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$^7$ and 10$^8$ 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

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

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

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 ($T$) and small pinning strength ($s$) is a topologically ordered Bragg glass. As $T$ or $s$ 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 $T$ and high $s$ 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 $\delta$, much smaller than the sample size $d$. We discuss here a novel perturbation method which exploits the smallness of $\epsilon \equiv \delta / d$ 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 issue