9,984 research outputs found
Weak-lensing shear estimates with general adaptive moments, and studies of bias by pixellation, PSF distortions, and noise
In weak gravitational lensing, weighted quadrupole moments of the brightness
profile in galaxy images are a common way to estimate gravitational shear. We
employ general adaptive moments (GLAM) to study causes of shear bias on a
fundamental level and for a practical definition of an image ellipticity. The
GLAM ellipticity has useful properties for any chosen weight profile: the
weighted ellipticity is identical to that of isophotes of elliptical images,
and in absence of noise and pixellation it is always an unbiased estimator of
reduced shear. We show that moment-based techniques, adaptive or unweighted,
are similar to a model-based approach in the sense that they can be seen as
imperfect fit of an elliptical profile to the image. Due to residuals in the
fit, moment-based estimates of ellipticities are prone to underfitting bias
when inferred from observed images. The estimation is fundamentally limited
mainly by pixellation which destroys information on the original, pre-seeing
image. We give an optimized estimator for the pre-seeing GLAM ellipticity and
quantify its bias for noise-free images. To deal with pixel noise, we consider
a Bayesian approach where the posterior of the GLAM ellipticity can be
inconsistent with the true ellipticity if we do not properly account for our
ignorance about fit residuals. This underfitting bias is S/N-independent but
changes with the pre-seeing brightness profile and the correlation or
heterogeneity of pixel noise over the post-seeing image. Furthermore, when
inferring a constant ellipticity or, more relevantly, constant shear from a
source sample with a distribution of intrinsic properties (sizes, centroid
positions, intrinsic shapes), an additional, now noise-dependent bias arises
towards low S/N if incorrect priors for the intrinsic properties are used. We
discuss the origin of this prior bias.Comment: 18 pages; 5 figures; accepted by A&A after major revision, especially
of Sect. 3.3 that corrects the previous discussion on the bias by
marginalizatio
Switching Costs, Firm Size, and Market Structure
In many markets homogenous goods are sold both by large global firms ("chain stores") and small local firms. Surprisingly, chain stores often charge higher prices. Examples include hotels, airlines, and coffe shops. We provide a simple model that can account for these pricing patterns. In this model, consumers face costs when switching from one supplier to another and change locations with a given probability. Consequently, chain stores insure consumers against switching costs. In equilibrium, chain stores charge higher prices, yet attract more consumers. Profits of local stores and chain stores increase with consumer mobility, but the latter do so faster.Firm size; switching costs; consumer mobility; market structure
Autonomous Fault Detection in Self-Healing Systems using Restricted Boltzmann Machines
Autonomously detecting and recovering from faults is one approach for
reducing the operational complexity and costs associated with managing
computing environments. We present a novel methodology for autonomously
generating investigation leads that help identify systems faults, and extends
our previous work in this area by leveraging Restricted Boltzmann Machines
(RBMs) and contrastive divergence learning to analyse changes in historical
feature data. This allows us to heuristically identify the root cause of a
fault, and demonstrate an improvement to the state of the art by showing
feature data can be predicted heuristically beyond a single instance to include
entire sequences of information.Comment: Published and presented in the 11th IEEE International Conference and
Workshops on Engineering of Autonomic and Autonomous Systems (EASe 2014
Inelastic Confinement-Induced Resonances in Low-Dimensional Quantum Systems
A theoretical model is presented describing the confinement-induced
resonances observed in the recent loss experiment of Haller et al. [Phys. Rev.
Lett. 104, 153203 (2010)]. These resonances originate from possible molecule
formation due to the coupling of center-of-mass and relative motion. A
corresponding model is verified by ab initio calculations and predicts the
resonance positions in 1D as well as in 2D confinement in agreement with the
experiment. This resolves the contradiction of the experimental observations to
previous theoretical predictions.Comment: 5 pages, 4 figure
The non-Gaussianity of the cosmic shear likelihood - or: How odd is the Chandra Deep Field South?
(abridged) We study the validity of the approximation of a Gaussian cosmic
shear likelihood. We estimate the true likelihood for a fiducial cosmological
model from a large set of ray-tracing simulations and investigate the impact of
non-Gaussianity on cosmological parameter estimation. We investigate how odd
the recently reported very low value of really is as derived from
the \textit{Chandra} Deep Field South (CDFS) using cosmic shear by taking the
non-Gaussianity of the likelihood into account as well as the possibility of
biases coming from the way the CDFS was selected.
We find that the cosmic shear likelihood is significantly non-Gaussian. This
leads to both a shift of the maximum of the posterior distribution and a
significantly smaller credible region compared to the Gaussian case. We
re-analyse the CDFS cosmic shear data using the non-Gaussian likelihood.
Assuming that the CDFS is a random pointing, we find
for fixed . In a
WMAP5-like cosmology, a value equal to or lower than this would be expected in
of the times. Taking biases into account arising from the way the
CDFS was selected, which we model as being dependent on the number of haloes in
the CDFS, we obtain . Combining the CDFS data
with the parameter constraints from WMAP5 yields and for a flat
universe.Comment: 18 pages, 16 figures, accepted for publication in A&A; New Bayesian
treatment of field selection bia
Confronting semi-analytic galaxy models with galaxy-matter correlations observed by CFHTLenS
Testing predictions of semi-analytic models of galaxy evolution against
observations help to understand the complex processes that shape galaxies. We
compare predictions from the Garching and Durham models implemented on the
Millennium Run with observations of galaxy-galaxy lensing (GGL) and
galaxy-galaxy-galaxy lensing (G3L) for various galaxy samples with stellar
masses in the range 0.5 < (M_* / 10^10 M_Sun) < 32 and photometric redshift
range 0.2 < z < 0.6 in the Canada-France-Hawaii Telescope Lensing Survey
(CFHTLenS). We find that the predicted GGL and G3L signals are in qualitative
agreement with CFHTLenS data. Quantitatively, the models succeed in reproducing
the observed signals in the highest stellar mass bin (16 < ( M_* / 10^10 M_Sun)
< 32) but show different degrees of tension for the other stellar mass samples.
The Durham models are strongly excluded at the 95% confidence level by the
observations as they largely over-predict the amplitudes of the GGL and G3L
signals, probably because they predict too many satellite galaxies in massive
halos.Comment: 9 pages, 8 figures, submitted to A&A. Comments welcom
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