2,576 research outputs found
Soft clustering analysis of galaxy morphologies: A worked example with SDSS
Context: The huge and still rapidly growing amount of galaxies in modern sky
surveys raises the need of an automated and objective classification method.
Unsupervised learning algorithms are of particular interest, since they
discover classes automatically. Aims: We briefly discuss the pitfalls of
oversimplified classification methods and outline an alternative approach
called "clustering analysis". Methods: We categorise different classification
methods according to their capabilities. Based on this categorisation, we
present a probabilistic classification algorithm that automatically detects the
optimal classes preferred by the data. We explore the reliability of this
algorithm in systematic tests. Using a small sample of bright galaxies from the
SDSS, we demonstrate the performance of this algorithm in practice. We are able
to disentangle the problems of classification and parametrisation of galaxy
morphologies in this case. Results: We give physical arguments that a
probabilistic classification scheme is necessary. The algorithm we present
produces reasonable morphological classes and object-to-class assignments
without any prior assumptions. Conclusions: There are sophisticated automated
classification algorithms that meet all necessary requirements, but a lot of
work is still needed on the interpretation of the results.Comment: 18 pages, 19 figures, 2 tables, submitted to A
Weak gravitational lensing with DEIMOS
We introduce a novel method for weak-lensing measurements, which is based on
a mathematically exact deconvolution of the moments of the apparent brightness
distribution of galaxies from the telescope's PSF. No assumptions on the shape
of the galaxy or the PSF are made. The (de)convolution equations are exact for
unweighted moments only, while in practice a compact weight function needs to
be applied to the noisy images to ensure that the moment measurement yields
significant results. We employ a Gaussian weight function, whose centroid and
ellipticity are iteratively adjusted to match the corresponding quantities of
the source. The change of the moments caused by the application of the weight
function can then be corrected by considering higher-order weighted moments of
the same source. Because of the form of the deconvolution equations, even an
incomplete weighting correction leads to an excellent shear estimation if
galaxies and PSF are measured with a weight function of identical size. We
demonstrate the accuracy and capabilities of this new method in the context of
weak gravitational lensing measurements with a set of specialized tests and
show its competitive performance on the GREAT08 challenge data. A complete C++
implementation of the method can be requested from the authors.Comment: 7 pages, 3 figures, fixed typo in Eq. 1
Deconvolution with Shapelets
We seek to find a shapelet-based scheme for deconvolving galaxy images from
the PSF which leads to unbiased shear measurements. Based on the analytic
formulation of convolution in shapelet space, we construct a procedure to
recover the unconvolved shapelet coefficients under the assumption that the PSF
is perfectly known. Using specific simulations, we test this approach and
compare it to other published approaches. We show that convolution in shapelet
space leads to a shapelet model of order
with and being the maximum orders of the intrinsic
galaxy and the PSF models, respectively. Deconvolution is hence a
transformation which maps a certain number of convolved coefficients onto a
generally smaller number of deconvolved coefficients. By inferring the latter
number from data, we construct the maximum-likelihood solution for this
transformation and obtain unbiased shear estimates with a remarkable amount of
noise reduction compared to established approaches. This finding is
particularly valid for complicated PSF models and low images, which
renders our approach suitable for typical weak-lensing conditions.Comment: 9 pages, 9 figures, submitted to A&
Limitations for shapelet-based weak-lensing measurements
We seek to understand the impact on shape estimators obtained from circular
and elliptical shapelet models under two realistic conditions: (a) only a
limited number of shapelet modes is available for the model, and (b) the
intrinsic galactic shapes are not restricted to shapelet models.
We create a set of simplistic simulations, in which the galactic shapes
follow a Sersic profile. By varying the Sersic index and applied shear, we
quantify the amount of bias on shear estimates which arises from insufficient
modeling. Additional complications due to PSF convolution, pixelation and pixel
noise are also discussed.
Steep and highly elliptical galaxy shapes cannot be accurately modeled within
the circular shapelet basis system and are biased towards shallower and less
elongated shapes. This problem can be cured partially by allowing elliptical
basis functions, but for steep profiles elliptical shapelet models still depend
critically on accurate ellipticity priors. As a result, shear estimates are
typically biased low. Independently of the particular form of the estimator,
the bias depends on the true intrinsic galaxy morphology, but also on the size
and shape of the PSF.
As long as the issues discussed here are not solved, the shapelet method
cannot provide weak-lensing measurements with an accuracy demanded by upcoming
missions and surveys, unless one can provide an accurate and reliable
calibration, specific for the dataset under investigation.Comment: 8 pages, 5 figures, submitted to A&
Star formation efficiency in galaxy interactions and mergers: a statistical study
We investigate the enhancement of star formation efficiency in galaxy
interactions and mergers, by numerical simulations of several hundred galaxy
collisions. All morphological types along the Hubble sequence are considered in
the initial conditions of the two colliding galaxies, with varying
bulge-to-disk ratios and gas mass fractions. Different types of orbits are
simulated, direct and retrograde, according to the initial relative energy and
impact parameter, and the resulting star formation history is compared to that
occuring in the two galaxies when they are isolated. Our principal results are:
(1) retrograde encounters have a larger star formation efficiency (SFE) than
direct encounters; (2) the amount of gas available in the galaxy is not the
main parameter governing the SFE in the burst phase; (3) there is an
anticorrelation between the amplitude of the star forming burst and the tidal
forces exerted per unit of time, which is due to the large amount of gas
dragged outside the galaxy by tidal tails in strong interactions; (4) globally,
the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies,
interacting pairs and mergers; (5) the enhanced star formation is essentially
occurring in nuclear starbursts, triggered by inward gas flows driven by
non-axisymmetries in the galaxy disks. Direct encounters develop more
pronounced asymmetries than retrograde ones. Based on these statistical
results, we derive general laws for the enhancement of star formation in galaxy
interactions and mergers, as a function of the main parameters of the
encounter.Comment: 22 pages, 37 figures, 4 tables. Accepted on Astronomy & Astrophysic
L'étude des phénomènes de marée gravimétrique
Cet article présente une description des phénomènes de marée terrestre et des méthodes de mesure. Les résultats des profils mondiaux de marée gravimétrique sont corrigés des effets indirects liés aux marées océaniques. Une corrélation entre marées gravimétriques, flux de chaleur et tectonique globale est proposé
Calibration biases in measurements of weak lensing
As recently shown by Viola et al., the common (KSB) method for measuring weak
gravitational shear creates a non-linear relation between the measured and the
true shear of objects. We investigate here what effect such a non-linear
calibration relation may have on cosmological parameter estimates from weak
lensing if a simpler, linear calibration relation is assumed. We show that the
non-linear relation introduces a bias in the shear-correlation amplitude and
thus a bias in the cosmological parameters Omega_matter and sigma_8. Its
direction and magnitude depends on whether the point-spread function is narrow
or wide compared to the galaxy images from which the shear is measured.
Substantial over- or underestimates of the cosmological parameters are equally
possible, depending also on the variant of the KSB method. Our results show
that for trustable cosmological-parameter estimates from measurements of weak
lensing, one must verify that the method employed is free from
ellipticity-dependent biases or monitor that the calibration relation inferred
from simulations is applicable to the survey at hand.Comment: 5 pages, 3 figures, submitted to A&
First measurement of gravitational lensing by cosmic voids in SDSS
We report the first measurement of the diminutive lensing signal arising from
matter underdensities associated with cosmic voids. While undetectable
individually, by stacking the weak gravitational shear estimates around 901
voids detected in SDSS DR7 by Sutter et al. (2012a), we find substantial
evidence for a depression of the lensing signal compared to the cosmic mean.
This depression is most pronounced at the void radius, in agreement with
analytical models of void matter profiles. Even with the largest void sample
and imaging survey available today, we cannot put useful constraints on the
radial dark-matter void profile. We invite independent investigations of our
findings by releasing data and analysis code to the public at
https://github.com/pmelchior/void-lensingComment: 6 pages, 5 figures, as accepted by MNRA
Biases in, and corrections to, KSB shear measurements
We analyse the KSB method to estimate gravitational shear from
surface-brightness moments of small and noisy galaxy images. We identify three
potentially problematic assumptions. These are: (1) While gravitational shear
must be estimated from averaged galaxy images, KSB derives a shear estimate
from each individual image and then takes the average. Since the two operations
do not commute, KSB gives biased results. (2) KSB implicitly assumes that
galaxy ellipticities are small, while weak gravitational lensing assures only
that the change in ellipticity due to the shear is small. (3) KSB does not
invert the convolution with the point-spread function, but gives an approximate
PSF correction which - even for a circular PSF - holds only in the limit of
circular sources. The effects of assumptions (2) and (3) partially counter-act
in a way dependent on the width of the weight function and of the PSF. We
quantitatively demonstrate the biases due to all assumptions, extend the KSB
approach consistently to third order in the shear and ellipticity and show that
this extension lowers the biases substantially. The issue of proper PSF
deconvolution will be addressed in a forthcoming paper.Comment: 12 pages, 10 figures, MNRAS submitte
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