3,244 research outputs found
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&
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
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&
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&
AGAPEROS: Searching for variable stars in the LMC Bar with the Pixel Method. I. Detection, astrometry and cross-identification
We extend the work developed in previous papers on microlensing with a
selection of variable stars. We use the Pixel Method to select variable stars
on a set of 2.5 x 10**6 pixel light curves in the LMC Bar presented elsewhere.
The previous treatment was done in order to optimise the detection of long
timescale variations (larger than a few days) and we further optimise our
analysis for the selection of Long Timescale and Long Period Variables
(LT&LPV). We choose to perform a selection of variable objects as comprehensive
as possible, independent of periodicity and of their position on the colour
magnitude diagram. We detail the different thresholds successively applied to
the light curves, which allow to produce a catalogue of 632 variable objects.
We present a table with the coordinate of each variable, its EROS magnitudes at
one epoch and an indicator of blending in both colours, together with a finding
chart.
A cross-correlation with various catalogues shows that 90% of those variable
objects were undetected before, thus enlarging the sample of LT&LPV previously
known in this area by a factor of 10. Due to the limitations of both the Pixel
Method and the data set, additional data -- namely a longer baseline and near
infrared photometry -- are required to further characterise these variable
stars, as will be addressed in subsequent papers.Comment: 11 pages with 10 figure
Detection of CO in the inner part of M31's bulge
We report the first detection of CO in M31's bulge. The 12CO (1-0) and (2-1)
lines are both detected in the dust complex D395A/393/384, at 1.3" (~0.35 kpc)
from the centre. From these data and from visual extinction data, we derive a
CO-luminosity to reddening ratio (and a CO-luminosity to H_2 column density
ratio) quite similar to that observed in the local Galactic clouds. The (2-1)
to (1-0) line intensity ratio points to a CO rotational temperature and a gas
kinetic temperature > 10 K. The molecular mass of the complex, inside a 25'
(100 pc) region, is 1.5 10^4 Mo.Comment: 5 pages including 4 figures (2 in colour
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
The Data Processing Pipeline for the Herschel-HIFI Instrument
The HIFI data processing pipeline was developed to systematically process
diagnostic, calibration and astronomical observations taken with the HIFI
science instrumentas part of the Herschel mission. The HIFI pipeline processed
data from all HIFI observing modes within the Herschel automated processing
environment, as well as, within an interactive environment. A common software
framework was developed to best support the use cases required by the
instrument teams and by the general astronomers. The HIFI pipeline was built on
top of that and was designed with a high degree of modularity. This modular
design provided the necessary flexibility and extensibility to deal with the
complexity of batch-processing eighteen different observing modes, to support
the astronomers in the interactive analysis and to cope with adjustments
necessary to improve the pipeline and the quality of the end-products. This
approach to the software development and data processing effort was arrived at
by coalescing the lessons learned from similar research based projects with the
understanding that a degree of foresight was required given the overall length
of the project. In this article, both the successes and challenges of the HIFI
software development process are presented. To support future similar projects
and retain experience gained lessons learned are extracted.Comment: 18 pages, 5 figure
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