5,074 research outputs found
GREAT3 results I: systematic errors in shear estimation and the impact of real galaxy morphology
We present first results from the third GRavitational lEnsing Accuracy
Testing (GREAT3) challenge, the third in a sequence of challenges for testing
methods of inferring weak gravitational lensing shear distortions from
simulated galaxy images. GREAT3 was divided into experiments to test three
specific questions, and included simulated space- and ground-based data with
constant or cosmologically-varying shear fields. The simplest (control)
experiment included parametric galaxies with a realistic distribution of
signal-to-noise, size, and ellipticity, and a complex point spread function
(PSF). The other experiments tested the additional impact of realistic galaxy
morphology, multiple exposure imaging, and the uncertainty about a
spatially-varying PSF; the last two questions will be explored in Paper II. The
24 participating teams competed to estimate lensing shears to within systematic
error tolerances for upcoming Stage-IV dark energy surveys, making 1525
submissions overall. GREAT3 saw considerable variety and innovation in the
types of methods applied. Several teams now meet or exceed the targets in many
of the tests conducted (to within the statistical errors). We conclude that the
presence of realistic galaxy morphology in simulations changes shear
calibration biases by per cent for a wide range of methods. Other
effects such as truncation biases due to finite galaxy postage stamps, and the
impact of galaxy type as measured by the S\'{e}rsic index, are quantified for
the first time. Our results generalize previous studies regarding sensitivities
to galaxy size and signal-to-noise, and to PSF properties such as seeing and
defocus. Almost all methods' results support the simple model in which additive
shear biases depend linearly on PSF ellipticity.Comment: 32 pages + 15 pages of technical appendices; 28 figures; submitted to
MNRAS; latest version has minor updates in presentation of 4 figures, no
changes in content or conclusion
Means of confusion: how pixel noise affects shear estimates for weak gravitational lensing
Weak-lensing shear estimates show a troublesome dependence on the apparent
brightness of the galaxies used to measure the ellipticity: In several studies,
the amplitude of the inferred shear falls sharply with decreasing source
significance. This dependence limits the overall ability of upcoming large
weak-lensing surveys to constrain cosmological parameters.
We seek to provide a concise overview of the impact of pixel noise on
weak-lensing measurements, covering the entire path from noisy images to shear
estimates. We show that there are at least three distinct layers, where pixel
noise not only obscures but biases the outcome of the measurements: 1) the
propagation of pixel noise to the non-linear observable ellipticity; 2) the
response of the shape-measurement methods to limited amount of information
extractable from noisy images; and 3) the reaction of shear estimation
statistics to the presence of noise and outliers in the measured ellipticities.
We identify and discuss several fundamental problems and show that each of
them is able to introduce biases in the range of a few tenths to a few percent
for galaxies with typical significance levels. Furthermore, all of these biases
do not only depend on the brightness of galaxies but also on their ellipticity,
with more elliptical galaxies often being harder to measure correctly. We also
discuss existing possibilities to mitigate and novel ideas to avoid the biases
induced by pixel noise. We present a new shear estimator that shows a more
robust performance for noisy ellipticity samples. Finally, we release the
open-source python code to predict and efficiently sample from the noisy
ellipticity distribution and the shear estimators used in this work at
https://github.com/pmelchior/epsnoiseComment: integrated the origin of the moment correlation (thanks to Alan
Heavens). source code at https://github.com/pmelchior/epsnois
Imfit: A Fast, Flexible New Program for Astronomical Image Fitting
I describe a new, open-source astronomical image-fitting program called
Imfit, specialized for galaxies but potentially useful for other sources, which
is fast, flexible, and highly extensible. A key characteristic of the program
is an object-oriented design which allows new types of image components (2D
surface-brightness functions) to be easily written and added to the program.
Image functions provided with Imfit include the usual suspects for galaxy
decompositions (Sersic, exponential, Gaussian), along with Core-Sersic and
broken-exponential profiles, elliptical rings, and three components which
perform line-of-sight integration through 3D luminosity-density models of disks
and rings seen at arbitrary inclinations.
Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead
simplex, and Differential Evolution, allowing trade-offs between speed and
decreased sensitivity to local minima in the fit landscape. Minimization can be
done using the standard chi^2 statistic (using either data or model values to
estimate per-pixel Gaussian errors, or else user-supplied error images) or
Poisson-based maximum-likelihood statistics; the latter approach is
particularly appropriate for cases of Poisson data in the low-count regime. I
show that fitting low-S/N galaxy images using chi^2 minimization and
individual-pixel Gaussian uncertainties can lead to significant biases in
fitted parameter values, which are avoided if a Poisson-based statistic is
used; this is true even when Gaussian read noise is present.Comment: pdflatex, 27 pages, 19 figures. Revised version, accepted by ApJ.
Programs, source code, and documentation available at:
http://www.mpe.mpg.de/~erwin/code/imfit
A novel prestack sparse azimuthal AVO inversion
In this paper we demonstrate a new algorithm for sparse prestack azimuthal
AVO inversion. A novel Euclidean prior model is developed to at once respect
sparseness in the layered earth and smoothness in the model of reflectivity.
Recognizing that methods of artificial intelligence and Bayesian computation
are finding an every increasing role in augmenting the process of
interpretation and analysis of geophysical data, we derive a generalized
matrix-variate model of reflectivity in terms of orthogonal basis functions,
subject to sparse constraints. This supports a direct application of machine
learning methods, in a way that can be mapped back onto the physical principles
known to govern reflection seismology. As a demonstration we present an
application of these methods to the Marcellus shale. Attributes extracted using
the azimuthal inversion are clustered using an unsupervised learning algorithm.
Interpretation of the clusters is performed in the context of the Ruger model
of azimuthal AVO
Local Statistical Modeling via Cluster-Weighted Approach with Elliptical Distributions
Cluster Weighted Modeling (CWM) is a mixture approach regarding the modelisation of the joint probability of data coming from a heterogeneous population. Under Gaussian assumptions, we investigate statistical properties of CWM from both the theoretical and numerical point of view; in particular, we show that CWM includes as special cases mixtures of distributions and mixtures of regressions. Further, we introduce CWM based on Student-t distributions providing more robust fitting for groups of observations with longer than normal tails or atypical observations. Theoretical results are illustrated using some empirical studies, considering both real and simulated data.Cluster-Weighted Modeling, Mixture Models, Model-Based Clustering
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