37 research outputs found
Breaking the Degeneracy: Optimal Use of Three-point Weak Lensing Statistics
We study the optimal use of third order statistics in the analysis of weak
lensing by large-scale structure. These higher order statistics have long been
advocated as a powerful tool to break measured degeneracies between
cosmological parameters. Using ray-tracing simulations, incorporating important
survey features such as a realistic depth-dependent redshift distribution, we
find that a joint two- and three-point correlation function analysis is a much
stronger probe of cosmology than the skewness statistic. We compare different
observing strategies, showing that for a limited survey time there is an
optimal depth for the measurement of third-order statistics, which balances
statistical noise and cosmic variance against signal amplitude. We find that
the chosen CFHTLS observing strategy was optimal and forecast that a joint two-
and three-point analysis of the completed CFHTLS-Wide will constrain the
amplitude of the matter power spectrum to 10% and the matter density
parameter to 17%, a factor of ~2.5 improvement on the two-point
analysis alone. Our error analysis includes all non-Gaussian terms, finding
that the coupling between cosmic variance and shot noise is a non-negligible
contribution which should be included in any future analytical error
calculations.Comment: 27 pages, 13 figures, 3 table
Cosmic variance of weak lensing surveys in the non-Gaussian regime
The results from weak gravitational lensing analyses are subject to a cosmic
variance error term that has previously been estimated assuming Gaussian
statistics. In this letter we address the issue of estimating cosmic variance
errors for weak lensing surveys in the non-Gaussian regime. Using standard cold
dark matter model ray-tracing simulations characterized by Omega_m=0.3,
Omega_Lambda=0.7, h=0.7, sigma_8=1.0 for different survey redshifts z_s, we
determine the variance of the two-point shear correlation function measured
across 64 independent lines of sight. We compare the measured variance to the
variance expected from a random Gaussian field and derive a redshift-dependent
non-Gaussian calibration relation. We find that the ratio can be as high as ~30
for a survey with source redshift z_s ~ 0.5 and ~10 for z_s ~ 1. The transition
scale theta_c above which the ratio is consistent with unity, is found to be
theta_c ~ 20 arcmin for z_s ~ 0.5 and theta_c ~ 10 arcmin for z_s ~ 1. We
provide fitting formula to our results permitting the estimation of
non-Gaussian cosmic variance errors for any weak lensing analysis, and discuss
the impact on current and future surveys. A more extensive set of simulations
will however be required to investigate the dependence of our results on
cosmology, specifically on the amplitude of clustering.Comment: 6 pages, 7 figures. MNRAS Accepted versio
Weak lensing from space: first cosmological constraints from three-point shear statistics
We use weak lensing data from the Hubble Space Telescope COSMOS survey to
measure the second- and third-moments of the cosmic shear field, estimated from
about 450,000 galaxies with average redshift ~ 1.3. We measure two- and
three-point shear statistics using a tree-code, dividing the signal in E, B and
mixed components. We present a detection of the third-order moment of the
aperture mass statistic and verify that the measurement is robust against
systematic errors caused by point spread function (PSF) residuals and by the
intrinsic alignments between galaxies. The amplitude of the measured
three-point cosmic shear signal is in very good agreement with the predictions
for a WMAP7 best-fit model, whereas the amplitudes of potential systematics are
consistent with zero. We make use of three sets of large Lambda CDM simulations
to test the accuracy of the cosmological predictions and to estimate the
influence of the cosmology-dependent covariance. We perform a likelihood
analysis using the measurement and find that the Omega_m-sigma_8 degeneracy
direction is well fitted by the relation: sigma_8
(Omega_m/0.30)^(0.49)=0.78+0.11/-0.26. We present the first measurement of a
more generalised three-point shear statistic and find a very good agreement
with the WMAP7 best-fit cosmology. The cosmological interpretation of this
measurement gives sigma_8 (Omega_m/0.30)^(0.46)=0.69 +0.08/-0.14. Furthermore,
the combined likelihood analysis of this measurement with the measurement of
the second order moment of the aperture mass improves the accuracy of the
cosmological constraints, showing the high potential of this combination of
measurements to infer cosmological constraints.Comment: 17 pages, 11 figures. MNRAS submitte
Sources of contamination to weak lensing tomography: redshift-dependent shear measurement bias
The current methods available to estimate gravitational shear from
astronomical images of galaxies introduce systematic errors which can affect
the accuracy of weak lensing cosmological constraints. We study the impact of
KSB shape measurement bias on the cosmological interpretation of tomographic
two-point weak lensing shear statistics.
We use a set of realistic image simulations produced by the STEP
collaboration to derive shape measurement bias as a function of redshift. We
define biased two-point weak lensing statistics and perform a likelihood
analysis for two fiducial surveys. We present a derivation of the covariance
matrix for tomography in real space and a fitting formula to calibrate it for
non-Gaussianity.
We find the biased aperture mass dispersion is reduced by ~20% at redshift
~1, and has a shallower scaling with redshift. This effect, if ignored in data
analyses, biases sigma_8 and w_0 estimates by a few percent. The power of
tomography is significantly reduced when marginalising over a range of
realistic shape measurement biases. For a CFHTLS-Wide-like survey, [Omega_m,
sigma_8] confidence regions are degraded by a factor of 2, whereas for a
KIDS-like survey the factor is 3.5. Our results are strictly valid only for KSB
methods but they demonstrate the need to marginalise over a redshift-dependent
shape measurement bias in all future cosmological analyses.Comment: 13 pages, 8 figures. Submitted MNRA
CFHTLenS: Co-evolution of galaxies and their dark matter haloes
Galaxy-galaxy weak lensing is a direct probe of the mean matter distribution
around galaxies. The depth and sky coverage of the CFHT Legacy Survey yield
statistically significant galaxy halo mass measurements over a much wider range
of stellar masses ( to ) and redshifts () than previous weak lensing studies. At redshift , the
stellar-to-halo mass ratio (SHMR) reaches a maximum of percent as a
function of halo mass at . We find, for the first
time from weak lensing alone, evidence for significant evolution in the SHMR:
the peak ratio falls as a function of cosmic time from percent at
to percent at , and shifts to lower
stellar mass haloes. These evolutionary trends are dominated by red galaxies,
and are consistent with a model in which the stellar mass above which star
formation is quenched "downsizes" with cosmic time. In contrast, the SHMR of
blue, star-forming galaxies is well-fit by a power law that does not evolve
with time. This suggests that blue galaxies form stars at a rate that is
balanced with their dark matter accretion in such a way that they evolve along
the SHMR locus. The redshift dependence of the SHMR can be used to constrain
the evolution of the galaxy population over cosmic time.Comment: 18 pages, MNRAS, in pres
CFHTLenS tomographic weak lensing: Quantifying accurate redshift distributions
The Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) comprises deep
multi-colour (u*g'r'i'z') photometry spanning 154 square degrees, with accurate
photometric redshifts and shape measurements. We demonstrate that the redshift
probability distribution function summed over galaxies provides an accurate
representation of the galaxy redshift distribution accounting for random and
catastrophic errors for galaxies with best fitting photometric redshifts z_p <
1.3.
We present cosmological constraints using tomographic weak gravitational
lensing by large-scale structure. We use two broad redshift bins 0.5 < z_p <=
0.85 and 0.85 < z_p <= 1.3 free of intrinsic alignment contamination, and
measure the shear correlation function on angular scales in the range ~1-40
arcmin. We show that the problematic redshift scaling of the shear signal,
found in previous CFHTLS data analyses, does not afflict the CFHTLenS data. For
a flat Lambda-CDM model and a fixed matter density Omega_m=0.27, we find the
normalisation of the matter power spectrum sigma_8=0.771 \pm 0.041. When
combined with cosmic microwave background data (WMAP7), baryon acoustic
oscillation data (BOSS), and a prior on the Hubble constant from the HST
distance ladder, we find that CFHTLenS improves the precision of the fully
marginalised parameter estimates by an average factor of 1.5-2. Combining our
results with the above cosmological probes, we find Omega_m=0.2762 \pm 0.0074
and sigma_8=0.802 \pm 0.013.Comment: 17 pages, 12 figures, submitted to MNRA
Cosmological Constraints From the 100 Square Degree Weak Lensing Survey
We present a cosmic shear analysis of the 100 square degree weak lensing
survey, combining data from the CFHTLS-Wide, RCS, VIRMOS-DESCART and GaBoDS
surveys. Spanning ~100 square degrees, with a median source redshift z~0.78,
this combined survey allows us to place tight joint constraints on the matter
density parameter Omega_m, and the amplitude of the matter power spectrum
sigma_8, finding sigma_8*(Omega_m/0.24)^0.59 = 0.84+/-0.05. Tables of the
measured shear correlation function and the calculated covariance matrix for
each survey are included.
The accuracy of our results is a marked improvement on previous work owing to
three important differences in our analysis; we correctly account for cosmic
variance errors by including a non-Gaussian contribution estimated from
numerical simulations; we correct the measured shear for a calibration bias as
estimated from simulated data; we model the redshift distribution, n(z), of
each survey from the largest deep photometric redshift catalogue currently
available from the CFHTLS-Deep. This catalogue is randomly sampled to reproduce
the magnitude distribution of each survey with the resulting survey dependent
n(z) parametrised using two different models. While our results are consistent
for the n(z) models tested, we find that our cosmological parameter constraints
depend weakly (at the 5% level) on the inclusion or exclusion of galaxies with
low confidence photometric redshift estimates (z>1.5). These high redshift
galaxies are relatively few in number but contribute a significant weak lensing
signal. It will therefore be important for future weak lensing surveys to
obtain near-infra-red data to reliably determine the number of high redshift
galaxies in cosmic shear analyses.Comment: 14 pages, 6 figures, accepted for publication by MNRA