121 research outputs found
The POINT-AGAPE Microlensing Survey: First Constraint on MACHOs towards M31
To reveal the galactic dark matter in the form of MACHOs ("Massive
Astrophysical Compact Halo Objects"), the POINT-AGAPE collaboration is carrying
out a search for gravitational microlensing towards M31. A clear microlensing
signal is detected. The high-threshold analysis of 3-year data leads to 7
bright and short microlensing candidates. The preliminary estimation of the
detection efficiency implies that less than 25% (60%) of standard halos can be
composed of objects with masses between 10^{-4} and 10^{-1} (10^{-1} and 1)
solar mass at the 95% C.L. This result is compatible with previous microlensing
results towards the Magellanic Clouds and gives the first constraints on MACHOs
for a distant spiral galaxy.Comment: 4 pages, 2 figures, to appear in the proceedings of the XXXIXth
Rencontres de Moriond "Exploring the Universe", La Thuile, Italy, March
28-April 4, 200
Cosmic Shear Systematics: Software-Hardware Balance
Cosmic shear measurements rely on our ability to measure and correct the
Point Spread Function (PSF) of the observations. This PSF is measured using
stars in the field, which give a noisy measure at random points in the field.
Using Wiener filtering, we show how errors in this PSF correction process
propagate into shear power spectrum errors. This allows us to test future
space-based missions, such as Euclid or JDEM, thereby allowing us to set clear
engineering specifications on PSF variability. For ground-based surveys, where
the variability of the PSF is dominated by the environment, we briefly discuss
how our approach can also be used to study the potential of mitigation
techniques such as correlating galaxy shapes in different exposures. To
illustrate our approach we show that for a Euclid-like survey to be statistics
limited, an initial pre-correction PSF ellipticity power spectrum, with a
power-law slope of -3 must have an amplitude at l =1000 of less than 2 x
10^{-13}. This is 1500 times smaller than the typical lensing signal at this
scale. We also find that the power spectrum of PSF size \dR^2) at this scale
must be below 2 x 10^{-12}. Public code available as part of iCosmo at
http://www.icosmo.orgComment: 5 pages, 3 figures. Submitted to MNRA
Optimal PSF modeling for weak lensing: complexity and sparsity
We investigate the impact of point spread function (PSF) fitting errors on
cosmic shear measurements using the concepts of complexity and sparsity.
Complexity, introduced in a previous paper, characterizes the number of degrees
of freedom of the PSF. For instance, fitting an underlying PSF with a model
with low complexity will lead to small statistical errors on the model
parameters, however these parameters could suffer from large biases.
Alternatively, fitting with a large number of parameters will tend to reduce
biases at the expense of statistical errors. We perform an optimisation of
scatters and biases by studying the mean squared error of a PSF model. We also
characterize a model sparsity, which describes how efficiently the model is
able to represent the underlying PSF using a limited number of free parameters.
We present the general case and illustrate it for a realistic example of PSF
fitted with shapelet basis sets. We derive the relation between complexity and
sparsity of the PSF model, signal-to-noise ratio of stars and systematic errors
on cosmological parameters. With the constraint of maintaining the systematics
below the statistical uncertainties, this lead to a relation between the
required number of stars to calibrate the PSF and the sparsity. We discuss the
impact of our results for current and future cosmic shear surveys. In the
typical case where the biases can be represented as a power law of the
complexity, we show that current weak lensing surveys can calibrate the PSF
with few stars, while future surveys will require hard constraints on the
sparsity in order to calibrate the PSF with 50 stars.Comment: accepted by A&A, 9 pages, 6 figure
Microlensing towards M31 with MDM data
We report the final analysis of a search for microlensing events in the
direction of the Andromeda galaxy, which aimed to probe the MACHO composition
of the M31 halo using data collected during the 1998-99 observational campaign
at the MDM observatory. In a previous paper, we discussed the results from a
first set of observations. Here, we deal with the complete data set, and we
take advantage of some INT observations in the 1999-2000 seasons. This merging
of data sets taken by different instruments turns out to be very useful, the
study of the longer baseline available allowing us to test the uniqueness
characteristic of microlensing events. As a result, all the candidate
microlensing events previously reported turn out to be variable stars. We
further discuss a selection based on different criteria, aimed at the detection
of short--duration events. We find three candidates whose positions are
consistent with self--lensing events, although the available data do not allow
us to conclude unambiguously that they are due to microlensing.Comment: Accepted for publication in Astronomy and Astrophysic
The Dark UNiverse Explorer (DUNE): Proposal to ESA's Cosmic Vision
The Dark UNiverse Explorer (DUNE) is a wide-field space imager whose primary
goal is the study of dark energy and dark matter with unprecedented precision.
For this purpose, DUNE is optimised for the measurement of weak gravitational
lensing but will also provide complementary measurements of baryonic accoustic
oscillations, cluster counts and the Integrated Sachs Wolfe effect. Immediate
auxiliary goals concern the evolution of galaxies, to be studied with
unequalled statistical power, the detailed structure of the Milky Way and
nearby galaxies, and the demographics of Earth-mass planets. DUNE is an
Medium-class mission which makes use of readily available components, heritage
from other missions, and synergy with ground based facilities to minimise cost
and risks. The payload consists of a 1.2m telescope with a combined visible/NIR
field-of-view of 1 deg^2. DUNE will carry out an all-sky survey, ranging from
550 to 1600nm, in one visible and three NIR bands which will form a unique
legacy for astronomy. DUNE will yield major advances in a broad range of fields
in astrophysics including fundamental cosmology, galaxy evolution, and
extrasolar planet search. DUNE was recently selected by ESA as one of the
mission concepts to be studied in its Cosmic Vision programme.Comment: Accepted in Experimental Astronom
Cosmic shear systematics: software-hardware balance
Cosmic shear measurements rely on our ability to measure and correct the point spread function (PSF) of the observations. This PSF is measured using stars in the field, which give a noisy measure at random points in the field. Using Wiener filtering, we show how errors in this PSF correction process propagate into shear power spectrum errors. This allows us to test future space-based missions, such as Euclid or the Joint Dark Energy Mission, thereby allowing us to set clear engineering specifications on PSF variability. For ground-based surveys, where the variability of the PSF is dominated by the environment, we briefly discuss how our approach can also be used to study the potential of mitigation techniques such as correlating galaxy shapes in different exposures. To illustrate our approach we show that for a Euclid-like survey to be statistics limited, an initial pre-correction PSF ellipticity power spectrum, with a power-law slope of −3, must have an amplitude of less than at ℓ= 1000. This is 200 times smaller than the typical lensing signal at this scale. We also find that the power spectrum of the PSF size () at this scale must be below . The public code is available as part of iCosmo at http://www.icosmo.or
On Point Spread Function modelling: towards optimal interpolation
Point Spread Function (PSF) modeling is a central part of any astronomy data
analysis relying on measuring the shapes of objects. It is especially crucial
for weak gravitational lensing, in order to beat down systematics and allow one
to reach the full potential of weak lensing in measuring dark energy. A PSF
modeling pipeline is made of two main steps: the first one is to assess its
shape on stars, and the second is to interpolate it at any desired position
(usually galaxies). We focus on the second part, and compare different
interpolation schemes, including polynomial interpolation, radial basis
functions, Delaunay triangulation and Kriging. For that purpose, we develop
simulations of PSF fields, in which stars are built from a set of basis
functions defined from a Principal Components Analysis of a real ground-based
image. We find that Kriging gives the most reliable interpolation,
significantly better than the traditionally used polynomial interpolation. We
also note that although a Kriging interpolation on individual images is enough
to control systematics at the level necessary for current weak lensing surveys,
more elaborate techniques will have to be developed to reach future ambitious
surveys' requirements.Comment: Accepted for publication in MNRA
The POINT-AGAPE Survey: Comparing Automated Searches of Microlensing Events toward M31
Searching for microlensing in M31 using automated superpixel surveys raises a
number of difficulties which are not present in more conventional techniques.
Here we focus on the problem that the list of microlensing candidates is
sensitive to the selection criteria or "cuts" imposed and some subjectivity is
involved in this. Weakening the cuts will generate a longer list of
microlensing candidates but with a greater fraction of spurious ones;
strengthening the cuts will produce a shorter list but may exclude some genuine
events. We illustrate this by comparing three analyses of the same data-set
obtained from a 3-year observing run on the INT in La Palma. The results of two
of these analyses have been already reported: Belokurov et al. (2005) obtained
between 3 and 22 candidates, depending on the strength of their cuts, while
Calchi Novati et al. (2005) obtained 6 candidates. The third analysis is
presented here for the first time and reports 10 microlensing candidates, 7 of
which are new. Only two of the candidates are common to all three analyses. In
order to understand why these analyses produce different candidate lists, a
comparison is made of the cuts used by the three groups...Comment: 28 pages, 24 figures, 9 table
The POINT-AGAPE survey II: An Unrestricted Search for Microlensing Events towards M31
An automated search is carried out for microlensing events using a catalogue
of 44554 variable superpixel lightcurves derived from our three-year monitoring
program of M31. Each step of our candidate selection is objective and
reproducible by a computer. Our search is unrestricted, in the sense that it
has no explicit timescale cut. So, it must overcome the awkward problem of
distinguishing long-timescale microlensing events from long-period stellar
variables. The basis of the selection algorithm is the fitting of the
superpixel lightcurves to two different theoretical models, using variable star
and blended microlensing templates. Only if microlensing is preferred is an
event retained as a possible candidate. Further cuts are made with regard to
(i) sampling, (ii) goodness of fit of the peak to a Paczynski curve, (iii)
consistency of the microlensing hypothesis with the absence of a resolved
source, (iv) achromaticity, (v) position in the colour-magnitude diagram and
(vi) signal-to-noise ratio. Our results are reported in terms of first-level
candidates, which are the most trustworthy, and second-level candidates, which
are possible microlensing but have lower signal-to-noise and are more
questionable. The pipeline leaves just 3 first-level candidates, all of which
have very short full-width half-maximum timescale (<5 days) and 3 second-level
candidates, which have timescales of 31, 36 and 51 days respectively. We also
show 16 third-level lightcurves, as an illustration of the events that just
fail the threshold for designation as microlensing candidates. They are almost
certainly mainly variable stars. Two of the 3 first-level candidates correspond
to known events (PA 00-S3 and PA 00-S4) already reported by the POINT-AGAPE
project. The remaining first-level candidate is new.Comment: 22 pages, 18 figures, MNRAS, to appea
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