327 research outputs found
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&
Modal decomposition of astronomical images with application to shapelets
The decomposition of an image into a linear combination of digitised basis
functions is an everyday task in astronomy. A general method is presented for
performing such a decomposition optimally into an arbitrary set of digitised
basis functions, which may be linearly dependent, non-orthogonal and
incomplete. It is shown that such circumstances may result even from the
digitisation of continuous basis functions that are orthogonal and complete. In
particular, digitised shapelet basis functions are investigated and are shown
to suffer from such difficulties. As a result the standard method of performing
shapelet analysis produces unnecessarily inaccurate decompositions. The optimal
method presented here is shown to yield more accurate decompositions in all
cases.Comment: 12 pages, 17 figures, submitted to MNRA
Reliable Shapelet Image Analysis
Aims: We discuss the applicability and reliability of the shapelet technique
for scientific image analysis. Methods: We quantify the effects of
non-orthogonality of sampled shapelet basis functions and misestimation of
shapelet parameters. We perform the shapelet decomposition on artificial galaxy
images with underlying shapelet models and galaxy images from the GOODS survey,
comparing the publicly available IDL implementation with our new C++
implementation. Results: Non-orthogonality of the sampled basis functions and
misestimation of the shapelet parameters can cause substantial
misinterpretation of the physical properties of the decomposed objects.
Additional constraints, image preprocessing and enhanced precision have to be
incorporated in order to achieve reliable decomposition results.Comment: 12 pages, 15 figures, revised version, accepted by A&
Radio Weak Gravitational Lensing with VLA and MERLIN
We carry out an exploratory weak gravitational lensing analysis on a combined
VLA and MERLIN radio data set: a deep (3.3 micro-Jy beam^-1 rms noise) 1.4 GHz
image of the Hubble Deep Field North. We measure the shear estimator
distribution at this radio sensitivity for the first time, finding a similar
distribution to that of optical shear estimators for HST ACS data in this
field. We examine the residual systematics in shear estimation for the radio
data, and give cosmological constraints from radio-optical shear
cross-correlation functions. We emphasize the utility of cross-correlating
shear estimators from radio and optical data in order to reduce the impact of
systematics. Unexpectedly we find no evidence of correlation between optical
and radio intrinsic ellipticities of matched objects; this result improves the
properties of optical-radio lensing cross-correlations. We explore the
ellipticity distribution of the radio counterparts to optical sources
statistically, confirming the lack of correlation; as a result we suggest a
connected statistical approach to radio shear measurements.Comment: 16 pages with 19 figures, accepted for publication in MNRAS; Minor
corrections to section 6.3; 2 references adde
Where are the Baryons?
New, high resolution, large-scale, cosmological hydrodynamic galaxy formation
simulations of a standard cold dark matter model (with a cosmological constant)
are utilized to predict the distribution of baryons at the present and at
moderate redshift. It is found that the average temperature of baryons is an
increasing function of time, with most of the baryons at the present time
having a temperature in the range 10^{5-7} K. Thus, not only is the universe
dominated by dark matter, but more than one half of the normal matter is yet to
be detected. Detection of this warm/hot gas poses an observational challenge,
requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmic
X-ray background, apparent warm components in hot clusters due to both
intrinsic warm intra-cluster gas and warm inter-cluster gas projected onto
clusters along the line of sight, absorption lines in X-ray and UV quasar
spectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emission
lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density
\lya absorption lines. We estimate that approximately 1/4 of the
extragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from the
warm/hot gas, half of it coming from and three-quarters from ,
so the source regions should be identifiable on deep optical images.Comment: ApJ in press, revised (fig 3 is in jpg). Whole paper including
fig3.ps can be obtained at
"http://astro.princeton.edu/~cen/PAPERS_TO_APPEAR/64
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
Aging in a topological spin glass
We have examined the nonconventional spin glass phase of the 2-dimensional
kagome antiferromagnet (H_3 O) Fe_3 (SO_4)_2 (OH)_6 by means of ac and dc
magnetic measurements. The frequency dependence of the ac susceptibility peak
is characteristic of a critical slowing down at Tg ~ 18K. At fixed temperature
below Tg, aging effects are found which obey the same scaling law as in spin
glasses or polymers. However, in clear contrast with conventional spin glasses,
aging is remarkably insensitive to temperature changes. This particular type of
dynamics is discussed in relation with theoretical predictions for highly
frustrated non-disordered systems.Comment: 4 pages, 4 figure
Cosmology with Weak Lensing Surveys
Weak gravitational lensing surveys measure the distortion of the image of
distant sources due to the deflections of light rays by the fluctuations of the
gravitational potential along the line of sight. Since they probe the
non-linear matter power spectrum itself at medium redshift such surveys are
complimentary to both galaxy surveys (which follow stellar light) and cosmic
microwave background observations (which probe the linear regime at high
redshift). Ongoing CMB experiments such as WMAP and the future Planck satellite
mission will measure the standard cosmological parameters with unprecedented
accuracy. The focus of attention will then shift to understanding the nature of
dark matter and vacuum energy: several recent studies suggest that lensing is
the best method for constraining the dark energy equation of state. During the
next 5 year period ongoing and future weak lensing surveys such as the Joint
Dark Energy Mission (JDEM, e.g. SNAP) or the Large-aperture Synoptic Survey
Telescope (LSST) will play a major role in advancing our understanding of the
universe in this direction. In this review article we describe various aspects
of weak lensing surveys and how they can help us in understanding our universe.Comment: 15 pages, review article to appear in 2005 Triennial Issue of Phil.
Trans.
An algorithm for the direct reconstruction of the dark matter correlation function from weak lensing and galaxy clustering
The clustering of matter on cosmological scales is an essential probe for
studying the physical origin and composition of our Universe. To date, most of
the direct studies have focused on shear-shear weak lensing correlations, but
it is also possible to extract the dark matter clustering by combining
galaxy-clustering and galaxy-galaxy-lensing measurements. In this study we
develop a method that can constrain the dark matter correlation function from
galaxy clustering and galaxy-galaxy-lensing measurements, by focusing on the
correlation coefficient between the galaxy and matter overdensity fields. To
generate a mock galaxy catalogue for testing purposes, we use the Halo
Occupation Distribution approach applied to a large ensemble of N-body
simulations to model pre-existing SDSS Luminous Red Galaxy sample observations.
Using this mock catalogue, we show that a direct comparison between the excess
surface mass density measured by lensing and its corresponding galaxy
clustering quantity is not optimal. We develop a new statistic that suppresses
the small-scale contributions to these observations and show that this new
statistic leads to a cross-correlation coefficient that is within a few percent
of unity down to 5 Mpc/h. Furthermore, the residual incoherence between the
galaxy and matter fields can be explained using a theoretical model for
scale-dependent bias, giving us a final estimator that is unbiased to within
1%. We also perform a comprehensive study of other physical effects that can
affect the analysis, such as redshift space distortions and differences in
radial windows between galaxy clustering and weak lensing observations. We
apply the method to a range of cosmological models and show the viability of
our new statistic to distinguish between cosmological models.Comment: 23 pages, 14 figures, accepted by PRD; minor changes to V1, 1 new
figure, more detailed discussion of the covariance of the new ADSD statisti
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