes at the Sun
and at the Earth can be approximately expressed in terms of phase-averaged
``vacuum'' oscillations, dominated by a single mixing parameter (the angle
theta_23).Comment: v2: 11 pages, 8 eps figures. Content added (Sec. III D and Fig. 6),
references updated. Matches the published versio
Steps on current-voltage characteristics of a silicon quantum dot covered by natural oxide
Considering a double-barrier structure formed by a silicon quantum dot
covered by natural oxide with two metallic terminals, we derive simple
conditions for a step-like voltage-current curve. Due to standard chemical
properties, doping phosphorus atoms located in a certain domain of the dot form
geometrically parallel current channels. The height of the current step
typically equals to (1.2 pA)N, where N=0,1,2,3... is the number of doping atoms
inside the domain, and only negligibly depends on the actual position of the
dopants. The found conditions are feasible in experimentally available
structures.Comment: 4 pages, 3 figure
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