8 research outputs found
Towards emulating cosmic shear data:Revisiting the calibration of the shear measurements for the Kilo-Degree Survey
Exploiting the full statistical power of future cosmic shear surveys will
necessitate improvements to the accuracy with which the gravitational lensing
signal is measured. We present a framework for calibrating shear with image
simulations that demonstrates the importance of including realistic
correlations between galaxy morphology, size and more importantly, photometric
redshifts. This realism is essential so that selection and shape measurement
biases can be calibrated accurately for a tomographic cosmic shear analysis. We
emulate Kilo-Degree Survey (KiDS) observations of the COSMOS field using
morphological information from {\it Hubble} Space Telescope imaging, faithfully
reproducing the measured galaxy properties from KiDS observations of the same
field. We calibrate our shear measurements from lensfit, and find through a
range of sensitivity tests that lensfit is robust and unbiased within the
allowed 2 per cent tolerance of our study. Our results show that the
calibration has to be performed by selecting the tomographic samples in the
simulations, consistent with the actual cosmic shear analysis, because the
joint distributions of galaxy properties are found to vary with redshift.
Ignoring this redshift variation could result in misestimating the shear bias
by an amount that exceeds the allowed tolerance. To improve the calibration for
future cosmic shear analyses, it will be essential to also correctly account
for the measurement of photometric redshifts, which requires simulating
multi-band observations.Comment: 31 pages, 17 figures and 2 tables. Accepted for publication in A&A.
Matches the published versio
GREAT3 results I: systematic errors in shear estimation and the impact of real galaxy morphology
We present first results from the third GRavitational lEnsing Accuracy
Testing (GREAT3) challenge, the third in a sequence of challenges for testing
methods of inferring weak gravitational lensing shear distortions from
simulated galaxy images. GREAT3 was divided into experiments to test three
specific questions, and included simulated space- and ground-based data with
constant or cosmologically-varying shear fields. The simplest (control)
experiment included parametric galaxies with a realistic distribution of
signal-to-noise, size, and ellipticity, and a complex point spread function
(PSF). The other experiments tested the additional impact of realistic galaxy
morphology, multiple exposure imaging, and the uncertainty about a
spatially-varying PSF; the last two questions will be explored in Paper II. The
24 participating teams competed to estimate lensing shears to within systematic
error tolerances for upcoming Stage-IV dark energy surveys, making 1525
submissions overall. GREAT3 saw considerable variety and innovation in the
types of methods applied. Several teams now meet or exceed the targets in many
of the tests conducted (to within the statistical errors). We conclude that the
presence of realistic galaxy morphology in simulations changes shear
calibration biases by per cent for a wide range of methods. Other
effects such as truncation biases due to finite galaxy postage stamps, and the
impact of galaxy type as measured by the S\'{e}rsic index, are quantified for
the first time. Our results generalize previous studies regarding sensitivities
to galaxy size and signal-to-noise, and to PSF properties such as seeing and
defocus. Almost all methods' results support the simple model in which additive
shear biases depend linearly on PSF ellipticity.Comment: 32 pages + 15 pages of technical appendices; 28 figures; submitted to
MNRAS; latest version has minor updates in presentation of 4 figures, no
changes in content or conclusion
GREAT3 results - I. Systematic errors in shear estimation and the impact of real galaxy morphology
We present first results from the third GRavitational lEnsing Accuracy Testing (GREAT3) challenge, the third in a sequence of challenges for testing methods of inferring weak gravitational lensing shear distortions from simulated galaxy images. GREAT3 was divided into experiments to test three specific questions, and included simulated space- and ground-based data with constant or cosmologically varying shear fields. The simplest (control) experiment included parametric galaxies with a realistic distribution of signal-to-noise, size, and ellipticity, and a complex point spread function (PSF). The other experiments tested the additional impact of realistic galaxy morphology, multiple exposure imaging, and the uncertainty about a spatially varying PSF; the last two questions will be explored in Paper II. The 24 participating teams competed to estimate lensing shears to within systematic error tolerances for upcoming Stage-IV dark energy surveys, making 1525 submissions overall. GREAT3 saw considerable variety and innovation in the types of methods applied. Several teams now meet or exceed the targets in many of the tests conducted (to within the statistical errors). We conclude that the presence of realistic galaxy morphology in simulations changes shear calibration biases by âŒ1percent for a wide range of methods. Other effects such as truncation biases due to finite galaxy postage stamps, and the impact of galaxy type as measured by the SĂ©rsic index, are quantified for the first time. Our results generalize previous studies regarding sensitivities to galaxy size and signal-to-noise, and to PSF properties such as seeing and defocus. Almost all methods' results support the simple model in which additive shear biases depend linearly on PSF ellipticit
The third data release of the Kilo-Degree Survey and associated data products
The Kilo-Degree Survey (KiDS) is an ongoing optical wide-field imaging survey
with the OmegaCAM camera at the VLT Survey Telescope. It aims to image 1500
square degrees in four filters (ugri). The core science driver is mapping the
large-scale matter distribution in the Universe, using weak lensing shear and
photometric redshift measurements. Further science cases include galaxy
evolution, Milky Way structure, detection of high-redshift clusters, and
finding rare sources such as strong lenses and quasars. Here we present the
third public data release (DR3) and several associated data products, adding
further area, homogenized photometric calibration, photometric redshifts and
weak lensing shear measurements to the first two releases. A dedicated pipeline
embedded in the Astro-WISE information system is used for the production of the
main release. Modifications with respect to earlier releases are described in
detail. Photometric redshifts have been derived using both Bayesian template
fitting, and machine-learning techniques. For the weak lensing measurements,
optimized procedures based on the THELI data reduction and lensfit shear
measurement packages are used. In DR3 stacked ugri images, weight maps, masks,
and source lists for 292 new survey tiles (~300 sq.deg) are made available. The
multi-band catalogue, including homogenized photometry and photometric
redshifts, covers the combined DR1, DR2 and DR3 footprint of 440 survey tiles
(447 sq.deg). Limiting magnitudes are typically 24.3, 25.1, 24.9, 23.8 (5 sigma
in a 2 arcsec aperture) in ugri, respectively, and the typical r-band PSF size
is less than 0.7 arcsec. The photometric homogenization scheme ensures accurate
colors and an absolute calibration stable to ~2% for gri and ~3% in u.
Separately released are a weak lensing shear catalogue and photometric
redshifts based on two different machine-learning techniques.Comment: small modifications; 27 pages, 12 figures, accepted for publication
in Astronomy & Astrophysic
KiDS-450: testing extensions to the standard cosmological model
We test extensions to the standard cosmological model with weak gravitational
lensing tomography using 450 deg of imaging data from the Kilo Degree
Survey (KiDS). In these extended cosmologies, which include massive neutrinos,
nonzero curvature, evolving dark energy, modified gravity, and running of the
scalar spectral index, we also examine the discordance between KiDS and cosmic
microwave background measurements from Planck. The discordance between the two
datasets is largely unaffected by a more conservative treatment of the lensing
systematics and the removal of angular scales most sensitive to nonlinear
physics. The only extended cosmology that simultaneously alleviates the
discordance with Planck and is at least moderately favored by the data includes
evolving dark energy with a time-dependent equation of state (in the form of
the parameterization). In this model, the respective constraints agree at the level, and there
is `substantial concordance' between the KiDS and Planck datasets when
accounting for the full parameter space. Moreover, the Planck constraint on the
Hubble constant is wider than in LCDM and in agreement with the Riess et al.
(2016) direct measurement of . The dark energy model is moderately favored
as compared to LCDM when combining the KiDS and Planck measurements, and
remains moderately favored after including an informative prior on the Hubble
constant. In both of these scenarios, marginalized constraints in the
plane are discrepant with a cosmological constant at the level.
Moreover, KiDS constrains the sum of neutrino masses to 4.0 eV (95% CL), finds
no preference for time or scale dependent modifications to the metric
potentials, and is consistent with flatness and no running of the spectral
index. The analysis code is public at https://github.com/sjoudaki/kids450Comment: 22 pages, 16 figures, results unchanged, version accepted for
publication by MNRA
A KiDS weak lensing analysis of assembly bias in GAMA galaxy groups
We investigate possible signatures of halo assembly bias for spectroscopically selected galaxy groups from the Galaxy And Mass Assembly (GAMA) survey using weak lensing measurements from the spatially overlapping regions of the deeper, high-imaging-quality photometric Kilo-Degree Survey.We use GAMA groups with an apparent richness larger than 4 to identify samples with comparable mean host halo masses but with a different radial distribution of satellite galaxies, which is a proxy for the formation time of the haloes. We measure the weak lensing signal for groups with a steeper than average and with a shallower than average satellite distribution and find no sign of halo assembly bias, with the bias ratio of 0.85+0.37 â0.25, which is consistent with the cold dark matter prediction. Our galaxy groups have typical masses of 1013M hâ1, naturally complementing previous studies of halo assembly bias on galaxy cluster scales