5,358 research outputs found
A Population of Metal-Poor Galaxies with ~L* Luminosities at Intermediate Redshifts
We present new spectroscopy and metallicity estimates for a sample of 15
star-forming galaxies with redshifts in the range 0.29 - 0.42. These objects
were selected in the KPNO International Spectroscopic Survey via their strong
emission lines seen in red objective-prism spectra. Originally thought to be
intermediate-redshift Seyfert 2 galaxies, our new spectroscopy in the far red
has revealed these objects to be metal-poor star-forming galaxies. These
galaxies follow a luminosity-metallicity (L-Z) relation that parallels the one
defined by low-redshift galaxies, but is offset by a factor of more than ten to
lower abundances. The amount of chemical and/or luminosity evolution required
to place these galaxies on the local L-Z relation is extreme, suggesting that
these galaxies are in a very special stage of their evolution. They may be
late-forming massive systems, which would challenge the current paradigm of
galaxy formation. Alternatively, they may represent intense starbursts in
dwarf-dwarf mergers or a major infall episode of pristine gas into a
pre-existing galaxy. In any case, these objects represent an extreme stage of
galaxy evolution taking place at relatively low redshift.Comment: 6 pages, 3 figures; to appear in 10 April 2009 ApJ
Chebyshev approximations for the Stumpff series of orders four and five Interim report
Chebyshev approximations for Stumpff series of orders four and fiv
Limits of three-dimensional gravity and metric kinematical Lie algebras in any dimension
We extend a recent classification of three-dimensional spatially isotropic
homogeneous spacetimes to Chern--Simons theories as three-dimensional gravity
theories on these spacetimes. By this we find gravitational theories for all
carrollian, galilean, and aristotelian counterparts of the lorentzian theories.
In order to define a nondegenerate bilinear form for each of the theories, we
introduce (not necessarily central) extensions of the original kinematical
algebras. Using the structure of so-called double extensions, this can be done
systematically. For homogeneous spaces that arise as a limit of (anti-)de
Sitter spacetime, we show that it is possible to take the limit on the level of
the action, after an appropriate extension. We extend our systematic
construction of nondegenerate bilinear forms also to all higher-dimensional
kinematical algebras.Comment: 52 pages, 2 figures, 11 tables; v2: matches published version,
additional references added and incorporated referee suggestion
Validation of Soft Classification Models using Partial Class Memberships: An Extended Concept of Sensitivity & Co. applied to the Grading of Astrocytoma Tissues
We use partial class memberships in soft classification to model uncertain
labelling and mixtures of classes. Partial class memberships are not restricted
to predictions, but may also occur in reference labels (ground truth, gold
standard diagnosis) for training and validation data.
Classifier performance is usually expressed as fractions of the confusion
matrix, such as sensitivity, specificity, negative and positive predictive
values. We extend this concept to soft classification and discuss the bias and
variance properties of the extended performance measures. Ambiguity in
reference labels translates to differences between best-case, expected and
worst-case performance. We show a second set of measures comparing expected and
ideal performance which is closely related to regression performance, namely
the root mean squared error RMSE and the mean absolute error MAE.
All calculations apply to classical crisp classification as well as to soft
classification (partial class memberships and/or one-class classifiers). The
proposed performance measures allow to test classifiers with actual borderline
cases. In addition, hardening of e.g. posterior probabilities into class labels
is not necessary, avoiding the corresponding information loss and increase in
variance.
We implement the proposed performance measures in the R package
"softclassval", which is available from CRAN and at
http://softclassval.r-forge.r-project.org.
Our reasoning as well as the importance of partial memberships for
chemometric classification is illustrated by a real-word application:
astrocytoma brain tumor tissue grading (80 patients, 37000 spectra) for finding
surgical excision borders. As borderline cases are the actual target of the
analytical technique, samples which are diagnosed to be borderline cases must
be included in the validation.Comment: The manuscript is accepted for publication in Chemometrics and
Intelligent Laboratory Systems. Supplementary figures and tables are at the
end of the pd
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