12,042 research outputs found
Source-lens clustering and intrinsic-alignment bias of weak-lensing estimators
We estimate the amplitude of the source-lens clustering bias and of the
intrinsic-alignment bias of weak lensing estimators of the two-point and
three-point convergence and cosmic-shear correlation functions. We use a linear
galaxy bias model for the galaxy-density correlations, as well as a linear
intrinsic-alignment model. For the three-point and four-point density
correlations, we use analytical or semi-analytical models, based on a
hierarchical ansatz or a combination of one-loop perturbation theory with a
halo model. For two-point statistics, we find that the source-lens clustering
bias is typically several orders of magnitude below the weak lensing signal,
except when we correlate a very low-redshift galaxy (z_2 \la 0.05) with a
higher redshift galaxy (z_1 \ga 0.5), where it can reach of the signal
for the shear. For three-point statistics, the source-lens clustering bias is
typically of order of the signal, as soon as the three galaxy source
redshifts are not identical. The intrinsic-alignment bias is typically about
of the signal for both two-point and three-point statistics. Thus, both
source-lens clustering bias and intrinsic-alignment bias must be taken into
account for three-point estimators aiming at a better than accuracy.Comment: 27 page
Applications of Multi-view Learning Approaches for Software Comprehension
Program comprehension concerns the ability of an individual to make an
understanding of an existing software system to extend or transform it.
Software systems comprise of data that are noisy and missing, which makes
program understanding even more difficult. A software system consists of
various views including the module dependency graph, execution logs,
evolutionary information and the vocabulary used in the source code, that
collectively defines the software system. Each of these views contain unique
and complementary information; together which can more accurately describe the
data. In this paper, we investigate various techniques for combining different
sources of information to improve the performance of a program comprehension
task. We employ state-of-the-art techniques from learning to 1) find a suitable
similarity function for each view, and 2) compare different multi-view learning
techniques to decompose a software system into high-level units and give
component-level recommendations for refactoring of the system, as well as
cross-view source code search. The experiments conducted on 10 relatively large
Java software systems show that by fusing knowledge from different views, we
can guarantee a lower bound on the quality of the modularization and even
improve upon it. We proceed by integrating different sources of information to
give a set of high-level recommendations as to how to refactor the software
system. Furthermore, we demonstrate how learning a joint subspace allows for
performing cross-modal retrieval across views, yielding results that are more
aligned with what the user intends by the query. The multi-view approaches
outlined in this paper can be employed for addressing problems in software
engineering that can be encoded in terms of a learning problem, such as
software bug prediction and feature location
Magnification bias as a novel probe for primordial magnetic fields
In this paper we investigate magnetic fields generated in the early Universe.
These fields are important candidates at explaining the origin of astrophysical
magnetism observed in galaxies and galaxy clusters, whose genesis is still by
and large unclear. Compared to the standard inflationary power spectrum,
intermediate to small scales would experience further substantial matter
clustering, were a cosmological magnetic field present prior to recombination.
As a consequence, the bias and redshift distribution of galaxies would also be
modified. Hitherto, primordial magnetic fields (PMFs) have been tested and
constrained with a number of cosmological observables, e.g. the cosmic
microwave background radiation, galaxy clustering and, more recently, weak
gravitational lensing. Here, we explore the constraining potential of the
density fluctuation bias induced by gravitational lensing magnification onto
the galaxy-galaxy angular power spectrum. Such an effect is known as
magnification bias. Compared to the usual galaxy clustering approach,
magnification bias helps in lifting the pathological degeneracy present amongst
power spectrum normalisation and galaxy bias. This is because magnification
bias cross-correlates galaxy number density fluctuations of nearby objects with
weak lensing distortions of high-redshift sources. Thus, it takes advantage of
the gravitational deflection of light, which is insensitive to galaxy bias but
powerful in constraining the density fluctuation amplitude. To scrutinise the
potentiality of this method, we adopt a deep and wide-field spectroscopic
galaxy survey. We show that magnification bias does contain important
information on primordial magnetism, which will be useful in combination with
galaxy clustering and shear. We find we shall be able to rule out at 95.4% CL
amplitudes of PMFs larger than 0.0005 nG for values of the PMF power spectral
index ~0.Comment: 21 pages, 9 figures; published on JCA
The Impact of Non-Gaussian Errors on Weak Lensing Surveys
The weak lensing power spectrum carries cosmological information via its
dependence on the growth of structure and on geometric factors. Since much of
the cosmological information comes from scales affected by nonlinear
clustering, measurements of the lensing power spectrum can be degraded by
non-Gaussian covariances. Recently there have been conflicting studies about
the level of this degradation. We use the halo model to estimate it and include
new contributions related to the finite size of lensing surveys, following
Rimes and Hamilton's study of 3D simulations. We find that non-Gaussian
correlations between different multipoles can degrade the cumulative
signal-to-noise for the power spectrum amplitude by up to a factor of 2 (or 5
for a worst-case model that exceeds current N-body simulation predictions).
However, using an eight-parameter Fisher analysis we find that the marginalized
errors on individual parameters are degraded by less than 10% (or 20% for the
worst-case model). The smaller degradation in parameter accuracy is primarily
because: individual parameters in a high-dimensional parameter space are
degraded much less than the volume of the full Fisher ellipsoid; lensing
involves projections along the line of sight, which reduce the non-Gaussian
effect; some of the cosmological information comes from geometric factors which
are not degraded at all. We contrast our findings with those of Lee & Pen
(2008) who suggested a much larger degradation in information content. Finally,
our results give a useful guide for exploring survey design by giving the
cosmological information returns for varying survey area, depth and the level
of some systematic errors.Comment: To appear in MNRAS, 22 pages, 12 figures. Minor modifications made
according to the referee comment
From Weak Lensing to non-Gaussianity via Minkowski Functionals
We present a new harmonic-domain approach for extracting morphological
information, in the form of Minkowski Functionals (MFs), from weak lensing (WL)
convergence maps. Using a perturbative expansion of the MFs, which is expected
to be valid for the range of angular scales probed by most current weak-lensing
surveys, we show that the study of three generalized skewness parameters is
equivalent to the study of the three MFs defined in two dimensions. We then
extend these skewness parameters to three associated skew-spectra which carry
more information about the convergence bispectrum than their one-point
counterparts. We discuss various issues such as noise and incomplete sky
coverage in the context of estimation of these skew-spectra from realistic
data. Our technique provides an alternative to the pixel-space approaches
typically used in the estimation of MFs, and it can be particularly useful in
the presence of masks with non-trivial topology. Analytical modeling of weak
lensing statistics relies on an accurate modeling of the statistics of
underlying density distribution. We apply three different formalisms to model
the underlying dark-matter bispectrum: the hierarchical ansatz, halo model and
a fitting function based on numerical simulations; MFs resulting from each of
these formalisms are computed and compared. We investigate the extent to witch
late-time gravity-induced non-Gaussianity (to which weak lensing is primarily
sensitive) can be separated from primordial non-Gaussianity and how this
separation depends on source redshift and angular scale.Comment: 22 Pages, 12 Figures. Submitting To MNRA
kernlab - An S4 Package for Kernel Methods in R
kernlab is an extensible package for kernel-based machine learning methods in R. It takes advantage of R's new S4 ob ject model and provides a framework for creating and using kernel-based algorithms. The package contains dot product primitives (kernels), implementations of support vector machines and the relevance vector machine, Gaussian processes, a ranking algorithm, kernel PCA, kernel CCA, and a spectral clustering algorithm. Moreover it provides a general purpose quadratic programming solver, and an incomplete Cholesky decomposition method.
Primordial Non-Gaussianity and Analytical Formula for Minkowski Functionals of the Cosmic Microwave Background and Large-scale Structure
We derive analytical formulae for the Minkowski Functions of the cosmic
microwave background (CMB) and large-scale structure (LSS) from primordial
non-Gaussianity. These formulae enable us to estimate a non-linear coupling
parameter, f_NL, directly from the CMB and LSS data without relying on
numerical simulations of non-Gaussian primordial fluctuations. One can use
these formulae to estimate statistical errors on f_NL from Gaussian
realizations, which are much faster to generate than non-Gaussian ones, fully
taking into account the cosmic/sampling variance, beam smearing, survey mask,
etc. We show that the CMB data from the Wilkinson Microwave Anisotropy Probe
should be sensitive to |f_NL|\simeq 40 at the 68% confidence level. The Planck
data should be sensitive to |f_NL|\simeq 20. As for the LSS data, the late-time
non-Gaussianity arising from gravitational instability and galaxy biasing makes
it more challenging to detect primordial non-Gaussianity at low redshifts. The
late-time effects obscure the primordial signals at small spatial scales.
High-redshift galaxy surveys at z>2 covering \sim 10Gpc^3 volume would be
required for the LSS data to detect |f_NL|\simeq 100. Minkowski Functionals are
nicely complementary to the bispectrum because the Minkowski Functionals are
defined in real space and the bispectrum is defined in Fourier space. This
property makes the Minksowski Functionals a useful tool in the presence of
real-world issues such as anisotropic noise, foreground and survey masks. Our
formalism can be extended to scale-dependent f_NL easily.Comment: 16 pages, 5 figures, accepted for publication in ApJ (Vol. 653, 2006
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