25 research outputs found
Photometric Redshift Calibration with Self Organising Maps
Accurate photometric redshift calibration is central to the robustness of all
cosmology constraints from cosmic shear surveys. Analyses of the KiDS
re-weighted training samples from all overlapping spectroscopic surveys to
provide a direct redshift calibration. Using self-organising maps (SOMs) we
demonstrate that this spectroscopic compilation is sufficiently complete for
KiDS, representing of the effective 2D cosmic shear sample. We use the
SOM to define a represented `gold' cosmic shear sample, per tomographic
bin. Using mock simulations of KiDS and the spectroscopic training set, we
estimate the uncertainty on the SOM redshift calibration, and find that
photometric noise, sample variance, and spectroscopic selection effects
(including redshift and magnitude incompleteness) induce a combined maximal
scatter on the bias of the redshift distribution reconstruction () of
in all tomographic bins. We show
that the SOM calibration is unbiased in the cases of noiseless photometry and
perfectly representative spectroscopic datasets, as expected from theory. The
inclusion of both photometric noise and spectroscopic selection effects in our
mock data introduces a maximal bias of , or at
confidence, once quality flags have been applied to the SOM. The method
presented here represents a significant improvement over the previously adopted
direct redshift calibration implementation for KiDS, owing to its diagnostic
and quality assurance capabilities. The implementation of this method in future
cosmic shear studies will allow better diagnosis, examination, and mitigation
of systematic biases in photometric redshift calibration.Comment: 22 pages, 10 figures, 4 appendices, accepted for publication in A&
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 ; smaller than, but otherwise consistent with,
previous work using this data set (). 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 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
; 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 estimates made with KV450
and Planck CMB probes.Comment: 10 pages, 3 figures, 4 appendices, accepted for publication in A&A
Letter
The effects of varying depth in cosmic shear surveys
We present a semi-analytic model for the shear two-point correlation function
of a cosmic shear survey with non-uniform depth. Ground-based surveys are
subject to depth variations that primarily arise through varying atmospheric
conditions. For a survey like the Kilo-Degree Survey (KiDS), we find that the
measured depth variation increases the amplitude of the observed shear
correlation function at the level of a few percent out to degree-scales,
relative to the assumed uniform-depth case. The impact on the inferred
cosmological parameters is shown to be insignificant for a KiDS-like survey.
For next-generation cosmic shear experiments, however, we conclude that
variable depth should be accounted for
Magnification bias in galaxy surveys with complex sample selection functions
Gravitational lensing magnification modifies the observed spatial
distribution of galaxies and can severely bias cosmological probes of
large-scale structure if not accurately modelled. Standard approaches to
modelling this magnification bias may not be applicable in practice as many
galaxy samples have complex, often implicit, selection functions. We propose
and test a procedure to quantify the magnification bias induced in clustering
and galaxy-galaxy lensing (GGL) signals in galaxy samples subject to a
selection function beyond a simple flux limit. The method employs realistic
mock data to calibrate an effective luminosity function slope,
, from observed galaxy counts, which can then be used with
the standard formalism. We demonstrate this method for two galaxy samples
derived from the Baryon Oscillation Spectroscopic Survey (BOSS) in the redshift
ranges and , complemented by mock data
built from the MICE2 simulation. We obtain
and for the two BOSS samples. For BOSS-like
lenses, we forecast a contribution of the magnification bias to the GGL signal
between the angular scales of and with a cumulative
signal-to-noise ratio between and for sources from the Kilo-Degree
Survey (KiDS), between and for sources from the Hyper Suprime-Cam
survey (HSC), and between and for ESA Euclid-like source samples.
These contributions are significant enough to require explicit modelling in
future analyses of these and similar surveys.Comment: 15 pages, 13 figure
Strong Lensing Model of SPT-CLJ0356-5337, a Major Merger Candidate at Redshift 1.0359
We present an analysis of the mass distribution inferred from strong lensing
by SPT-CL J0356-5337, a cluster of galaxies at redshift z = 1.0359 revealed in
the follow-up of the SPT-SZ clusters. The cluster has an Einstein radius of
Erad=14 for a source at z = 3 and a mass within 500 kpc of M_500kpc =
4.0+-0.8x10^14Msol. Our spectroscopic identification of three multiply-imaged
systems (z = 2.363, z = 2.364, and z = 3.048), combined with HST F606W-band
imaging allows us to build a strong lensing model for this cluster with an rms
of <0.3'' between the predicted and measured positions of the multiple images.
Our modeling reveals a two-component mass distribution in the cluster. One mass
component is dominated by the brightest cluster galaxy and the other component,
separated by ~170 kpc, contains a group of eight red elliptical galaxies
confined in a ~9'' (~70 kpc) diameter circle. We estimate the mass ratio
between the two components to be between 1:1.25 and 1:1.58. In addition,
spectroscopic data reveal that these two near-equal mass cores have only a
small velocity difference of 300 km/s between the two components. This small
radial velocity difference suggests that most of the relative velocity takes
place in the plane of the sky, and implies that SPT-CL J0356-5337 is a major
merger with a small impact parameter seen face-on. We also assess the relative
contributions of galaxy-scale halos to the overall mass of the core of the
cluster and find that within 800 kpc from the brightest cluster galaxy about
27% of the total mass can be attributed to visible and dark matter associated
with galaxies, whereas only 73% of the total mass in the core comes from
cluster-scale dark matter halos.Comment: 19 pages, 11 figures. Submitted to Ap
KiDS-Legacy calibration: unifying shear and redshift calibration with the SKiLLS multi-band image simulations
We present SKiLLS, a suite of multi-band image simulations for the weak
lensing analysis of the complete Kilo-Degree Survey (KiDS), dubbed KiDS-Legacy
analysis. The resulting catalogues enable joint shear and redshift calibration,
enhancing the realism and hence accuracy over previous efforts. To create a
large volume of simulated galaxies with faithful properties and to a sufficient
depth, we integrated cosmological simulations with high-quality imaging
observations. We also improved the realism of simulated images by allowing the
point spread function (PSF) to differ between CCD images, including stellar
density variations and varying noise levels between pointings. Using realistic
variable shear fields, we accounted for the impact of blended systems at
different redshifts. Although the overall correction is minor, we found a clear
redshift-bias correlation in the blending-only variable shear simulations,
indicating the non-trivial impact of this higher-order blending effect. We also
explored the impact of the PSF modelling errors and found a small yet
noticeable effect on the shear bias. Finally, we conducted a series of
sensitivity tests, including changing the input galaxy properties. We conclude
that our fiducial shape measurement algorithm, lensfit, is robust within the
requirements of lensing analyses with KiDS. As for future weak lensing surveys
with tighter requirements, we suggest further investments in understanding the
impact of blends at different redshifts, improving the PSF modelling algorithm
and developing the shape measurement method to be less sensitive to the galaxy
properties.Comment: 28 pages, 31 figures, 2 tables, minor revisions to match the final
accepted versio
KiDS-1000: cross-correlation with Planck cosmic microwave background lensing and intrinsic alignment removal with self-calibration
Galaxy shear - cosmic microwave background (CMB) lensing convergence
cross-correlations contain additional information on cosmology to
auto-correlations. While being immune to certain systematic effects, they are
affected by the galaxy intrinsic alignments (IA). This may be responsible for
the reported low lensing amplitude of the galaxy shear CMB convergence
cross-correlations, compared to the standard Planck CDM (cosmological
constant and cold dark matter) cosmology prediction. In this work, we
investigate how IA affects the Kilo-Degree Survey (KiDS) galaxy lensing shear -
Planck CMB lensing convergence cross-correlation and compare it to previous
treatments with or without IA taken into consideration. More specifically, we
compare marginalization over IA parameters and the IA self-calibration (SC)
method (with additional observables defined only from the source galaxies) and
prove that SC can efficiently break the degeneracy between the CMB lensing
amplitude and the IA amplitude . We further
investigate how different systematics affect the resulting and
, and validate our results with the MICE2 simulation. We find
that by including the SC method to constrain IA, the information loss due to
the degeneracy between CMB lensing and IA is strongly reduced. The best-fit
values are and , while different angular scale cuts can affect
by . We show that appropriate treatment of the boost
factor, cosmic magnification, and photometric redshift modeling is important
for obtaining the correct IA and cosmological results.Comment: match version accepted by A&
KiDS-1000: Cosmology with improved cosmic shear measurements
We present refined cosmological parameter constraints derived from a cosmic
shear analysis of the fourth data release of the Kilo-Degree Survey
(KiDS-1000). Our main improvements include enhanced galaxy shape measurements
made possible by an updated version of the lensfit code and improved shear
calibration achieved with a newly developed suite of multi-band image
simulations. Additionally, we incorporated recent advancements in cosmological
inference from the joint Dark Energy Survey Year 3 and KiDS-1000 cosmic shear
analysis. Assuming a spatially flat standard cosmological model, we constrain
, where the second set of uncertainties
accounts for the systematic uncertainties within the shear calibration. These
systematic uncertainties stem from minor deviations from realism in the image
simulations and the sensitivity of the shear measurement algorithm to the
morphology of the galaxy sample. Despite these changes, our results align with
previous KiDS studies and other weak lensing surveys, and we find a
level of tension with the Planck cosmic microwave background
constraints on .Comment: 20 pages, 13 figures, 4 tables, minor revisions to match the final
accepted versio
KiDS+VIKING-450 and DES-Y1 combined::Mitigating baryon feedback uncertainty with COSEBIs
We present cosmological constraints from a joint cosmic shear analysis of the
Kilo-Degree Survey (KV450) and the Dark Energy Survey (DES-Y1), conducted using
Complete Orthogonal Sets of E/B-Integrals (COSEBIs). With COSEBIs we isolate
any B-modes which have a non-cosmic shear origin and demonstrate the robustness
of our cosmological E-mode analysis as no significant B-modes are detected. We
highlight how COSEBIs are fairly insensitive to the amplitude of the non-linear
matter power spectrum at high -scales, mitigating the uncertain impact of
baryon feedback in our analysis. COSEBIs, therefore, allow us to utilise
additional small-scale information, improving the DES-Y1 joint constraints on
and by .
Adopting a flat CDM model we find , which
is in tension with the Planck Legacy analysis of the cosmic
microwave background.Comment: Accepted for publication in A&A. 15 pages, 7 figure
KiDS-1000 Cosmology:Multi-probe weak gravitational lensing and spectroscopic galaxy clustering constraints
We present a joint cosmological analysis of weak gravitational lensing
observations from the Kilo-Degree Survey (KiDS-1000), with redshift-space
galaxy clustering observations from the Baryon Oscillation Spectroscopic Survey
(BOSS), and galaxy-galaxy lensing observations from the overlap between
KiDS-1000, BOSS and the spectroscopic 2-degree Field Lensing Survey (2dFLenS).
This combination of large-scale structure probes breaks the degeneracies
between cosmological parameters for individual observables, resulting in a
constraint on the structure growth parameter , that has the same overall precision as that
reported by the full-sky cosmic microwave background observations from Planck.
The recovered amplitude is low, however, by % relative to
Planck. This result builds from a series of KiDS-1000 analyses where we
validate our methodology with variable depth mock galaxy surveys, our lensing
calibration with image simulations and null-tests, and our
optical-to-near-infrared redshift calibration with multi-band mock catalogues
and a spectroscopic-photometric clustering analysis. The systematic
uncertainties identified by these analyses are folded through as nuisance
parameters in our cosmological analysis. Inspecting the offset between the
marginalised posterior distributions, we find that the -difference with
Planck is driven by a tension in the matter fluctuation amplitude parameter,
. We quantify the level of agreement between the CMB and our
large-scale structure constraints using a series of different metrics, finding
differences with a significance ranging between , when
considering the offset in , and , when considering the
full multi-dimensional parameter space.Comment: 25 pages, 11 figures, 5 tables, A&A accepted, including a new
appendix on Intrinsic Alignments. The KiDS-1000 data products are available
for download at http://kids.strw.leidenuniv.nl/DR4/lensing.php. This data
release includes open source software, the shear-photo-z catalogue, the
cosmic shear and 3x2pt data vectors and covariances, and posteriors in the
form of Multinest chain