145,196 research outputs found
Spectro-Perfectionism: An Algorithmic Framework for Photon Noise-Limited Extraction of Optical Fiber Spectroscopy
We describe a new algorithm for the "perfect" extraction of one-dimensional
spectra from two-dimensional (2D) digital images of optical fiber
spectrographs, based on accurate 2D forward modeling of the raw pixel data. The
algorithm is correct for arbitrarily complicated 2D point-spread functions
(PSFs), as compared to the traditional optimal extraction algorithm, which is
only correct for a limited class of separable PSFs. The algorithm results in
statistically independent extracted samples in the 1D spectrum, and preserves
the full native resolution of the 2D spectrograph without degradation. Both the
statistical errors and the 1D resolution of the extracted spectrum are
accurately determined, allowing a correct chi-squared comparison of any model
spectrum with the data. Using a model PSF similar to that found in the red
channel of the Sloan Digital Sky Survey spectrograph, we compare the
performance of our algorithm to that of cross-section based optimal extraction,
and also demonstrate that our method allows coaddition and foreground
estimation to be carried out as an integral part of the extraction step. This
work demonstrates the feasibility of current- and next-generation multi-fiber
spectrographs for faint galaxy surveys even in the presence of strong night-sky
foregrounds. We describe the handling of subtleties arising from fiber-to-fiber
crosstalk, discuss some of the likely challenges in deploying our method to the
analysis of a full-scale survey, and note that our algorithm could be
generalized into an optimal method for the rectification and combination of
astronomical imaging data.Comment: 9 pages, 4 figures, emulateapj; minor corrections and clarifications;
to be published in the PAS
Numerical implementation of the eXtended Finite Element Method for dynamic crack analysis
A numerical implementation of the eXtended Finite Element Method (X-FEM) to analyze crack propagation in a structure under dynamic loading is presented in this paper. The arbitrary crack is treated by the X-FEM method without re-meshing but using an enrichment of the classical displacement-based finite element approximation in the framework of the partition of unity method. Several algorithms have been implemented, within an Oriented Object framework in C++, in the home made explicit FEM code. The new module, called DynaCrack, included in the dynamic FEM code DynELA, evaluates the crack geometry, the propagation of the crack and allow the post-processing of the numerical results. The module solves the system of discrete equations using an explicit integration scheme. Some numerical examples illustrating the main features and the computational efficiency of the DynaCrack module for dynamic crack propagation are presented in the last section of the paper
Direct and Inverse Computational Methods for Electromagnetic Scattering in Biological Diagnostics
Scattering theory has had a major roll in twentieth century mathematical
physics. Mathematical modeling and algorithms of direct,- and inverse
electromagnetic scattering formulation due to biological tissues are
investigated. The algorithms are used for a model based illustration technique
within the microwave range. A number of methods is given to solve the inverse
electromagnetic scattering problem in which the nonlinear and ill-posed nature
of the problem are acknowledged.Comment: 61 pages, 5 figure
The stellar populations of massive galaxies in the local Universe
I present a brief review of the stellar population properties of massive
galaxies, focusing on early-type galaxies in particular, with emphasis on
recent results from the ATLAS3D Survey. I discuss the occurrence of young
stellar ages, cold gas, and ongoing star formation in early-type galaxies, the
presence of which gives important clues to the evolutionary path of these
galaxies. Consideration of empirical star formation histories gives a
meaningful picture of galaxy stellar population properties, and allows accurate
comparison of mass estimates from populations and dynamics. This has recently
provided strong evidence of a non-universal IMF, as supported by other recent
evidences. Spatially-resolved studies of stellar populations are also crucial
to connect distinct components within galaxies to spatial structures seen in
other wavelengths or parameters. Stellar populations in the faint outer
envelopes of early-type galaxies are a formidable frontier for observers, but
promise to put constraints on the ratio of accreted stellar mass versus that
formed 'in situ' - a key feature of recent galaxy formation models. Galaxy
environment appears to play a key role in controlling the stellar population
properties of low mass galaxies. Simulations remind us, however, that current
day galaxies are the product of a complex assembly and environment history,
which gives rise to the trends we see. This has strong implications for our
interpretation of environmental trends.Comment: 10 pages, 4 figures, LaTeX. Invited talk for the IAU Symposium 295
"The Intriguing Life of Massive Galaxies". To appear in Proc. of the XXVIII
IAU General Assembly, Beijing, China, August 2012, eds. D. Thomas, A.
Pasquali; I. Ferreras. Cambridge University Pres
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