736 research outputs found
A method for spatial deconvolution of spectra
A method for spatial deconvolution of spectra is presented. It follows the
same fundamental principles as the ``MCS image deconvolution algorithm''
(Magain, Courbin, Sohy, 1998) and uses information contained in the spectrum of
a reference Point Spread Function (PSF) to spatially deconvolve spectra of very
blended sources. An improved resolution rather than an infinite one is aimed
at, overcoming the well known problem of ``deconvolution artefacts''. As in the
MCS algorithm, the data are decomposed into a sum of analytical point sources
and a numerically deconvolved background, so that the spectrum of extended
sources in the immediate vicinity of bright point sources may be accurately
extracted and sharpened. The algorithm has been tested on simulated data
including seeing variation as a function of wavelength and atmospheric
refraction. It is shown that the spectra of severely blended point sources can
be resolved while fully preserving the spectrophotometric properties of the
data. Extended objects ``hidden'' by bright point sources (up to 4-5 magnitudes
brighter) can be accurately recovered as well, provided the data have a
sufficiently high total signal-to-noise ratio (200-300 per spectral resolution
element). Such spectra are relatively easy to obtain, even down to faint
magnitudes, within a few hours of integration time with 10m class telescopes.Comment: 18 pages, 6 postscript figures, in press in Ap
A deconvolution-based algorithm for crowded field photometry with unknown Point Spread Function
A new method is presented for determining the Point Spread Function (PSF) of
images that lack bright and isolated stars. It is based on the same principles
as the MCS (Magain, Courbin, Sohy, 1998) image deconvolution algorithm. It uses
the information contained in all stellar images to achieve the double task of
reconstructing the PSFs for single or multiple exposures of the same field and
to extract the photometry of all point sources in the field of view. The use of
the full information available allows to construct an accurate PSF. The
possibility to simultaneously consider several exposures makes it very well
suited to the measurement of the light curves of blended point sources from
data that would be very difficult or even impossible to analyse with
traditional PSF fitting techniques. The potential of the method for the
analysis of ground-based and space-based data is tested on artificial images
and illustrated by several examples, including HST/NICMOS images of a lensed
quasar and VLT/ISAAC images of a faint blended Mira star in the halo of the
giant elliptical galaxy NGC5128 (Cen A).Comment: Institutes: (1) Institut d'Astrophysique et de Geophysique,
Universite de Liege, allee du 6 Aout 17, B-4000 Liege, Belgium; (2) Ecole
Polytechnique Federale de Lausanne (EPFL), Laboratoire d'Astrophysique,
Observatoire, CH-1290 Sauverny, Switzerland; (3) Observatoire de Geneve, 51
Chemin des Maillettes, CH-1290 Sauverny, Switzerland. 8 pages, 8 figures.
Accepted for publication in A&
A deep, narrow J-band search for proto-galactic Lyman Alpha emission at redshifts z~9
We present a deep, narrow J-band search for proto--galactic Lyman Alpha
emission at redshifts z~9. We combine an exceptionally deep image of the Hubble
Deep Field South, obtained using a narrow band filter centred on the wavelength
1.187 microns using the VLT/ISAAC facility, with existing deep, broad band
images covering optical to near infrared wavelengths. We search for z~9 Lyman
Alpha emitting galaxies displaying a significant narrow band excess relative to
the Js-band that are undetected at optical wavelengths. We detect no sources
consistent with this criterion to the 90% point source flux limit of the NB
image, F_{NB} = 3.28 x 10^{-18} ergs/s/cm2. The survey selection function
indicates that we have sampled a volume of approximately 340 h^{-3} Mpc^3 to a
Lyman Alpha emission luminosity of 10^{43} h^{-2} ergs/s. We conclude by
considering the potential implications for the physics of the high--redshift
universe.Comment: 11 pages, accepted MNRAS. Please note that the automatic postscript
generation may result in a corrupted postscript file. Please use
ftp://astroftp.phys.uvic.ca/pub/jwillis/jwillis_zen1.ps.gz to obtain an
uncorrupted versio
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
Stellar classification from single-band imaging using machine learning
Information on the spectral types of stars is of great interest in view of
the exploitation of space-based imaging surveys. In this article, we
investigate the classification of stars into spectral types using only the
shape of their diffraction pattern in a single broad-band image. We propose a
supervised machine learning approach to this endeavour, based on principal
component analysis (PCA) for dimensionality reduction, followed by artificial
neural networks (ANNs) estimating the spectral type. Our analysis is performed
with image simulations mimicking the Hubble Space Telescope (HST) Advanced
Camera for Surveys (ACS) in the F606W and F814W bands, as well as the Euclid
VIS imager. We first demonstrate this classification in a simple context,
assuming perfect knowledge of the point spread function (PSF) model and the
possibility of accurately generating mock training data for the machine
learning. We then analyse its performance in a fully data-driven situation, in
which the training would be performed with a limited subset of bright stars
from a survey, and an unknown PSF with spatial variations across the detector.
We use simulations of main-sequence stars with flat distributions in spectral
type and in signal-to-noise ratio, and classify these stars into 13 spectral
subclasses, from O5 to M5. Under these conditions, the algorithm achieves a
high success rate both for Euclid and HST images, with typical errors of half a
spectral class. Although more detailed simulations would be needed to assess
the performance of the algorithm on a specific survey, this shows that stellar
classification from single-band images is well possible.Comment: 10 pages, 9 figures, 2 tables, accepted in A&
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&
Chandra discovery of the intracluster medium around UM425 at redshift 1.47
We report on a discovery of a candidate cluster of galaxies at redshift
z=1.47 based on Chandra observations in the field of quasars UM425 A & B. We
detect with high significance diffuse emission due the intracluster hot gas
around the quasar pair. This is the second highest redshift cluster candidate
after 3C294 at z=1.786. The diffuse emission is elliptical in shape with about
17" extent. If indeed at z=1.47, this corresponds to a physical size of 140
h_{70}^{-1} Kpc and 2--10 keV luminosity of about 3 times 10^{43} erg/s. The
cluster is unlikely to be the long sought gravitational lens invoked to explain
unusual brightness of UM425 A and the close quasar pair. Coexistence of the
quasars with the cluster suggests a link of activity to cluster environment.
The unusual brightness of UM425 A may then be due to a higher accretion rate.
We also comment briefly on the X-ray spectra of UM 425 A & B which also happen
to be broad absorption line quasars. We argue that present evidence suggests
that the quasars are just a pair and not lensed images of the same quasar.Comment: Submitted to ApJ Letter
POX 186: the ultracompact Blue Compact Dwarf Galaxy reveals its nature
High resolution, ground based R and I band observations of the ultra compact
dwarf galaxy POX 186 are presented. The data, obtained with the ESO New
Technology Telescope (NTT), are analyzed using a new deconvolution algorithm
which allows one to resolve the innermost regions of this stellar-like object
into three Super-Star Clusters (SSC). Upper limits to both masses (M\sim 10^5
M_{\odot}) and the physical sizes (\le 60pc) of the SSCs are set. In addition,
and maybe most importantly, extended light emission underlying the compact
star-forming region is clearly detected in both bands. The R-I color rules out
nebular H\alpha contamination and is consistent with an old stellar population.
This casts doubt on the hypothesis that Blue Compact Dwarf Galaxies (BCDG) are
young galaxies.Comment: 4 figures postscript, 2 tables, to appear in A&A main journa
Confirmation of two extended objects along the line of sight to PKS1830-211 with ESO-VLT adaptive optics imaging
We report on new high-resolution near-infrared images of the gravitationally
lensed radio source PKS1830-211, a quasar at z=2.507. These adaptive optics
observations, taken with the Very Large Telescope (VLT), are further improved
through image deconvolution. They confirm the presence of a second object along
the line of sight to the quasar, in addition to the previously known spiral
galaxy. This additional object is clearly extended in our images. However, its
faint luminosity does not allow to infer any photometric redshift. If this
galaxy is located in the foreground of PKS1830-211, it complicates the modeling
of this system and decreases the interest in using PKS1830-211 as a means to
determine H0 via the time delay between the two lensed images of the quasar.Comment: Accepted in A&A Letter
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