1,648 research outputs found
Interpolating point spread function anisotropy
Planned wide-field weak lensing surveys are expected to reduce the
statistical errors on the shear field to unprecedented levels. In contrast,
systematic errors like those induced by the convolution with the point spread
function (PSF) will not benefit from that scaling effect and will require very
accurate modeling and correction. While numerous methods have been devised to
carry out the PSF correction itself, modeling of the PSF shape and its spatial
variations across the instrument field of view has, so far, attracted much less
attention. This step is nevertheless crucial because the PSF is only known at
star positions while the correction has to be performed at any position on the
sky. A reliable interpolation scheme is therefore mandatory and a popular
approach has been to use low-order bivariate polynomials. In the present paper,
we evaluate four other classical spatial interpolation methods based on splines
(B-splines), inverse distance weighting (IDW), radial basis functions (RBF) and
ordinary Kriging (OK). These methods are tested on the Star-challenge part of
the GRavitational lEnsing Accuracy Testing 2010 (GREAT10) simulated data and
are compared with the classical polynomial fitting (Polyfit). We also test all
our interpolation methods independently of the way the PSF is modeled, by
interpolating the GREAT10 star fields themselves (i.e., the PSF parameters are
known exactly at star positions). We find in that case RBF to be the clear
winner, closely followed by the other local methods, IDW and OK. The global
methods, Polyfit and B-splines, are largely behind, especially in fields with
(ground-based) turbulent PSFs. In fields with non-turbulent PSFs, all
interpolators reach a variance on PSF systematics better than
the upper bound expected by future space-based surveys, with
the local interpolators performing better than the global ones
Evaluating the effect of stellar multiplicity on the PSF of space-based weak lensing surveys
The next generation of space-based telescopes used for weak lensing surveys
will require exquisite point spread function (PSF) determination. Previously
negligible effects may become important in the reconstruction of the PSF, in
part because of the improved spatial resolution. In this paper, we show that
unresolved multiple star systems can affect the ellipticity and size of the PSF
and that this effect is not cancelled even when using many stars in the
reconstruction process. We estimate the error in the reconstruction of the PSF
due to the binaries in the star sample both analytically and with image
simulations for different PSFs and stellar populations. The simulations support
our analytical finding that the error on the size of the PSF is a function of
the multiple stars distribution and of the intrinsic value of the size of the
PSF, i.e. if all stars were single. Similarly, the modification of each of the
complex ellipticity components (e1,e2) depends on the distribution of multiple
stars and on the intrinsic complex ellipticity. Using image simulations, we
also show that the predicted error in the PSF shape is a theoretical limit that
can be reached only if large number of stars (up to thousands) are used
together to build the PSF at any desired spatial position. For a lower number
of stars, the PSF reconstruction is worse. Finally, we compute the effect of
binarity for different stellar magnitudes and show that bright stars alter the
PSF size and ellipticity more than faint stars. This may affect the design of
PSF calibration strategies and the choice of the related calibration fields.Comment: 10 pages, 6 figures, accepted in A&
Holomorphic extension of smooth CR-mappings between real-analytic and real-algebraic CR-manifolds
We establish results on holomorphic extension of CR-mappings of class
between a real-analytic CR-submanifold of \C^N and a
real-algebraic CR-submanifold of \C^{N'}
Palomar 13: a velocity dispersion inflated by binaries ?
Recently, combining radial velocities from Keck/HIRES echelle spectra with
published proper motion membership probabilities, Cote et al (2002) observed a
sample of 21 stars, probable members of Palomar 13, a globular cluster in the
Galactic halo. Their projected velocity dispersion sigma_p = 2.2 +/-0.4 km/s
gives a mass-to-light ratio M/L_V = 40 +24/-17, about one order of magnitude
larger than the usual estimate for globular clusters. We present here radial
velocities measured from three different CCD frames of commissioning
observations obtained with the new ESO/VLT instrument FLAMES (Fibre Large Array
Multi Element Spectrograph). From these data, now publicly available, we
measure the homogeneous radial velocities of eight probable members of this
globular cluster. A new projected velocity dispersion sigma_p = 0.6-0.9 +/-0.3
km/s implies Palomar 13 mass-to-light ratio M/L_V = 3-7, similar to the usual
value for globular clusters. We discuss briefly the two most obvious reasons
for the previous unusual mass-to-light ratio finding: binaries, now clearly
detected, and more homogeneous data from the multi-fibre FLAMES spectrograph.Comment: 9 pages, 2 Postscript figure
Microlensing of the broad line region in 17 lensed quasars
When an image of a strongly lensed quasar is microlensed, the different
components of its spectrum are expected to be differentially magnified owing to
the different sizes of the corresponding emitting region. Chromatic changes are
expected to be observed in the continuum while the emission lines should be
deformed as a function of the size, geometry and kinematics of the regions from
which they originate. Microlensing of the emission lines has been reported only
in a handful of systems so far. In this paper we search for microlensing
deformations of the optical spectra of pairs of images in 17 lensed quasars.
This sample is composed of 13 pairs of previously unpublished spectra and four
pairs of spectra from literature. Our analysis is based on a spectral
decomposition technique which allows us to isolate the microlensed fraction of
the flux independently of a detailed modeling of the quasar emission lines.
Using this technique, we detect microlensing of the continuum in 85% of the
systems. Among them, 80% show microlensing of the broad emission lines.
Focusing on the most common lines in our spectra (CIII] and MgII) we detect
microlensing of either the blue or the red wing, or of both wings with the same
amplitude. This observation implies that the broad line region is not in
general spherically symmetric. In addition, the frequent detection of
microlensing of the blue and red wings independently but not simultaneously
with a different amplitude, does not support existing microlensing simulations
of a biconical outflow. Our analysis also provides the intrinsic flux ratio
between the lensed images and the magnitude of the microlensing affecting the
continuum. These two quantities are particularly relevant for the determination
of the fraction of matter in clumpy form in galaxies and for the detection of
dark matter substructures via the identification of flux ratio anomalies.Comment: Accepted for publication in Astronomy and Astrophysics. Main data set
available via the German virtual observatory
http://dc.g-vo.org/mlqso/q/web/form and soon via CDS. Additional material
available on reques
COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses X. Modeling based on high-precision astrometry of a sample of 25 lensed quasars: consequences for ellipticity, shear, and astrometric anomalies
(abridged) Gravitationally lensed quasars can be used as powerful
cosmological and astrophysical probes. We can (i) infer the Hubble constant
based on the time-delay technique, (ii) unveil substructures along the l.o.s.
toward distant galaxies, and (iii) compare the shape and the slope of baryons
and dark matter distributions in galaxies. To reach these goals, we need
high-accuracy astrometry and morphology measurements of the lens. In this work,
we first present new astrometry for 11 lenses with measured time delays. Using
MCS deconvolution on NIC2 HST images, we reached an astrometric accuracy of
about 1-2.5 mas and an accurate shape measurement of the lens galaxy. Second,
we combined these measurements with those of 14 other systems to present new
mass models of these lenses. This led to the following results: 1) In 4
double-image quasars, we show that the influence of the lens environment on the
time delay can easily be quantified and modeled, hence putting these lenses
with high priority for time-delay determination. 2) For quadruple-image
quasars, the difficulty often encountered in reproducing the image positions to
milli-arcsec accuracy (astrometric anomaly) is overcome by explicitly including
the nearest visible galaxy in the model. However, one anomalous system
(J1131-1231) does not show any luminous perturber in its vicinity, and three
others (WFI2026-4536, WFI2033-4723, and B2045+265) have problematic modeling.
These 4 systems are the best candidates for a pertubation by a dark matter
substructure. 3) We find a significant correlation between the PA of the light
and of the mass distributions in lensing galaxies. In contrast with other
studies, we find that the ellipticity of the light and of the mass also
correlate well, suggesting that the overall spatial distribution of matter is
not very different from the baryon distribution in the inner \sim 5 kpc of
lensing galaxies.Comment: Accepted for publication in Astronomy and Astrophysics abridged
abstrac
Firedec: a two-channel finite-resolution image deconvolution algorithm
We present a two-channel deconvolution method that decomposes images into a
parametric point-source channel and a pixelized extended-source channel. Based
on the central idea of the deconvolution algorithm proposed by Magain, Courbin
& Sohy (1998), the method aims at improving the resolution of the data rather
than at completely removing the point spread function (PSF). Improvements over
the original method include a better regularization of the pixel channel of the
image, based on wavelet filtering and multiscale analysis, and a better
controlled separation of the point source vs. the extended source. In addition,
the method is able to simultaneously deconvolve many individual frames of the
same object taken with different instruments under different PSF conditions.
For this purpose, we introduce a general geometric transformation between
individual images. This transformation allows the combination of the images
without having to interpolate them. We illustrate the capability of our
algorithm using real and simulated images with complex diffraction-limited PSF.Comment: Accepted in A&A. An application of the technique to real data is
available in Cantale et al. http://arxiv.org/abs/1601.05192v
Probing the inner structure of distant AGNs with gravitational lensing
Microlensing is a powerful technique which can be used to study the continuum
and the broad line emitting regions in distant AGNs. After a brief description
of the methods and required data, we present recent applications of this
technique. We show that microlensing allows one to measure the temperature
profile of the accretion disc, estimate the size and study the geometry of the
region emitting the broad emission lines.Comment: 6 pages, Proceedings of the Seyfert 2012 conferenc
Exploring the gravitationally lensed system HE 1104-1805: Near-IR Spectroscopy
(Abridged) A new technique for the spatial deconvolution of spectra is
applied to near-IR (0.95 - 2.50 micron) NTT/SOFI spectra of the lensed,
radio-quiet quasar HE 1104-1805. The continuum of the lensing galaxy is
revealed between 1.5 and 2.5 micron. It is used in combination with previous
optical and IR photometry to infer a plausible redshift in the range 0.8 < z <
1.2. Modeling of the system shows that the lens is probably composed of the red
galaxy seen between the quasar images and a more extended component associated
with a galaxy cluster with fairly low velocity dispersion (~ 575 km/s). The
spectra of the two lensed images of the source show no trace of reddening at
the redshift of the lens nor at the redshift of the source. Additionally, the
difference between the spectrum of the brightest component a nd that of a
scaled version of the faintest component is a featureless continuum. Broad and
narrow emission lines, including the FeII features, are perfectly subtracted.
The very good quality of our spectrum makes it possible to fit precisely the
optical Fe II feature, taking into account the underlying continuum over a wide
wavelength range. HE 1104-1805 can be classified as a weak Fe II emitter.
Finally, the slope of the continuum in the brightest image is steeper than the
continuum in the faintest image and supports the finding by Wisotzki et al.
(1993) that the brightest image is microlensed. This is particularly
interesting in view of the new source reconstruction methods from
multiwavelength photometric monitoring.Comment: to be published in A&A, 8 pages, 9 postscript figure
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