48,775 research outputs found
Reconstructing Galaxy Spectral Energy Distributions from Broadband Photometry
We present a novel approach to photometric redshifts, one that merges the
advantages of both the template fitting and empirical fitting algorithms,
without any of their disadvantages. This technique derives a set of templates,
describing the spectral energy distributions of galaxies, from a catalog with
both multicolor photometry and spectroscopic redshifts. The algorithm is
essentially using the shapes of the templates as the fitting parameters. From
simulated multicolor data we show that for a small training set of galaxies we
can reconstruct robustly the underlying spectral energy distributions even in
the presence of substantial errors in the photometric observations. We apply
these techniques to the multicolor and spectroscopic observations of the Hubble
Deep Field building a set of template spectra that reproduced the observed
galaxy colors to better than 10%. Finally we demonstrate that these improved
spectral energy distributions lead to a photometric-redshift relation for the
Hubble Deep Field that is more accurate than standard template-based
approaches.Comment: 23 pages, 8 figures, LaTeX AASTeX, accepted for publication in A
The self-consistent quantum-electrostatic problem in strongly non-linear regime
The self-consistent quantum-electrostatic (also known as
Poisson-Schr\"odinger) problem is notoriously difficult in situations where the
density of states varies rapidly with energy. At low temperatures, these
fluctuations make the problem highly non-linear which renders iterative schemes
deeply unstable. We present a stable algorithm that provides a solution to this
problem with controlled accuracy. The technique is intrinsically convergent
including in highly non-linear regimes. We illustrate our approach with (i) a
calculation of the compressible and incompressible stripes in the integer
quantum Hall regime and (ii) a calculation of the differential conductance of a
quantum point contact geometry. Our technique provides a viable route for the
predictive modeling of the transport properties of quantum nanoelectronics
devices.Comment: 28 pages. 14 figures. Added solution to a potential failure mode of
the algorith
Very Singular Similarity Solutions and Hermitian Spectral Theory for Semilinear Odd-Order PDEs
Very singular self-similar solutions of semilinear odd-order PDEs are studied
on the basis of a Hermitian-type spectral theory for linear rescaled odd-order
operators.Comment: 49 pages, 12 Figure
Influence of wall thickness and diameter on arterial shear wave elastography: a phantom and finite element study
Quantitative, non-invasive and local measurements of arterial mechanical
properties could be highly beneficial for early diagnosis of cardiovascular
disease and follow up of treatment. Arterial shear wave elastography (SWE)
and wave velocity dispersion analysis have previously been applied to
measure arterial stiffness. Arterial wall thickness (h) and inner diameter (D)
vary with age and pathology and may influence the shear wave propagation.
Nevertheless, the effect of arterial geometry in SWE has not yet been
systematically investigated. In this study the influence of geometry on the
estimated mechanical properties of plates (h = 0.5–3 mm) and hollow
cylinders (h = 1, 2 and 3 mm, D = 6 mm) was assessed by experiments in
phantoms and by finite element method simulations. In addition, simulations
in hollow cylinders with wall thickness difficult to achieve in phantoms
were performed (h = 0.5–1.3 mm, D = 5–8 mm). The phase velocity curves obtained from experiments and simulations were compared in the frequency
range 200–1000 Hz and showed good agreement (R2 = 0.80 ± 0.07 for plates
and R2 = 0.82 ± 0.04 for hollow cylinders). Wall thickness had a larger effect
than diameter on the dispersion curves, which did not have major effects above
400 Hz. An underestimation of 0.1–0.2 mm in wall thickness introduces an
error 4–9 kPa in hollow cylinders with shear modulus of 21–26 kPa. Therefore,
wall thickness should correctly be measured in arterial SWE applications for
accurate mechanical properties estimation
Electron and phonon dispersions of the two dimensional Holstein model: Effects of vertex and non-local corrections
I apply the newly developed dynamical cluster approximation (DCA) to the
calculation of the electron and phonon dispersions in the two dimensional
Holstein model. In contrast to previous work, the DCA enables the effects of
spatial fluctuations (non-local corrections) to be examined. Approximations
neglecting and incorporating lowest-order vertex corrections are investigated.
I calculate the phonon density of states, the renormalised phonon dispersion,
the electron dispersion and electron spectral functions. I demonstrate how
vertex corrections stabilise the solution, stopping a catastrophic softening of
the phonon mode. A kink in the electron dispersion is found in the
normal state along the symmetry direction in both the vertex-
and non-vertex-corrected theories for low phonon frequencies, corresponding
directly to the renormalised phonon frequency at the point. This kink
is accompanied by a sudden drop in the quasi-particle lifetime. Vertex and
non-local corrections enhance the effects at large bare phonon frequencies.Comment: I am posting reprints of the final submitted versions of previous
articles to improve access. Here ARPES "kinks" are discussed. Article was
published in 2003. 17 pages, 9 figure
- …