58 research outputs found

### The impact of cosmic variance on simulating weak lensing surveys

Upcoming weak lensing surveys will survey large cosmological volumes to
measure the growth of cosmological structure with time and thereby constrain
dark energy. One major systematic uncertainty in this process is the
calibration of the weak lensing shape distortions, or shears. Most upcoming
surveys plan to test several aspects of their shear estimation algorithms using
sophisticated image simulations that include realistic galaxy populations based
on high-resolution data from the Hubble Space Telescope (HST). However,
existing datasets from the (HST) cover very small cosmological volumes, so
cosmic variance could cause the galaxy populations in them to be atypical. A
narrow redshift slice from such surveys could be dominated by a single large
overdensity or underdensity. In that case, the morphology-density relation
could alter the local galaxy populations and yield an incorrect calibration of
shear estimates as a function of redshift. We directly test this scenario using
the COSMOS survey, the largest-area (HST) survey to date, and show how the
statistical distributions of galaxy shapes and morphological parameters (e.g.,
S\'{e}rsic $n$) are influenced by redshift-dependent cosmic variance. The
typical variation in RMS ellipticity due to environmental effects is 5 per cent
(absolute, not relative) for redshift bins of width $\Delta z=0.05$, which
could result in uncertain shear calibration at the 1 per cent level. We
conclude that the cosmic variance effects are large enough to exceed the
systematic error budget of future surveys, but can be mitigated with careful
choice of training dataset and sufficiently large redshift binning.Comment: 18 pages, 16 figures, 3 tables. v2 matches the accepted version for
MNRA

### Persistent entanglement in a class of eigenstates of quantum Heisenberg spin glasses

The eigenstates of a quantum spin glass Hamiltonian with long-range
interaction are examined from the point of view of localisation and
entanglement. In particular, low particle sectors are examined and an anomalous
family of eigenstates is found that is more delocalised but also has larger
inter-spin entanglement. These are then identified as particle-added
eigenstates from the one-particle sector. This motivates the introduction and
the study of random promoted two-particle states, and it is shown that they may
have large delocalisation such as generic ran- dom states and scale exactly
like them. However, the entanglement as measured by two-spin concurrence
displays different scaling with the total number of spins. This shows how for
different classes of complex quantum states entanglement can be qualitatively
different even if localisation measures such as participation ratio are not.Comment: 7 pages, 3 figures, 1 tabl

### Mitigating the effects of undersampling in weak lensing shear estimation with metacalibration

Metacalibration is a state-of-the-art technique for measuring weak
gravitational lensing shear from well-sampled galaxy images. We investigate the
accuracy of shear measured with metacalibration from fitting elliptical
Gaussians to undersampled galaxy images. In this case, metacalibration
introduces aliasing effects leading to an ensemble multiplicative shear bias
about 0.01 for Euclid, and even larger for the Roman Space Telescope, well
exceeding the missions' requirements. We find that this aliasing bias can be
mitigated by computing shapes from weighted moments with wider Gaussians as
weight functions, thereby trading bias for a slight increase in variance of the
measurements. We show that this approach is robust to the point-spread function
in consideration and meets the stringent requirements of Euclid for galaxies
with moderate to high signal-to-noise ratios. We therefore advocate
metacalibration as a viable shear measurement option for weak lensing from
upcoming space missions.Comment: 17 pages, 12 figures, 3 tables; matches the published version in
MNRA

### Accounting for object detection bias in weak gravitational lensing studies

Weak lensing by large-scale structure is a powerful probe of cosmology if the
apparent alignments in the shapes of distant galaxies can be accurately
measured. Most studies have therefore focused on improving the fidelity of the
shape measurements themselves, but the preceding step of object detection has
been largely ignored. In this paper we study the impact of object detection for
a Euclid-like survey and show that it leads to biases that exceed requirements
for the next generation of cosmic shear surveys. In realistic scenarios,
blending of galaxies is an important source of detection bias. We find that
MetaDetection is able to account for blending, leading to average
multiplicative biases that meet requirements for Stage IV surveys, provided a
sufficiently accurate model for the point spread function is available. Further
work is needed to estimate the performance for actual surveys. Combined with
sufficiently realistic image simulations, this provides a viable way forward
towards accurate shear estimates for Stage IV surveys.Comment: version accepted by A&A; 24 pages; included reference to companion
paper Kannawadi et al. arXiv:2010.0416

### KiDS+VIKING-450: An internal-consistency test for cosmic shear tomography with a colour-based split of source galaxies

We performed an internal-consistency test of the KiDS+VIKING-450 (KV450)
cosmic shear analysis with a colour-based split of source galaxies. Utilising
the same measurements and calibrations for both sub-samples, we inspected the
characteristics of the shear measurements and the performance of the
calibration pipelines. On the modelling side, we examined the observational
nuisance parameters, specifically those for the redshift calibration and
intrinsic alignments, using a Bayesian analysis with dedicated test parameters.
We verified that the current nuisance parameters are sufficient for the KV450
data to capture residual systematics, with slight deviations seen in the second
and the third redshift tomographic bins. Our test also showcases the degeneracy
between the inferred amplitude of intrinsic alignments and the redshift
uncertainties in low redshift tomographic bins. The test is rather insensitive
to the background cosmology and, therefore, can be implemented before any
cosmological inference is made.Comment: 12 pages, 8 figures, 3 tables. Revised to match version published in
A&

### Luminous red galaxies in the Kilo Degree Survey: selection with broad-band photometry and weak lensing measurements

We use the overlap between multiband photometry of the Kilo-Degree Survey
(KiDS) and spectroscopic data based on the Sloan Digital Sky Survey (SDSS) and
Galaxy And Mass Assembly (GAMA) to infer the colour-magnitude relation of
red-sequence galaxies. We then use this inferred relation to select luminous
red galaxies (LRGs) in the redshift range of $0.1<z<0.7$ over the entire KiDS
Data Release 3 footprint. We construct two samples of galaxies with different
constant comoving densities and different luminosity thresholds. The selected
red galaxies have photometric redshifts with typical photo-z errors of
$\sigma_z \sim 0.014 (1+z)$ that are nearly uniform with respect to
observational systematics. This makes them an ideal set of galaxies for lensing
and clustering studies. As an example, we use the KiDS-450 cosmic shear
catalogue to measure the mean tangential shear signal around the selected LRGs.
We detect a significant weak lensing signal for lenses out to $z \sim 0.7$

### KiDS+VIKING-450:Improved cosmological parameter constraints from redshift calibration with self-organising maps

We present updated cosmological constraints for the KiDS+VIKING-450 cosmic
shear data set (KV450), estimated using redshift distributions and photometric
samples defined using self-organising maps (SOMs). Our fiducial analysis finds
marginal posterior constraints of $S_8\equiv\sigma_8\sqrt{\Omega_{\rm
m}/0.3}=0.716^{+0.043}_{-0.038}$; smaller than, but otherwise consistent with,
previous work using this data set ($|\Delta S_8| = 0.023$). We analyse
additional samples and redshift distributions constructed in three ways:
excluding certain spectroscopic surveys during redshift calibration, excluding
lower-confidence spectroscopic redshifts in redshift calibration, and
considering only photometric sources which are jointly calibrated by at least
three spectroscopic surveys. In all cases, the method utilised here proves
robust: we find a maximal deviation from our fiducial analysis of $|\Delta S_8|
\leq 0.011$ for all samples defined and analysed using our SOM. To demonstrate
the reduction in systematic biases found within our analysis, we highlight our
results when performing redshift calibration without the DEEP2 spectroscopic
data set. In this case we find marginal posterior constraints of
$S_8=0.707_{-0.042}^{+0.046}$; a difference with respect to the fiducial that
is both significantly smaller than, and in the opposite direction to, the
equivalent shift from previous work. These results suggest that our improved
cosmological parameter estimates are insensitive to pathological
misrepresentation of photometric sources by the spectroscopy used for direct
redshift calibration, and therefore that this systematic effect cannot be
responsible for the observed difference between $S_8$ estimates made with KV450
and Planck CMB probes.Comment: 10 pages, 3 figures, 4 appendices, accepted for publication in A&A
Letter

### Impact of Point Spread Function Higher Moments Error on Weak Gravitational Lensing II: A Comprehensive Study

Weak gravitational lensing, or weak lensing, is one of the most powerful
probes for dark matter and dark energy science, although it faces increasing
challenges in controlling systematic uncertainties as \edit{the statistical
errors become smaller}. The Point Spread Function (PSF) needs to be precisely
modeled to avoid systematic error on the weak lensing measurements. The weak
lensing biases induced by errors in the PSF model second moments, i.e., its
size and shape, are well-studied. However, Zhang et al. (2021) showed that
errors in the higher moments of the PSF may also be a significant source of
systematics for upcoming weak lensing surveys. Therefore, the goal of this work
is to comprehensively investigate the modeling quality of PSF moments from the
$3^{\text{rd}}$ to $6^{\text{th}}$ order, and estimate their impact on
cosmological parameter inference. We propagate the \textsc{PSFEx} higher
moments modeling error in the HSC survey dataset to the weak lensing
\edit{shear-shear correlation functions} and their cosmological analyses. We
find that the overall multiplicative shear bias associated with errors in PSF
higher moments can cause a $\sim 0.1 \sigma$ shift on the cosmological
parameters for LSST Y10. PSF higher moment errors also cause additive biases in
the weak lensing shear, which, if not accounted for in the cosmological
parameter analysis, can induce cosmological parameter biases comparable to
their $1\sigma$ uncertainties for LSST Y10. We compare the \textsc{PSFEx} model
with PSF in Full FOV (\textsc{Piff}), and find similar performance in modeling
the PSF higher moments. We conclude that PSF higher moment errors of the future
PSF models should be reduced from those in current methods to avoid a need to
explicitly model these effects in the weak lensing analysis.Comment: 24 pages, 17 figures, 3 tables; Submitted to MNRAS; Comments welcome

### PSFs of coadded images

We provide a detailed exploration of the connection between choice of
coaddition schemes and the point-spread function (PSF) of the resulting coadded
images. In particular, we investigate what properties of the coaddition
algorithm lead to the final coadded image having a well-defined PSF. The key
elements of this discussion are as follows:
1. We provide an illustration of how linear coaddition schemes can produce a
coadd that lacks a well-defined PSF even for relatively simple scenarios and
choices of weight functions.
2. We provide a more formal demonstration of the fact that a linear coadd
only has a well-defined PSF in the case that either (a) each input image has
the same PSF or (b) the coadd is produced with weights that are independent of
the signal.
3. We discuss some reasons that two plausible nonlinear coaddition algorithms
(median and clipped-mean) fail to produce a consistent PSF profile for stars.
4. We demonstrate that all nonlinear coaddition procedures fail to produce a
well-defined PSF for extended objects.
In the end, we conclude that, for any purpose where a well-defined PSF is
desired, one should use a linear coaddition scheme with weights that do not
correlate with the signal and are approximately uniform across typical objects
of interest.Comment: 13 pages, 4 figures; pedagogical article for submission to the Open
Journal of Astrophysic

### The dependence of intrinsic alignment of galaxies on wavelength using KiDS and GAMA

The outer regions of galaxies are more susceptible to the tidal interactions that lead to intrinsic alignments of galaxies. The resulting alignment signal may therefore depend on the passband if the colours of galaxies vary spatially. To quantify this, we measured the shapes of galaxies with spectroscopic redshifts from the GAMA survey using deep gri imaging data from the KiloDegree Survey. The performance of the moment-based shape measurement algorithm DEIMOS was assessed using dedicated image simulations, which showed that the ellipticities could be determined with an accuracy better than 1% in all bands. Additional tests for potential systematic errors did not reveal any issues. We measure a significant difference of the alignment signal between the g,r and i-band observations. This difference exceeds the amplitude of the linear alignment model on scales below 2 Mpc h -1 . Separating the sample into central/satellite and red/blue galaxies, we find that the difference is dominated by red satellite galaxies

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