10,875 research outputs found
The far-infrared - radio correlation in dwarf galaxies
The far-infrared - radio correlation connects star formation and magnetic
fields in galaxies, and has been confirmed over a large range of far-infrared
luminosities. Recent investigations indicate that it may even hold in the
regime of local dwarf galaxies, and we explore here the expected behavior in
the regime of star formation surface densities below 0.1 M_sun kpc^{-2}
yr^{-1}. We derive two conditions that can be particularly relevant for
inducing a change in the expected correlation: a critical star formation
surface density to maintain the correlation between star formation rate and the
magnetic field, and a critical star formation surface density below which
cosmic ray diffusion losses dominate over their injection via supernova
explosions. For rotation periods shorter than 1.5x10^7 (H/kpc)^2 yrs, with H
the scale height of the disk, the first correlation will break down before
diffusion losses are relevant, as higher star formation rates are required to
maintain the correlation between star formation rate and magnetic field
strength. For high star formation surface densities Sigma_SFR, we derive a
characteristic scaling of the non-thermal radio to the far-infrared / infrared
emission with Sigma_SFR^{1/3}, corresponding to a scaling of the non-thermal
radio luminosity L_s with the infrared luminosity L_{th} as L_{th}^{4/3}. The
latter is expected to change when the above processes are no longer steadily
maintained. In the regime of long rotation periods, we expect a transition
towards a steeper scaling with Sigma_SFR^{2/3}, implying L_s~L_th^{5/3}, while
the regime of fast rotation is expected to show a considerably enhanced
scatter. These scaling relations explain the increasing thermal fraction of the
radio emission observed within local dwarfs, and can be tested with future
observations by the SKA and its precursor radio telescopes.Comment: 16 pages, 11 figures, accepted at A&
A new interpretation of the far-infrared - radio correlation and the expected breakdown at high redshift
(Abrigded) Observations of galaxies up to z 2 show a tight correlation
between far-infrared and radio continuum emission. We explain the far-infrared
- radio continuum correlation by relating star formation and magnetic field
strength in terms of turbulent magnetic field amplification, where turbulence
is injected by supernova explosions from massive stars. We calculate the
expected amount of turbulence in galaxies based on their star formation rates,
and infer the expected magnetic field strength due to turbulent dynamo
amplification. We estimate the timescales for cosmic ray energy losses via
synchrotron emission, inverse Compton scattering, ionization and bremsstrahlung
emission, probing up to which redshift strong synchrotron emission can be
maintained. We find that the correlation between star formation rate and
magnetic field strength in the local Universe can be understood as a result of
turbulent magnetic field amplification. If the typical gas density in the
interstellar medium increases at high z, we expect an increase of the magnetic
field strength and the radio emission, as indicated by current observations.
Such an increase would imply a modification of the far-infrared - radio
correlation. We expect a breakdown when inverse Compton losses start dominating
over synchrotron emission. For a given star formation surface density, we
calculate the redshift where the breakdown occurs, yielding z (Sigma_SFR/0.0045
M_solar kpc^{-2} yr^{-1})^{1/(6-alpha/2)}. In this relation, the parameter
\alpha describes the evolution of the characteristic ISM density in galaxies as
(1+z)^\alpha. Both the possible raise of the radio emission at high redshift
and the final breakdown of the far-infrared -- radio correlation at a critical
redshift will be probed by the Square Kilometre Array (SKA) and its
pathfinders, while the typical ISM density in galaxies will be probed with
ALMA.Comment: 13 pages, 14 figures, 1 table, accepted at A&A (proof corrections
included
Algebraic Quantum Theory on Manifolds: A Haag-Kastler Setting for Quantum Geometry
Motivated by the invariance of current representations of quantum gravity
under diffeomorphisms much more general than isometries, the Haag-Kastler
setting is extended to manifolds without metric background structure. First,
the causal structure on a differentiable manifold M of arbitrary dimension
(d+1>2) can be defined in purely topological terms, via cones (C-causality).
Then, the general structure of a net of C*-algebras on a manifold M and its
causal properties required for an algebraic quantum field theory can be
described as an extension of the Haag-Kastler axiomatic framework.
An important application is given with quantum geometry on a spatial slice
within the causally exterior region of a topological horizon H, resulting in a
net of Weyl algebras for states with an infinite number of intersection points
of edges and transversal (d-1)-faces within any neighbourhood of the spatial
boundary S^2.Comment: 15 pages, Latex; v2: several corrections, in particular in def. 1 and
in sec.
Cosmological particle creation in states of low energy
The recently proposed states of low energy provide a well-motivated class of
reference states for the quantized linear scalar field on cosmological
Friedmann-Robertson-Walker spacetimes. The low energy property of a state is
localized close to some value of the cosmological time coordinate. We present
calculations of the relative cosmological particle production between a state
of low energy at early time and another such state at later time. In an
exponentially expanding Universe, we find that the particle production shows
oscillations in the spatial frequency modes. The basis of the method for
calculating the relative particle production is completely rigorous.
Approximations are only used at the level of numerical calculation.Comment: 24 pages, 7 figure
Controlling the energy of defects and interfaces in the amplitude expansion of the phase-field crystal model
One of the major difficulties in employing phase field crystal (PFC) modeling
and the associated amplitude (APFC) formulation is the ability to tune model
parameters to match experimental quantities. In this work we address the
problem of tuning the defect core and interface energies in the APFC
formulation. We show that the addition of a single term to the free energy
functional can be used to increase the solid-liquid interface and defect
energies in a well-controlled fashion, without any major change to other
features. The influence of the newly added term is explored in two-dimensional
triangular and honeycomb structures as well as bcc and fcc lattices in three
dimensions. In addition, a finite element method (FEM) is developed for the
model that incorporates a mesh refinement scheme. The combination of the FEM
and mesh refinement to simulate amplitude expansion with a new energy term
provides a method of controlling microscopic features such as defect and
interface energies while simultaneously delivering a coarse-grained examination
of the system.Comment: 14 pages, 9 figure
RETURN ON INVESTMENT IN SOCIAL NETWORKS
This review focuses on electrochemical metallization memory cells (ECM), highlighting their advantages as the next generation memories. In a brief introduction, the basic switching mechanism of ECM cells is described and the historical development is sketched. In a second part, the full spectra of materials and material combinations used for memory device prototypes and for dedicated studies are presented. In a third part, the specific thermodynamics and kinetics of nanosized electrochemical cells are described. The overlapping of the space charge layers is found to be most relevant for the cell properties at rest. The major factors determining the functionality of the ECM cells are the electrode reaction and the transport kinetics. Depending on electrode and/or electrolyte material electron transfer, electro-crystallization or slow diffusion under strong electric fields can be rate determining. In the fourth part, the major device characteristics of ECM cells are explained. Emphasis is placed on switching speed, forming and SET/RESET voltage, R(ON) to R(OFF) ratio, endurance and retention, and scaling potentials. In the last part, circuit design aspects of ECM arrays are discussed, including the pros and cons of active and passive arrays. In the case of passive arrays, the fundamental sneak path problem is described and as well as a possible solution by two anti-serial (complementary) interconnected resistive switches per cell. Furthermore, the prospects of ECM with regard to further scalability and the ability for multi-bit data storage are addressed
Instanton filtering for the stochastic Burgers equation
We address the question whether one can identify instantons in direct
numerical simulations of the stochastically driven Burgers equation. For this
purpose, we first solve the instanton equations using the Chernykh-Stepanov
method [Phys. Rev. E 64, 026306 (2001)]. These results are then compared to
direct numerical simulations by introducing a filtering technique to extract
prescribed rare events from massive data sets of realizations. Using this
approach we can extract the entire time history of the instanton evolution
which allows us to identify the different phases predicted by the direct method
of Chernykh and Stepanov with remarkable agreement
Nonlinear magnetic field dependence of the conductance in d-wave NIS tunnel junctions
The ab-plane NIS-tunnelling conductance in d-wave superconductors shows a
zero-bias conductance peak which is predicted to split in a magnetic field. In
a pure d-wave superconductor the splitting is linear for fields small on the
scale of the thermodynamic critical field. The field dependence is shown to be
nonlinear, even at low fields, in the vicinity of a surface phase transition
into a local time-reversal symmetry breaking state. The field evolution of the
conductance is sensitive to temperature, doping, and the symmetry of the
sub-dominant pairing channel.Comment: 4 pages, 4 figure
Organizational intelligence and negotiation based DAI systems – theoretical foundations and experimental results
A steadily increasing number of researchers believes that so-called ’organizational’ multi agent systems are a key technology to support information and knowledge processing activities in cooperative, networked organizations. This, in turn, necessitates their integration with the underlying human-centred organization.The concept of an ’organization’ has emerged as central to the structuring of activities of both decentralized industrial and commercial conglomerates and collections of intelligent problem solvers within Distributed Artificial Intelligence (DAI) systems. Of late a new discipline has begun to emerge, that of Organizational Intelligence (OI). Organizational Intelligence demands a greater synthesis between the principles of Organization Theory (OT) and DAI, by the explicit incorporation of theories of both organizations and DAI into the field of OI. This paper concentrates on two rather important features of OI, namely organizational memory and learning capabilities. It will first discuss the theoretical foundations. Then it will be shown how the contract net approach can be extended to meet these demands. Finally, it will be proved by some experimental results that the increased "intellectual" capabilities of the extended contract net will substantially contribute to the performance as well as the quality of solution processes.<br/
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