23,723 research outputs found
On the Lyman-alpha Emission of Starburst Galaxies
Nearby starburst galaxies have consistently shown anomalous Ly-alpha/H-beta
ratios. By re-analysing the published IUE/optical observations, we show that
most starbursts present a normal Ly-alpha emission, consistent with case B
recombination theory, provided extinction laws appropriate to their
metallicities are used. This implies that extinction is more important than
multiple resonant scattering effects. The anomalous emission and absorption
lines present in a few remaining galaxies are simply explained if they are
observed in the post-burst phase, between about 10 and 10 yrs after the
start of the burst. We use updated stellar population synthesis models to show
that anomalous ratios are produced by the aging of stellar populations, since
the underlying stellar Ly-alpha line is important in the cooler massive stars.
The inferred low-duty cycle of massive star formation accounts naturally for
the failure to detect large numbers of Ly-alpha--emitting galaxies in deep
surveys and at high redshift. Some testable predictions of the proposed
scenario are also discussed.Comment: 7 PostScript pages with 4 Figures (included), astro-ph/yymmnn
Max-Weight Revisited: Sequences of Non-Convex Optimisations Solving Convex Optimisations
We investigate the connections between max-weight approaches and dual
subgradient methods for convex optimisation. We find that strong connections
exist and we establish a clean, unifying theoretical framework that includes
both max-weight and dual subgradient approaches as special cases. Our analysis
uses only elementary methods, and is not asymptotic in nature. It also allows
us to establish an explicit and direct connection between discrete queue
occupancies and Lagrange multipliers.Comment: convex optimisation, max-weight scheduling, backpressure, subgradient
method
Kernel Manifold Alignment
We introduce a kernel method for manifold alignment (KEMA) and domain
adaptation that can match an arbitrary number of data sources without needing
corresponding pairs, just few labeled examples in all domains. KEMA has
interesting properties: 1) it generalizes other manifold alignment methods, 2)
it can align manifolds of very different complexities, performing a sort of
manifold unfolding plus alignment, 3) it can define a domain-specific metric to
cope with multimodal specificities, 4) it can align data spaces of different
dimensionality, 5) it is robust to strong nonlinear feature deformations, and
6) it is closed-form invertible which allows transfer across-domains and data
synthesis. We also present a reduced-rank version for computational efficiency
and discuss the generalization performance of KEMA under Rademacher principles
of stability. KEMA exhibits very good performance over competing methods in
synthetic examples, visual object recognition and recognition of facial
expressions tasks
Phase diagram of randomly pinned vortex matter in layered superconductors: dependence on the details of the point pinning
We study the thermodynamic and structural properties of the superconducting
vortex system in high temperature layered superconductors, with magnetic field
normal to the layers, in the presence of a small concentration of strong random
point pinning defects via numerical minimization of a model free energy
functional in terms of the time-averaged local density of pancake vortices.
Working at constant magnetic induction and point pinning center concentration,
we find that the equilibrium phase at low temperature () and small pinning
strength () is a topologically ordered Bragg glass. As or is
increased, the Bragg glass undergoes a first order transition to a disordered
phase which we characterize as a ``vortex slush'' with polycrystalline
structure within the layers and interlayer correlations extending to about
twenty layers. This is in contrast with the pinned vortex liquid phase into
which the Bragg glass was found to melt, using the same methods, in the case of
a large concentration of weak pinning centers: that phase was amorphous with
very little interlayer correlation. The value of the second moment of the
random pinning potential at which the Bragg glass melts for a fixed temperature
is very different in the two systems. These results imply that the effects of
random point pinning can not be described only in terms of the second moment of
the pinning potential, and that some of the unresolved contradictions in the
literature concerning the nature of the low and high phase in this
system are likely to arise from differences in the nature of the pinning in
different samples, or from assumptions made about the pinning potential.Comment: 13 pages including 11 figures. Typos in HTML abstract corrected in v
Computation of the Nonlinear Magnetic Response of a Three Dimensional Anisotropic Superconductor
Many problems in computational magnetics involve computation of fields which
decay within a skin depth , much smaller than the sample size . We
discuss here a novel perturbation method which exploits the smallness of
and the asymptotic behavior of the solution in the
exterior and interior of a sample. To illustrate this procedure we consider the
computation of the magnetic dipole and quadrupole moments of an anisotropic,
unconventional, three dimensional superconductor. The method significantly
reduces the required numerical work and can be implemented in different
numerical algorithms.Comment: Three pages. To appear in Journal of Applied Physics (MMM-Intermag
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