# Publications

by categories in reversed chronological order. Generated by NASA ADS with jekyll-scholar.

A lot of my research is collaborative in nature due to the complexity of the projects. A complete list of my publications is below and could also be accessed from any of the links below. All of my papers are available for free on arXiv, which match the published versions. They are categorized into four groups:

- White papers
- First or second author papers
- Co-authored papers with significant contribution
- Co-authored papers with minor or infrastructure contributions

### Statistics

I do not think that the impact of one’s research, based on publications, can be captured in one or a handful number of parameters. However, if you must know, my **h-index** is 21 with a total citation of 2522 (according to Google Scholar). According to ADS, my papers have been cited over 2304 times, with an **h-index** of 20.

# White papers I contributed to

- From Data to Software to Science with the Rubin Observatory LSSTKatelyn Breivik, Andrew J. Connolly, K. E. Saavik Ford, and 97 more authors
*arXiv e-prints*, Aug 2022The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) dataset will dramatically alter our understanding of the Universe, from the origins of the Solar System to the nature of dark matter and dark energy. Much of this research will depend on the existence of robust, tested, and scalable algorithms, software, and services. Identifying and developing such tools ahead of time has the potential to significantly accelerate the delivery of early science from LSST. Developing these collaboratively, and making them broadly available, can enable more inclusive and equitable collaboration on LSST science. To facilitate such opportunities, a community workshop entitled “From Data to Software to Science with the Rubin Observatory LSST” was organized by the LSST Interdisciplinary Network for Collaboration and Computing (LINCC) and partners, and held at the Flatiron Institute in New York, March 28-30th 2022. The workshop included over 50 in-person attendees invited from over 300 applications. It identified seven key software areas of need: (i) scalable cross-matching and distributed joining of catalogs, (ii) robust photometric redshift determination, (iii) software for determination of selection functions, (iv) frameworks for scalable time-series analyses, (v) services for image access and reprocessing at scale, (vi) object image access (cutouts) and analysis at scale, and (vii) scalable job execution systems. This white paper summarizes the discussions of this workshop. It considers the motivating science use cases, identified cross-cutting algorithms, software, and services, their high-level technical specifications, and the principles of inclusive collaborations needed to develop them. We provide it as a useful roadmap of needs, as well as to spur action and collaboration between groups and individuals looking to develop reusable software for early LSST science.

# First or second author papers

- MNRASMitigating the effects of undersampling in weak lensing shear estimation with metacalibration
*Arun Kannawadi*, Erik Rosenberg, and Henk Hoekstra*Monthly Notices of the Royal Astronomical Society*, Apr 2021METACALIBRATION 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.

- A&AAccounting for object detection bias in weak gravitational lensing studiesHenk Hoekstra,
*Arun Kannawadi*, and Thomas D. Kitching*Astronomy and Astrophysics*, Feb 2021Weak 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, the 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.

- A&ATowards emulating cosmic shear data: revisiting the calibration of the shear measurements for the Kilo-Degree Survey
*Arun Kannawadi*, Henk Hoekstra, Lance Miller, and 9 more authors*Astronomy and Astrophysics*, Apr 2019Exploiting 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 to ensure 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 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 two 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 also be essential to correctly account for the measurement of photometric redshifts, which requires simulating multi-band observations.

- The Impact of Interpixel Capacitance in CMOS Detectors on PSF Shapes and Implications for WFIRST
*Arun Kannawadi*, Charles A. Shapiro, Rachel Mandelbaum, and 3 more authors*Publications of the ASP*, Sep 2016Unlike optical CCDs, near-infrared detectors, which are based on CMOS hybrid readout technology, typically suffer from electrical crosstalk between the pixels. The interpixel capacitance (IPC) responsible for the crosstalk affects the point-spread function (PSF) of the telescope, increasing the size and modifying the shape of all objects in the images while correlating the Poisson noise. Upcoming weak lensing surveys that use these detectors, such as WFIRST, place stringent requirements on the PSF size and shape (and the level at which these are known), which in turn must be translated into requirements on IPC. To facilitate this process, we present a first study of the effect of IPC on WFIRST PSF sizes and shapes. Realistic PSFs are forward-simulated from physical principles for each WFIRST bandpass. We explore how the PSF size and shape depends on the range of IPC coupling with pixels that are connected along an edge or corner; for the expected level of IPC in WFIRST, IPC increases the PSF sizes by \ensuremath∼5%. We present a linear fitting formula that describes the uncertainty in the PSF size or shape due to uncertainty in the IPC, which could arise for example due to unknown time evolution of IPC as the detectors age or due to spatial variation of IPC across the detector. We also study of the effect of a small anisotropy in the IPC, which further modifies the PSF shapes. Our results are a first, critical step in determining the hardware and characterization requirements for the detectors used in the WFIRST survey.

- Persistent entanglement in a class of eigenstates of quantum Heisenberg spin glasses
*Arun Kannawadi*, Auditya Sharma, and Arul Lakshminarayan*EPL (Europhysics Letters)*, Sep 2016The 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 random 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.

- MNRASThe impact of cosmic variance on simulating weak lensing surveys
*Arun Kannawadi*, Rachel Mandelbaum, and Claire Lackner*Monthly Notices of the Royal Astronomical Society*, Jun 2015Upcoming 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 data sets 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. Sérsic 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 \ensuremath∆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 data set and sufficiently large redshift binning.

# Co-authored papers with significant contribution

## 2022

- MNRASImpact of point spread function higher moments error on weak gravitational lensing II: A comprehensive studyTianqing Zhang, Husni Almoubayyed, Rachel Mandelbaum, and 6 more authors
*Monthly Notices of the Royal Astronomical Society*, Nov 2022Weak 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 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, this work comprehensively investigate the modeling quality of PSF moments from the 3^rd to 6^th order, and propagate the PSFEX higher moments modeling error in the HSC survey dataset to the weak lensing shear-shear correlation functions and their cosmological analyses. The overall multiplicative shear bias associated with errors in PSF higher moments can cause a \raisebox-0.5ex\textasciitilde0.1\ensuremathσ shift on the cosmological parameters for LSST Y10, while the associated additive biases can induce 1\ensuremathσ uncertainties in cosmology parameter inference for LSST Y10, if not accounted. We compare the PSFEX model with PSF in Full FOV (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, otherwise needed to be explicitly modeled in the weak lensing analysis.

- PSFs of coadded imagesRachel Mandelbaum, Mike Jarvis, Robert H. Lupton, and 5 more authors
*arXiv e-prints*, Sep 2022We 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.

- Faro: a framework for measuring the scientific performance of petascale Rubin Observatory data productsLeanne P. Guy, Keith Bechtol, Jeffrey L. Carlin, and 14 more authors
*In Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series*, Aug 2022The Vera C. Rubin Observatory will advance many areas of astronomy over the next decade with its unique widefast- deep multi-color imaging survey, the Legacy Survey of Space and Time (LSST).1 The LSST will produce approximately 20TB of raw data per night, which will be automatically processed by the LSST Science Pipelines to generate science-ready data products - processed images, catalogs and alerts. To ensure that these data products enable transformative science with LSST, stringent requirements have been placed on their quality and scientific fidelity, for example on image quality and depth, astrometric and photometric performance, and object recovery completeness. In this paper we introduce faro, a framework for automatically and efficiently computing scientific performance metrics on the LSST data products for units of data of varying granularity, ranging from single-detector to full-survey summary statistics. By measuring and monitoring metrics, we are able to evaluate trends in algorithmic performance and conduct regression testing during development, compare the performance of one algorithm against another, and verify that the LSST data products will meet performance requirements by comparing to specifications. We present initial results using faro to characterize the performance of the data products produced on simulated and precursor data sets, and discuss plans to use faro to verify the performance of the LSST commissioning data products.

## 2021

- A&AKiDS-1000: Constraints on the intrinsic alignment of luminous red galaxiesMaria Cristina Fortuna, Henk Hoekstra, Harry Johnston, and 11 more authors
*Astronomy and Astrophysics*, Oct 2021We constrain the luminosity and redshift dependence of the intrinsic alignment (IA) of a nearly volume-limited sample of luminous red galaxies selected from the fourth public data release of the Kilo-Degree Survey (KiDS-1000). To measure the shapes of the galaxies, we used two complementary algorithms, finding consistent IA measurements for the overlapping galaxy sample. The global significance of IA detection across our two independent luminous red galaxy samples, with our favoured method of shape estimation, is \ensuremath∼10.7\ensuremathσ. We find no significant dependence with redshift of the IA signal in the range 0.2 < z < 0.8, nor a dependence with luminosity below L_r \ensuremath≲ 2.9 \texttimes 10^10 h^\ensuremath-2L_r, \ensuremath⊙. Above this luminosity, however, we find that the IA signal increases as a power law, although our results are also compatible with linear growth within the current uncertainties. This behaviour motivates the use of a broken power law model when accounting for the luminosity dependence of IA contamination in cosmic shear studies.

- A&AStrong detection of the CMB lensing and galaxy weak lensing cross-correlation from ACT-DR4, Planck Legacy, and KiDS-1000Naomi Clare Robertson, David Alonso, Joachim Harnois-Déraps, and 48 more authors
*Astronomy and Astrophysics*, May 2021We measured the cross-correlation between galaxy weak lensing data from the Kilo Degree Survey (KiDS-1000, DR4) and cosmic microwave background (CMB) lensing data from the Atacama Cosmology Telescope (ACT, DR4) and the Planck Legacy survey. We used two samples of source galaxies, selected with photometric redshifts, (0.1 < z_B < 1.2) and (1.2 < z_B < 2), which produce a combined detection significance of the CMB lensing and weak galaxy lensing cross-spectrum of 7.7\ensuremathσ. With the lower redshift galaxy sample, for which the cross- correlation was detected at a significance of 5.3\ensuremathσ, we present joint cosmological constraints on the matter density parameter, \ensuremathΩ_m, and the matter fluctuation amplitude parameter, \ensuremathσ_8, marginalising over three nuisance parameters that model our uncertainty in the redshift and shear calibration as well as the intrinsic alignment of galaxies. We find our measurement to be consistent with the best-fitting flat \ensuremathΛCDM cosmological models from both Planck and KiDS-1000. We demonstrate the capacity of CMB weak lensing cross-correlations to set constraints on either the redshift or shear calibration by analysing a previously unused high-redshift KiDS galaxy sample (1.2 < z_B < 2), with the cross-correlation detected at a significance of 7\ensuremathσ. This analysis provides an independent assessment for the accuracy of redshift measurements in a regime that is challenging to calibrate directly owing to known incompleteness in spectroscopic surveys.

- A&AKiDS+VIKING-450: An internal-consistency test for cosmic shear tomography with a colour-based split of source galaxiesShun-Sheng Li, Konrad Kuijken, Henk Hoekstra, and 3 more authors
*Astronomy and Astrophysics*, Feb 2021We 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.

- A&AKiDS-1000 catalogue: Weak gravitational lensing shear measurementsBenjamin Giblin, Catherine Heymans, Marika Asgari, and 20 more authors
*Astronomy and Astrophysics*, Jan 2021We present weak lensing shear catalogues from the fourth data release of the Kilo-Degree Survey, KiDS-1000, spanning 1006 square degrees of deep and high-resolution imaging. Our ‘gold-sample’ of galaxies, with well-calibrated photometric redshift distributions, consists of 21 million galaxies with an effective number density of 6.17 galaxies per square arcminute. We quantify the accuracy of the spatial, temporal, and flux- dependent point-spread function (PSF) model, verifying that the model meets our requirements to induce less than a 0.1\ensuremathσ change in the inferred cosmic shear constraints on the clustering cosmological parameter S_8 = \ensuremathσ_8 \ensuremath\sqrt\ensuremathΩ_m/0.3.. Through a series of two-point null-tests, we validate the shear estimates, finding no evidence for significant non-lensing B-mode distortions in the data. The PSF residuals are detected in the highest-redshift bins, originating from object selection and/or weight bias. The amplitude is, however, shown to be sufficiently low and within our stringent requirements. With a shear-ratio null-test, we verify the expected redshift scaling of the galaxy-galaxy lensing signal around luminous red galaxies. We conclude that the joint KiDS-1000 shear and photometric redshift calibration is sufficiently robust for combined-probe gravitational lensing and spectroscopic clustering analyses.

- A&AKiDS-1000 cosmology: Cosmic shear constraints and comparison between two point statisticsMarika Asgari, Chieh-An Lin, Benjamin Joachimi, and 22 more authors
*Astronomy and Astrophysics*, Jan 2021We present cosmological constraints from a cosmic shear analysis of the fourth data release of the Kilo-Degree Survey (KiDS-1000), which doubles the survey area with nine-band optical and near-infrared photometry with respect to previous KiDS analyses. Adopting a spatially flat standard cosmological model, we find S_8 = \ ensuremathσ_8(\ensuremathΩ_m/0.3)^0.5 = 0.759_-0.021^+0.024 for our fiducial analysis, which is in 3\ensuremathσ tension with the prediction of the Planck Legacy analysis of the cosmic microwave background. We compare our fiducial COSEBIs (Complete Orthogonal Sets of E/B-Integrals) analysis with complementary analyses of the two-point shear correlation function and band power spectra, finding the results to be in excellent agreement. We investigate the sensitivity of all three statistics to a number of measurement, astrophysical, and modelling systematics, finding our S_8 constraints to be robust and dominated by statistical errors. Our cosmological analysis of different divisions of the data passes the Bayesian internal consistency tests, with the exception of the second tomographic bin. As this bin encompasses low-redshift galaxies, carrying insignificant levels of cosmological information, we find that our results are unchanged by the inclusion or exclusion of this sample.

## 2020

- A&AKiDS+VIKING-450: Improved cosmological parameter constraints from redshift calibration with self-organising mapsAngus H. Wright, Hendrik Hildebrandt, Jan Luca van den Busch, and 4 more authors
*Astronomy and Astrophysics*, Aug 2020We present updated cosmological constraints for the KiDS+VIKING-450 cosmic shear data set (KV450) estimated through redshift distributions and photometric samples defined using self- organising maps (SOMs). Our fiducial analysis finds marginal posterior constraints of S_8 \ensuremath≡ \ensuremathσ_8 \ensuremathΩ_m/0.3 = 0.716_-0.038^+0.043, which are smaller but otherwise consistent with previous works that have applied this data set (|\ensuremath∆S_8| = 0.023). We analysed additional samples and redshift distributions set up in three ways: (1) by excluding certain spectroscopic surveys during redshift calibration; (2) by excluding lower-confidence spectroscopic redshifts in redshift calibration; and (3) by considering only those photometric sources which are jointly calibrated by at least three spectroscopic surveys. In all cases, the method utilised here has been proven to be robust: we find a maximal deviation from our fiducial analysis of |\ensuremath∆S_8| \ensuremath≤ 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; this is a difference, with respect to the fiducial, that is both significantly smaller and in the opposite direction with regard to the equivalent shift from previous works. These results suggest that our improved cosmological parameter estimates are not sensitive to pathological misrepresentations 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.

# Co-authored papers with minor or infrastructure contributions

## 2023

- MNRASConsistent lensing and clustering in a low-S_8 Universe with BOSS, DES Year 3, HSC Year 1, and KiDS-1000A. Amon, N. C. Robertson, H. Miyatake, and 105 more authors
*Monthly Notices of the Royal Astronomical Society*, Jan 2023We evaluate the consistency between lensing and clustering based on measurements from Baryon Oscillation Spectroscopic Survey combined with galaxy-galaxy lensing from Dark Energy Survey (DES) Year 3, Hyper Suprime-Cam Subaru Strategic Program (HSC) Year 1, and Kilo-Degree Survey (KiDS)-1000. We find good agreement between these lensing data sets. We model the observations using the DARK EMULATOR and fit the data at two fixed cosmologies: Planck (S_8 = 0.83), and a Lensing cosmology (S_8 = 0.76). For a joint analysis limited to large scales, we find that both cosmologies provide an acceptable fit to the data. Full utilization of the higher signal-to-noise small-scale measurements is hindered by uncertainty in the impact of baryon feedback and assembly bias, which we account for with a reasoned theoretical error budget. We incorporate a systematic inconsistency parameter for each redshift bin, A, that decouples the lensing and clustering. With a wide range of scales, we find different results for the consistency between the two cosmologies. Limiting the analysis to the bins for which the impact of the lens sample selection is expected to be minimal, for the Lensing cosmology, the measurements are consistent with A = 1; A = 0.91 \ensuremath\pm 0.04 (A = 0.97 \ensuremath\pm 0.06) using DES+KiDS (HSC). For the Planck case, we find a discrepancy: A = 0.79 \ensuremath\pm 0.03 (A = 0.84 \ensuremath\pm 0.05) using DES+KiDS (HSC). We demonstrate that a kinematic Sunyaev- Zeldovich-based estimate for baryonic effects alleviates some of the discrepancy in the Planck cosmology. This analysis demonstrates the statistical power of small-scale measurements; however, caution is still warranted given modelling uncertainties and foreground sample selection effects.

## 2022

- A General Framework for Removing Point Spread Function Additive Systematics in Cosmological Weak Lensing AnalysisTianqing Zhang, Xiangchong Li, Roohi Dalal, and 9 more authors
*arXiv e-prints*, Dec 2022Cosmological weak lensing measurements rely on a precise measurement of the shear two-point correlation function (2PCF) along with a deep understanding of systematics that affect it. In this work, we demonstrate a general framework for describing the impact of PSF systematics on the cosmic shear 2PCF, and mitigating its impact on cosmological analysis. Our framework can describe leakage and modeling error from all spin-2 quantities contributed by the PSF second and higher moments, rather than just the second moments. We interpret null tests using the HSC Year 3 (Y3) catalogs with this formalism, and find that leakage from the spin-2 combination of PSF fourth moments is the leading contributor to additive shear systematics, with total contamination that is an order of magnitude higher than that contributed by PSF second moments alone. We conducted a mock cosmic shear analysis for HSC Y3, and find that, if uncorrected, PSF systematics can bias the cosmological parameters \Omega_m and S_8 by \sim0.3σ. The traditional second moment- based model can only correct for a 0.1σbias, leaving the contamination largely uncorrected. We conclude it is necessary to model both PSF second and fourth moment contamination for HSC Y3 cosmic shear analysis. We also reanalyze the HSC Y1 cosmic shear analysis with our updated systematics model, and identify a 0.07σbias on \Omega_m when using the more restricted second moment model from the original analysis. We demonstrate how to self-consistently use the method in both real space and Fourier space, assess shear systematics in tomographic bins, and test for PSF model overfitting.

- A&AKiDS-1000: Cosmic shear with enhanced redshift calibrationJ. L. van den Busch, A. H. Wright, H. Hildebrandt, and 8 more authors
*Astronomy and Astrophysics*, Aug 2022We present a cosmic shear analysis with an improved redshift calibration for the fourth data release of the Kilo-Degree Survey (KiDS-1000) using self-organising maps (SOMs). Compared to the previous analysis of the KiDS-1000 data, we expand the redshift calibration sample to more than twice its size, now consisting of data of 17 spectroscopic redshift campaigns, and significantly extending the fraction of KiDS galaxies we are able to calibrate with our SOM redshift methodology. We then enhanced the calibration sample with precision photometric redshifts from COSMOS2015 and the Physics of the Accelerated Universe Survey (PAUS), allowing us to fill gaps in the spectroscopic coverage of the KiDS data. Finally we performed a Complete Orthogonal Sets of E/B-Integrals (COSEBIs) cosmic shear analysis of the newly calibrated KiDS sample. We found S_8 = 0.748_\ensuremath-0.025^+0.021, which is in good agreement with previous KiDS studies and increases the tension with measurements of the cosmic microwave background to 3.4\ensuremathσ. We repeated the redshift calibration with different subsets of the full calibration sample and obtained, in all cases, agreement within at most 0.5\ensuremathσ in S_8 compared to our fiducial analysis. Including additional photometric redshifts allowed us to calibrate an additional 6% of the source galaxy sample. Even though further systematic testing with simulated data is necessary to quantify the impact of redshift outliers, precision photometric redshifts can be beneficial at high redshifts and to mitigate selection effects commonly found in spectroscopically selected calibration samples.

- PASJThe three-year shear catalog of the Subaru Hyper Suprime-Cam SSP SurveyXiangchong Li, Hironao Miyatake, Wentao Luo, and 15 more authors
*Publications of the ASJ*, Apr 2022We present the galaxy shear catalog that will be used for the three-year cosmological weak gravitational lensing analyses using data from the Wide layer of the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) Survey. The galaxy shapes are measured from the i-band imaging data acquired from 2014 to 2019 and calibrated with image simulations that resemble the observing conditions of the survey based on training galaxy images from the Hubble Space Telescope in the COSMOS region. The catalog covers an area of 433.48 deg^2 of the northern sky, split into six fields. The mean i-band seeing is 0^\prime \prime_.59. With conservative galaxy selection criteria (e.g., i-band magnitude brighter than 24.5), the observed raw galaxy number density is 22.9 arcmin^-2, and the effective galaxy number density is 19.9 arcmin^-2. The calibration removes the galaxy property- dependent shear estimation bias to the level |\ensuremathδm| < 9 \texttimes 10^-3. The bias residual \ensuremathδm shows no dependence on redshift in the range 0 < z \ensuremath≤ 3. We define the requirements for cosmological weak-lensing science for this shear catalog, and quantify potential systematics in the catalog using a series of internal null tests for systematics related to point-spread function modelling and shear estimation. A variety of the null tests are statistically consistent with zero or within requirements, but (i) there is evidence for PSF model shape residual correlations; and (ii) star-galaxy shape correlations reveal additive systematics. Both effects become significant on >1\textdegree scales and will require mitigation during the inference of cosmological parameters using cosmic shear measurements.

- A&AJoint constraints on cosmology and the impact of baryon feedback: Combining KiDS-1000 lensing with the thermal Sunyaev-Zeldovich effect from Planck and ACTTilman Tröster, Alexander J. Mead, Catherine Heymans, and 13 more authors
*Astronomy and Astrophysics*, Apr 2022We conduct a pseudo-C_\ensuremath\ell analysis of the tomographic cross-correlation between 1000 deg^2 of weak- lensing data from the Kilo-Degree Survey (KiDS-1000) and the thermal Sunyaev-Zeldovich (tSZ) effect measured by Planck and the Atacama Cosmology Telescope (ACT). Using HMX, a halo-model- based approach that consistently models the gas, star, and dark matter components, we are able to derive constraints on both cosmology and baryon feedback for the first time from these data, marginalising over redshift uncertainties, intrinsic alignment of galaxies, and contamination by the cosmic infrared background (CIB). We find our results to be insensitive to the CIB, while intrinsic alignment provides a small but significant contribution to the lensing-tSZ cross-correlation. The cosmological constraints are consistent with those of other low- redshift probes and prefer strong baryon feedback. The inferred amplitude of the lensing-tSZ cross-correlation signal, which scales as \ensuremathσ_8(\ensuremathΩ_m /0.3)^0.2, is low by \ensuremath∼2 \ensuremathσ compared to the primary cosmic microwave background constraints by Planck. The lensing-tSZ measurements are then combined with pseudo-C_\ensuremath\ell measurements of KiDS-1000 cosmic shear into a novel joint analysis, accounting for the full cross-covariance between the probes, providing tight cosmological constraints by breaking parameter degeneracies inherent to both probes. The joint analysis gives an improvement of 40% on the constraint of S_8 = \ensuremathσ_8\ensuremathΩ_m/0.3 over cosmic shear alone, while providing constraints on baryon feedback consistent with hydrodynamical simulations, demonstrating the potential of such joint analyses with baryonic tracers such as the tSZ effect. We discuss remaining modelling challenges that need to be addressed if these baryonic probes are to be included in future precision-cosmology analyses.

- MNRASLensing without borders - I. A blind comparison of the amplitude of galaxy-galaxy lensing between independent imaging surveysA. Leauthaud, A. Amon, S. Singh, and 104 more authors
*Monthly Notices of the Royal Astronomical Society*, Mar 2022Lensing without borders is a cross-survey collaboration created to assess the consistency of galaxy-galaxy lensing signals (\ensuremath∆\ensuremathΣ) across different data sets and to carry out end-to-end tests of systematic errors. We perform a blind comparison of the amplitude of \ensuremath∆\ensuremathΣ using lens samples from BOSS and six independent lensing surveys. We find good agreement between empirically estimated and reported systematic errors which agree to better than 2.3\ensuremathσ in four lens bins and three radial ranges. For lenses with z_L > 0.43 and considering statistical errors, we detect a 3-4\ensuremathσ correlation between lensing amplitude and survey depth. This correlation could arise from the increasing impact at higher redshift of unrecognized galaxy blends on shear calibration and imperfections in photometric redshift calibration. At z_L > 0.54, amplitudes may additionally correlate with foreground stellar density. The amplitude of these trends is within survey-defined systematic error budgets that are designed to include known shear and redshift calibration uncertainty. Using a fully empirical and conservative method, we do not find evidence for large unknown systematics. Systematic errors greater than 15 per cent (25 per cent) ruled out in three lens bins at 68 per cent (95 per cent) confidence at z < 0.54. Differences with respect to predictions based on clustering are observed to be at the 20-30 per cent level. Our results therefore suggest that lensing systematics alone are unlikely to fully explain the ’lensing is low’ effect at z < 0.54. This analysis demonstrates the power of cross- survey comparisons and provides a promising path for identifying and reducing systematics in future lensing analyses.

## 2021

- A&AGeometry versus growth. Internal consistency of the flat \ensuremathΛCDM model with KiDS-1000Jaime Ruiz-Zapatero, Benjamin Stölzner, Benjamin Joachimi, and 11 more authors
*Astronomy and Astrophysics*, Nov 2021We carry out a multi-probe self-consistency test of the flat Lambda Cold Dark Matter (\ensuremathΛCDM) model with the aim of exploring potential causes of the reported tensions between high- and low-redshift cosmological observations. We divide the model into two theory regimes determined by the smooth background (geometry) and the evolution of matter density fluctuations (growth), each governed by an independent set of \ensuremathΛCDM cosmological parameters. This extended model is constrained by a combination of weak gravitational lensing measurements from the Kilo-Degree Survey, galaxy clustering signatures extracted from Sloan Digital Sky Survey campaigns and the Six-Degree Field Galaxy Survey, and the angular baryon acoustic scale and the primordial scalar fluctuation power spectrum measured in Planck cosmic microwave background (CMB) data. For both the weak lensing data set individually and the combined probes, we find strong consistency between the geometry and growth parameters, as well as with the posterior of standard \ensuremathΛCDM analysis. In the non-split analysis, for which one single set of parameters was used, tension in the amplitude of matter density fluctuations as measured by the parameter S_8 persists at around 3\ensuremathσ, with a 1.5% constraint of S_8 = 0.776_\ensuremath-0.008^+0.016 for the combined probes. We also observe a less significant preference (at least 2\ensuremathσ) for higher values of the Hubble constant, H_0 = 70.5_\ensuremath-1.5^+0.7 km s^\ensuremath-1 Mpc^\ensuremath-1, as well as for lower values of the total matter density parameter \ensuremathΩ_m = 0.289_\ensuremath-0.005^+0.007 compared to the full Planck analysis. Including the subset of the CMB information in the probe combination enhances these differences rather than alleviate them, which we link to the discrepancy between low and high multipoles in Planck data. Our geometry versus growth analysis does not yet yield clear signs regarding whether the origin of the discrepancies lies in \ensuremathΛCDM structure growth or expansion history but holds promise as an insightful test for forthcoming, more powerful data.

- A&ABright galaxy sample in the Kilo-Degree Survey Data Release 4. Selection, photometric redshifts, and physical propertiesM. Bilicki, A. Dvornik, H. Hoekstra, and 16 more authors
*Astronomy and Astrophysics*, Sep 2021We present a bright galaxy sample with accurate and precise photometric redshifts (photo-zs), selected using ugriZYJHK_s photometry from the Kilo-Degree Survey (KiDS) Data Release 4. The highly pure and complete dataset is flux-limited at r < 20 mag, covers \ensuremath∼1000 deg^2, and contains about 1 million galaxies after artifact masking. We exploit the overlap with Galaxy And Mass Assembly spectroscopy as calibration to determine photo-zs with the supervised machine learning neural network algorithm implemented in the ANNz2 software. The photo- zs have a mean error of |⟨\ensuremathδz⟩|\ensuremath∼5 \texttimes 10^\ensuremath-4 and low scatter (scaled mean absolute deviation of \ensuremath∼0.018(1 + z)); they are both practically independent of the r-band magnitude and photo-z at 0.05 < z_phot < 0.5. Combined with the 9-band photometry, these allow us to estimate robust absolute magnitudes and stellar masses for the full sample. As a demonstration of the usefulness of these data, we split the dataset into red and blue galaxies, used them as lenses, and measured the weak gravitational lensing signal around them for five stellar mass bins. We fit a halo model to these high-precision measurements to constrain the stellar-mass-halo-mass relations for blue and red galaxies. We find that for high stellar mass (M_\ensuremath⋆ > 5 \texttimes 10^11 M_\ensuremath⊙), the red galaxies occupy dark matter halos that are much more massive than those occupied by blue galaxies with the same stellar mass. \\textbackslashData available from <A href=“http:/ /kids.strw.leidenuniv.nl/DR4/brightsample.php”>http://kids.strw .leidenuniv.nl/DR4/brightsample.php</A>

- A&AProbing galaxy bias and intergalactic gas pressure with KiDS Galaxies-tSZ-CMB lensing cross-correlationsZiang Yan, Ludovic van Waerbeke, Tilman Tröster, and 14 more authors
*Astronomy and Astrophysics*, Jul 2021We constrain the redshift dependence of gas pressure bias ⟨b_yP_e⟩ (bias-weighted average electron pressure), which characterises the thermodynamics of intergalactic gas, through a combination of cross-correlations between galaxy positions and the thermal Sunyaev-Zeldovich (tSZ) effect, as well as galaxy positions and the gravitational lensing of the cosmic microwave background (CMB). The galaxy sample is from the fourth data release of the Kilo-Degree Survey (KiDS). The tSZ y map and the CMB lensing map are from the Planck 2015 and 2018 data releases, respectively. The measurements are performed in five redshift bins with z \ensuremath≲ 1. With these measurements, combining galaxy-tSZ and galaxy-CMB lensing cross- correlations allows us to break the degeneracy between galaxy bias and gas pressure bias, and hence constrain them simultaneously. In all redshift bins, the best-fit values of ⟨b_yP_e⟩ are at a level of \ensuremath∼0.3 meV cm^\ensuremath-3 and increase slightly with redshift. The galaxy bias is consistent with unity in all the redshift bins. Our results are not sensitive to the non-linear details of the cross-correlation, which are smoothed out by the Planck beam. Our measurements are in agreement with previous measurements as well as with theoretical predictions. We also show that our conclusions are not changed when CMB lensing is replaced by galaxy lensing, which shows the consistency of the two lensing signals despite their radically different redshift ranges. This study demonstrates the feasibility of using CMB lensing to calibrate the galaxy distribution such that the galaxy distribution can be used as a mass proxy without relying on the precise knowledge of the matter distribution.

- A&AThe weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000Margot M. Brouwer, Kyle A. Oman, Edwin A. Valentijn, and 22 more authors
*Astronomy and Astrophysics*, Jun 2021We present measurements of the radial gravitational acceleration around isolated galaxies, comparing the expected gravitational acceleration given the baryonic matter (g_bar) with the observed gravitational acceleration (g_obs), using weak lensing measurements from the fourth data release of the Kilo- Degree Survey (KiDS-1000). These measurements extend the radial acceleration relation (RAR), traditionally measured using galaxy rotation curves, by 2 decades in g_obs into the low- acceleration regime beyond the outskirts of the observable galaxy. We compare our RAR measurements to the predictions of two modified gravity (MG) theories: modified Newtonian dynamics and Verlinde’s emergent gravity (EG). We find that the measured relation between g_obs and g_bar agrees well with the MG predictions. In addition, we find a difference of at least 6\ensuremathσ between the RARs of early- and late-type galaxies (split by Sérsic index and u \ensuremath- r colour) with the same stellar mass. Current MG theories involve a gravity modification that is independent of other galaxy properties, which would be unable to explain this behaviour, although the EG theory is still limited to spherically symmetric static mass models. The difference might be explained if only the early-type galaxies have significant (M_gas \ensuremath≈ M_\ensuremath⋆) circumgalactic gaseous haloes. The observed behaviour is also expected in \ensuremathΛ-cold dark matter (\ensuremathΛCDM) models where the galaxy-to-halo mass relation depends on the galaxy formation history. We find that MICE, a \ensuremathΛCDM simulation with hybrid halo occupation distribution modelling and abundance matching, reproduces the observed RAR but significantly differs from BAHAMAS, a hydrodynamical cosmological galaxy formation simulation. Our results are sensitive to the amount of circumgalactic gas; current observational constraints indicate that the resulting corrections are likely moderate. Measurements of the lensing RAR with future cosmological surveys (such as Euclid) will be able to further distinguish between MG and \ensuremathΛCDM models if systematic uncertainties in the baryonic mass distribution around galaxies are reduced.

- A&AKiDS-1000 Cosmology: Constraints beyond flat \ensuremathΛCDMTilman Tröster, Marika Asgari, Chris Blake, and 26 more authors
*Astronomy and Astrophysics*, May 2021We present constraints on extensions to the standard cosmological model of a spatially flat Universe governed by general relativity, a cosmological constant (\ensuremathΛ), and cold dark matter (CDM) by varying the spatial curvature \ensuremathΩ_K, the sum of the neutrino masses \ensuremath∑m_\ensuremathν, the dark energy equation of state parameter w, and the Hu-Sawicki f(R) gravity f_R0 parameter. With the combined 3 \texttimes 2 pt measurements of cosmic shear from the Kilo-Degree Survey (KiDS-1000), galaxy clustering from the Baryon Oscillation Spectroscopic Survey (BOSS), and galaxy-galaxy lensing from the overlap between KiDS-1000, BOSS, and the spectroscopic 2-degree Field Lensing Survey, we find results that are fully consistent with a flat \ensuremathΛCDM model with \ensuremathΩ_K = 0.011_\ensuremath-0.057^+0.054, \ensuremath∑m_\ensuremathν < 1.76 eV (95% CL), and w = \ensuremath-0.99_\ensuremath-0.13^+0.11. The f_R0 parameter is unconstrained in our fully non-linear f(R) cosmic shear analysis. Considering three different model selection criteria, we find no clear preference for either the fiducial flat \ensuremathΛCDM model or any of the considered extensions. In addition to extensions to the flat \ensuremathΛCDM parameter space, we also explore restrictions to common subsets of the flat \ensuremathΛCDM parameter space by fixing the amplitude of the primordial power spectrum to the Planck best- fit value, as well as adding external data from supernovae and lensing of the cosmic microwave background (CMB). Neither the beyond-\ensuremathΛCDM models nor the imposed restrictions explored in this analysis are able to resolve the \ensuremath∼3\ensuremathσ tension in S_8 between the 3 \texttimes 2 pt constraints and the Planck temperature and polarisation data, with the exception of wCDM, where the S_8 tension is resolved. The tension in the wCDM case persists, however, when considering the joint S_8 \ensuremath- w parameter space. The joint flat \ensuremathΛCDM CMB lensing and 3 \texttimes 2 pt analysis is found to yield tight constraints on \ensuremathΩ_m = 0.307_\ensuremath-0.013^+0.008, \ensuremathσ_8 = 0.769_\ensuremath-0.010^+0.022, and S_8 = 0.779_\ensuremath-0.013^+0.013.

- A&APhotometric selection and redshifts for quasars in the Kilo-Degree Survey Data Release 4S. J. Nakoneczny, M. Bilicki, A. Pollo, and 10 more authors
*Astronomy and Astrophysics*, May 2021We present a catalog of quasars with their corresponding redshifts derived from the photometric Kilo-Degree Survey (KiDS) Data Release 4. We achieved it by training machine learning (ML) models, using optical ugri and near-infrared ZYJHK_s bands, on objects known from Sloan Digital Sky Survey (SDSS) spectroscopy. We define inference subsets from the 45 million objects of the KiDS photometric data limited to 9-band detections, based on a feature space built from magnitudes and their combinations. We show that projections of the high- dimensional feature space on two dimensions can be successfully used, instead of the standard color-color plots, to investigate the photometric estimations, compare them with spectroscopic data, and efficiently support the process of building a catalog. The model selection and fine-tuning employs two subsets of objects: those randomly selected and the faintest ones, which allowed us to properly fit the bias versus variance trade-off. We tested three ML models: random forest (RF), XGBoost (XGB), and artificial neural network (ANN). We find that XGB is the most robust and straightforward model for classification, while ANN performs the best for combined classification and redshift. The ANN inference results are tested using number counts, Gaia parallaxes, and other quasar catalogs that are external to the training set. Based on these tests, we derived the minimum classification probability for quasar candidates which provides the best purity versus completeness trade-off: p(QSO_cand) > 0.9 for r < 22 and p(QSO_cand) > 0.98 for 22 < r < 23.5. We find 158 000 quasar candidates in the safe inference subset (r < 22) and an additional 185 000 candidates in the reliable extrapolation regime (22 < r < 23.5). Test-data purity equals 97% and completeness is 94%; the latter drops by 3% in the extrapolation to data fainter by one magnitude than the training set. The photometric redshifts were derived with ANN and modeled with Gaussian uncertainties. The test-data redshift error (mean and scatter) equals 0.009 \ensuremath\pm 0.12 in the safe subset and \ensuremath-0.0004 \ensuremath\pm 0.19 in the extrapolation, averaged over a redshift range of 0.14 < z < 3.63 (first and 99th percentiles). Our success of the extrapolation challenges the way that models are optimized and applied at the faint data end. The resulting catalog is ready for cosmology and active galactic nucleus (AGN) studies. \\textbackslashA copy of the catalog is only available at the CDS via anonymous ftp to <A href=“http://cdsarc.u-strasbg.fr/”>cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href=“http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/649/A81”>http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/649/A81</A> \\textbackslashWe publicly release the catalog at <A href=“https://kids.strw.leid enuniv.nl/DR4/quasarcatalog.php”>https://kids.strw.leidenuniv.n l/DR4/quasarcatalog.php</A> and the code at <A href=“https://github.com/snakoneczny/kids- quasars”>github.com/snakoneczny/kids-quasars</A>

- A&AHalo shapes constrained from a pure sample of central galaxies in KiDS-1000Christos Georgiou, Henk Hoekstra, Konrad Kuijken, and 12 more authors
*Astronomy and Astrophysics*, Mar 2021We present measurements of f_h, the ratio of the aligned components of the projected halo and galaxy ellipticities, for a sample of central galaxies using weak gravitational lensing data from the Kilo-Degree Survey (KiDS). Using a lens galaxy shape estimation that is more sensitive to outer galaxy regions, we find f_h = 0.50 \ensuremath\pm 0.20 for our full sample and f_h = 0.55 \ensuremath\pm 0.19 for an intrinsically red sub-sample (that therefore has a higher stellar mass), rejecting the hypothesis that round halos and/or galaxies are not aligned with their parent halo at 2.5\ensuremathσ and 2.9\ensuremathσ, respectively. We quantify the 93.4% purity of our central galaxy sample using numerical simulations and overlapping spectroscopy from the Galaxy and Mass Assembly survey. This purity ensures that the interpretation of our measurements is not complicated by the presence of a significant fraction of satellite galaxies. Restricting our central galaxy ellipticity measurement to the inner isophotes, we find f_h = 0.34 \ensuremath\pm 0.17 for our red sub-sample, suggesting that the outer galaxy regions are more aligned with their dark matter halos than the inner regions. Our results are in agreement with previous studies and suggest that lower mass halos are rounder and/or less aligned with their host galaxy than samples of more massive galaxies, studied in galaxy groups and clusters.

- A&AKiDS-1000 catalogue: Redshift distributions and their calibrationH. Hildebrandt, J. L. van den Busch, A. H. Wright, and 15 more authors
*Astronomy and Astrophysics*, Mar 2021We present redshift distribution estimates of galaxies selected from the fourth data release of the Kilo-Degree Survey over an area of \ensuremath∼1000 deg^2 (KiDS-1000). These redshift distributions represent one of the crucial ingredients for weak gravitational lensing measurements with the KiDS-1000 data. The primary estimate is based on deep spectroscopic reference catalogues that are re-weighted with the help of a self- organising map (SOM) to closely resemble the KiDS-1000 sources, split into five tomographic redshift bins in the photometric redshift range 0.1 < z_B \ensuremath≤ 1.2. Sources are selected such that they only occupy that volume of nine- dimensional magnitude-space that is also covered by the reference samples (‘gold’ selection). Residual biases in the mean redshifts determined from this calibration are estimated from mock catalogues to be \ensuremath≲0.01 for all five bins with uncertainties of \ensuremath∼0.01. This primary SOM estimate of the KiDS-1000 redshift distributions is complemented with an independent clustering redshift approach. After validation of the clustering-z on the same mock catalogues and a careful assessment of systematic errors, we find no significant bias of the SOM redshift distributions with respect to the clustering-z measurements. The SOM redshift distributions re-calibrated by the clustering-z represent an alternative calibration of the redshift distributions with only slightly larger uncertainties in the mean redshifts of \ensuremath∼0.01 \ensuremath- 0.02 to be used in KiDS-1000 cosmological weak lensing analyses. As this includes the SOM uncertainty, clustering-z are shown to be fully competitive on KiDS-1000 data.

- A&AKiDS-1000 Cosmology: Multi-probe weak gravitational lensing and spectroscopic galaxy clustering constraintsCatherine Heymans, Tilman Tröster, Marika Asgari, and 31 more authors
*Astronomy and Astrophysics*, Feb 2021We 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. 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 S_8 = \ensuremathσ_8\ensuremath\sqr t(\ensuremathΩ_m/0.3) = 0.766_-0.014^+0.020, which has the same overall precision as that reported by the full-sky cosmic microwave background observations from Planck. The recovered S_8 amplitude is low, however, by 8.3 \ensuremath\pm 2.6% 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 S_8-difference with Planck is driven by a tension in the matter fluctuation amplitude parameter, \ensuremathσ_8. We quantify the level of agreement between the cosmic microwave background and our large- scale structure constraints using a series of different metrics, finding differences with a significance ranging between \ensuremath∼3\ensuremathσ, when considering the offset in S_8, and \ensuremath∼2\ensuremathσ, when considering the full multi-dimensional parameter space.

- A&AKiDS-1000 methodology: Modelling and inference for joint weak gravitational lensing and spectroscopic galaxy clustering analysisB. Joachimi, C. -A. Lin, M. Asgari, and 24 more authors
*Astronomy and Astrophysics*, Feb 2021We present the methodology for a joint cosmological analysis of weak gravitational lensing from the fourth data release of the ESO Kilo-Degree Survey (KiDS-1000) and galaxy clustering from the partially overlapping Baryon Oscillation Spectroscopic Survey (BOSS) and the 2-degree Field Lensing Survey (2dFLenS). Cross- correlations between BOSS and 2dFLenS galaxy positions and source galaxy ellipticities have been incorporated into the analysis, necessitating the development of a hybrid model of non-linear scales that blends perturbative and non-perturbative approaches, and an assessment of signal contributions by astrophysical effects. All weak lensing signals were measured consistently via Fourier-space statistics that are insensitive to the survey mask and display low levels of mode mixing. The calibration of photometric redshift distributions and multiplicative gravitational shear bias has been updated, and a more complete tally of residual calibration uncertainties was propagated into the likelihood. A dedicated suite of more than 20 000 mocks was used to assess the performance of covariance models and to quantify the impact of survey geometry and spatial variations of survey depth on signals and their errors. The sampling distributions for the likelihood and the \ensuremathχ^2 goodness-of-fit statistic have been validated, with proposed changes for calculating the effective number of degrees of freedom. The prior volume was explicitly mapped, and a more conservative, wide top-hat prior on the key structure growth parameter S_8 = \ensuremathσ_8 (\ensuremathΩ_m/0.3)^1/2 was introduced. The prevalent custom of reporting S_8 weak lensing constraints via point estimates derived from its marginal posterior is highlighted to be easily misinterpreted as yielding systematically low values of S_8, and an alternative estimator and associated credible interval are proposed. Known systematic effects pertaining to weak lensing modelling and inference are shown to bias S_8 by no more than 0.1 standard deviations, with the caveat that no conclusive validation data exist for models of intrinsic galaxy alignments. Compared to the previous KiDS analyses, S_8 constraints are expected to improve by 20% for weak lensing alone and by 29% for the joint analysis.

- A&ATightening weak lensing constraints on the ellipticity of galaxy-scale dark matter haloesTim Schrabback, Henk Hoekstra, Ludovic Van Waerbeke, and 19 more authors
*Astronomy and Astrophysics*, Feb 2021Cosmological simulations predict that galaxies are embedded into triaxial dark matter haloes, which appear approximately elliptical in projection. Weak gravitational lensing allows us to constrain these halo shapes and thereby test the nature of dark matter. Weak lensing has already provided robust detections of the signature of halo flattening at the mass scales of groups and clusters, whereas results for galaxies have been somewhat inconclusive. Here we combine data from five weak lensing surveys (NGVSLenS, KiDS/KV450, CFHTLenS, CS82, and RCSLenS, listed in order of most to least constraining) in order to tighten observational constraints on galaxy-scale halo ellipticity for photometrically selected lens samples. We constrain f_h, the average ratio between the aligned component of the halo ellipticity and the ellipticity of the light distribution, finding f_h = 0.303_-0.079^+0.080 for red lens galaxies and f_h = 0.217_-0.159^+0.160 for blue lens galaxies when assuming elliptical Navarro-Frenk-White density profiles and a linear scaling between halo ellipticity and galaxy ellipticity. Our constraints for red galaxies constitute the currently most significant (3.8\ensuremathσ) systematics-corrected detection of the signature of halo flattening at the mass scale of galaxies. Our results are in good agreement with expectations from the Millennium Simulation that apply the same analysis scheme and incorporate models for galaxy-halo misalignment. Assuming these misalignment models and the analysis assumptions stated above are correct, our measurements imply an average dark matter halo ellipticity for the studied red galaxy samples of ⟨|ϵ_h|⟩ = 0.174 \ensuremath\pm 0.046, where |ϵ_h| = (1 - q)/(1 + q) relates to the ratio q = b/a of the minor and major axes of the projected mass distribution. Similar measurements based on larger upcoming weak lensing data sets can help to calibrate models for intrinsic galaxy alignments, which constitute an important source of systematic uncertainty in cosmological weak lensing studies.

## 2020

- A&ATesting gravity using galaxy-galaxy lensing and clustering amplitudes in KiDS-1000, BOSS, and 2dFLenSChris Blake, Alexandra Amon, Marika Asgari, and 18 more authors
*Astronomy and Astrophysics*, Oct 2020The physics of gravity on cosmological scales affects both the rate of assembly of large-scale structure and the gravitational lensing of background light through this cosmic web. By comparing the amplitude of these different observational signatures, we can construct tests that can distinguish general relativity from its potential modifications. We used the latest weak gravitational lensing dataset from the Kilo-Degree Survey, KiDS-1000, in conjunction with overlapping galaxy spectroscopic redshift surveys, BOSS and 2dFLenS, to perform the most precise existing amplitude-ratio test. We measured the associated E_G statistic with 15 - 20% errors in five \ensuremath∆z = 0.1 tomographic redshift bins in the range 0.2 < z < 0.7 on projected scales up to 100 h^-1 Mpc. The scale-independence and redshift-dependence of these measurements are consistent with the theoretical expectation of general relativity in a Universe with matter density \ensuremathΩ_m = 0.27 \ensuremath\pm 0.04. We demonstrate that our results are robust against different analysis choices, including schemes for correcting the effects of source photometric redshift errors, and we compare the performance of angular and projected galaxy- galaxy lensing statistics.

- A&AKiDS+GAMA: The weak lensing calibrated stellar-to-halo mass relation of central and satellite galaxiesAndrej Dvornik, Henk Hoekstra, Konrad Kuijken, and 13 more authors
*Astronomy and Astrophysics*, Oct 2020We simultaneously present constraints on the stellar-to-halo mass relation for central and satellite galaxies through a weak lensing analysis of spectroscopically classified galaxies. Using overlapping data from the fourth data release of the Kilo-Degree Survey (KiDS), and the Galaxy And Mass Assembly survey (GAMA), we find that satellite galaxies are hosted by halo masses that are 0.53 \ensuremath\pm 0.39 dex (68% confidence, 3\ensuremathσ detection) smaller than those of central galaxies of the same stellar mass (for a stellar mass of log(M_\ensuremath⋆/M_\ensuremath⊙) = 10.6). This is consistent with galaxy formation models, whereby infalling satellite galaxies are preferentially stripped of their dark matter. We find consistent results with similar uncertainties when comparing constraints from a standard azimuthally averaged galaxy-galaxy lensing analysis and a two- dimensional likelihood analysis of the full shear field. As the latter approach is somewhat biased due to the lens incompleteness and as it does not provide any improvement to the precision when applied to actual data, we conclude that stacked tangential shear measurements are best-suited for studies of the galaxy-halo connection.

- Clustering of red-sequence galaxies in the fourth data release ofthe Kilo-Degree SurveyMohammadjavad Vakili, Henk Hoekstra, Maciej Bilicki, and 13 more authors
*arXiv e-prints*, Aug 2020We present a sample of luminous red-sequence galaxies to study the large-scale structure in the fourth data release of the Kilo- Degree Survey. The selected galaxies are defined by a red- sequence template, in the form of a data-driven model of the colour-magnitude relation conditioned on redshift. In this work, the red-sequence template is built using the broad-band optical+near infrared photometry of KiDS-VIKING and the overlapping spectroscopic data sets. The selection process involves estimating the red-sequence redshifts, assessing the purity of the sample, and estimating the underlying redshift distributions of redshift bins. After performing the selection, we mitigate the impact of survey properties on the observed number density of galaxies by assigning photometric weights to the galaxies. We measure the angular two-point correlation function of the red galaxies in four redshift bins, and constrain the large scale bias of our red-sequence sample assuming a fixed \LambdaCDM cosmology. We find consistent linear biases for two luminosity-threshold samples (dense and luminous). We find that our constraints are well characterized by the passive evolution model.

- A&AKiDS+VIKING+GAMA: Testing semi-analytic models of galaxy evolution with galaxy-galaxy-galaxy lensingLaila Linke, Patrick Simon, Peter Schneider, and 9 more authors
*Astronomy and Astrophysics*, Aug 2020Context. Several semi-analytic models (SAMs) try to explain how galaxies form, evolve, and interact inside the dark matter large-scale structure. These SAMs can be tested by comparing their predictions for galaxy-galaxy-galaxy lensing (G3L), which is weak gravitational lensing around galaxy pairs, with observations. \Aims: We evaluate the SAMs by Henriques et al. (2015, MNRAS, 451, 2663, hereafter H15) and by Lagos et al. (2012, MNRAS, 426, 2142, hereafter L12), which were implemented in the Millennium Run, by comparing their predictions for G3L to observations at smaller scales than previous studies and also for pairs of lens galaxies from different populations. \Methods: We compared the G3L signal predicted by the SAMs to measurements in the overlap of the Galaxy And Mass Assembly survey (GAMA), the Kilo-Degree Survey (KiDS), and the VISTA Kilo-degree Infrared Galaxy survey (VIKING) by splitting lens galaxies into two colour and five stellar-mass samples. Using an improved G3L estimator, we measured the three-point correlation of the matter distribution with “mixed lens pairs” with galaxies from different samples, and with “unmixed lens pairs” with galaxies from the same sample. \Results: Predictions by the H15 SAM for the G3L signal agree with the observations for all colour- selected samples and all but one stellar-mass-selected sample with 95% confidence. Deviations occur for lenses with stellar masses below 9.5 h^-2 M_\ensuremath⊙ at scales below 0.2 h^-1 Mpc. Predictions by the L12 SAM for stellar- mass selected samples and red galaxies are significantly higher than observed, while the predicted signal for blue galaxy pairs is too low. \Conclusions: The L12 SAM predicts more pairs of low stellar mass and red galaxies than the H15 SAM and the observations, as well as fewer pairs of blue galaxies. This difference increases towards the centre of the galaxies’ host halos. Likely explanations are different treatments of environmental effects by the SAMs and different models of the initial mass function. We conclude that G3L provides a stringent test for models of galaxy formation and evolution.

- A&AKiDS+VIKING-450 and DES-Y1 combined: Cosmology with cosmic shearS. Joudaki, H. Hildebrandt, D. Traykova, and 12 more authors
*Astronomy and Astrophysics*, Jun 2020We present a combined tomographic weak gravitational lensing analysis of the Kilo Degree Survey (KV450) and the Dark Energy Survey (DES-Y1). We homogenize the analysis of these two public cosmic shear datasets by adopting consistent priors and modeling of nonlinear scales, and determine new redshift distributions for DES-Y1 based on deep public spectroscopic surveys. Adopting these revised redshifts results in a 0.8\ensuremathσ reduction in the DES-inferred value for S_8, which decreases to a 0.5\ensuremathσ reduction when including a systematic redshift calibration error model from mock DES data based on the MICE2 simulation. The combined KV450+DES-Y1 constraint on S_8 = 0.762_-0.024^+0.025 is in tension with the Planck 2018 constraint from the cosmic microwave background at the level of 2.5\ensuremathσ. This result highlights the importance of developing methods to provide accurate redshift calibration for current and future weak-lensing surveys.

- A&AEuclid preparation. VI. Verifying the Performance of Cosmic Shear Experiments (Corrigendum)Euclid Collaboration, P. Paykari, T. Kitching, and 132 more authors
*Astronomy and Astrophysics*, Jun 2020 - Constraining the masses of galaxy overdensities at z > 1 in CANDELS and COSMOS through weak lensing in the NIRBomee Lee, Ranga-Ram Chary, Gabriel Brammer, and 8 more authorsMay 2020
- A&AEuclid preparation. VI. Verifying the performance of cosmic shear experimentsEuclid Collaboration, P. Paykari, T. Kitching, and 127 more authors
*Astronomy and Astrophysics*, Mar 2020\Aims: Our aim is to quantify the impact of systematic effects on the inference of cosmological parameters from cosmic shear. \Methods: We present an “end-to-end” approach that introduces sources of bias in a modelled weak lensing survey on a galaxy-by-galaxy level. We propagated residual biases through a pipeline from galaxy properties at one end to cosmic shear power spectra and cosmological parameter estimates at the other end. We did this to quantify how imperfect knowledge of the pipeline changes the maximum likelihood values of dark energy parameters. \Results: We quantify the impact of an imperfect correction for charge transfer inefficiency and modelling uncertainties of the point spread function for Euclid, and find that the biases introduced can be corrected to acceptable levels.

- A&AKiDS+VIKING-450 and DES-Y1 combined: Mitigating baryon feedback uncertainty with COSEBIsMarika Asgari, Tilman Tröster, Catherine Heymans, and 12 more authors
*Astronomy and Astrophysics*, Feb 2020We present cosmological constraints from a joint cosmic shear analysis of the Kilo-Degree Survey (KV450) and the Dark Energy Survey (DES-Y1), which were conducted using Complete Orthogonal Sets of E/B-Integrals (COSEBIs). With COSEBIs, we isolated any B-modes that have a non-cosmic shear origin and demonstrate the robustness of our cosmological E-mode analysis as no significant B-modes were detected. We highlight how COSEBIs are fairly insensitive to the amplitude of the non-linear matter power spectrum at high k-scales, mitigating the uncertain impact of baryon feedback in our analysis. COSEBIs, therefore, allowed us to utilise additional small-scale information, improving the DES-Y1 joint constraints on S_8 = \ensuremathσ_8 (\ensuremathΩ_m/0.3)^0.5 and \ensuremathΩ_m by 20%. By adopting a flat \ensuremathΛCDM model we find S_8 = 0.755_-0.021^+0.019, which is in 3.2\ensuremathσ tension with the Planck Legacy analysis of the cosmic microwave background.

- A&ACosmology from large-scale structure. Constraining \ensuremathΛCDM with BOSSTilman Tröster, Ariel. G. Sánchez, Marika Asgari, and 9 more authors
*Astronomy and Astrophysics*, Jan 2020We reanalyse the anisotropic galaxy clustering measurement from the Baryon Oscillation Spectroscopic Survey (BOSS), demonstrating that using the full shape information provides cosmological constraints that are comparable to other low-redshift probes. We find \ensuremathΩ_m = 0.317^+0.015_-0.019, \ensuremathσ_8 = 0.710\ensuremath\pm0.049, and h = 0.704 \ensuremath\pm 0.024 for flat \ensuremathΛCDM cosmologies using uninformative priors on \ensuremathΩ_ch^2, 100\ensuremathθ_MC, ln10^10A_s, and n_s, and a prior on \ensuremathΩ_bh^2 that is much wider than current constraints. We quantify the agreement between the Planck 2018 constraints from the cosmic microwave background and BOSS, finding the two data sets to be consistent within a flat \ensuremathΛCDM cosmology using the Bayes factor as well as the prior-insensitive suspiciousness statistic. Combining two low-redshift probes, we jointly analyse the clustering of BOSS galaxies with weak lensing measurements from the Kilo-Degree Survey (KV450). The combination of BOSS and KV450 improves the measurement by up to 45%, constraining \ensuremathσ_8 = 0.702 \ensuremath\pm 0.029 and S_8 = \ensuremathσ_8 \ensuremathΩ_m/0.3 = 0.728 \ensuremath\pm 0.026. Over the full 5D parameter space, the odds in favour of a single cosmology describing galaxy clustering, lensing, and the cosmic microwave background are 7 \ensuremath\pm 2. The suspiciousness statistic signals a 2.1 \ensuremath\pm 0.3\ensuremathσ tension between the combined low-redshift probes and measurements from the cosmic microwave background.

- A&AA gravitational lensing detection of filamentary structures connecting luminous red galaxiesQianli Xia, Naomi Robertson, Catherine Heymans, and 12 more authors
*Astronomy and Astrophysics*, Jan 2020We present a weak lensing detection of filamentary structures in the cosmic web, combining data from the Kilo-Degree Survey, the Red Cluster Sequence Lensing Survey, and the Canada-France-Hawaii Telescope Lensing Survey. The line connecting luminous red galaxies with a separation of 3 - 5 h^-1 Mpc was chosen as a proxy for the location of filaments. We measured the average weak lensing shear around \ensuremath∼11 000 candidate filaments selected in this way from the Sloan Digital Sky Survey. After nulling the shear induced by the dark matter haloes around each galaxy, we reported a 3.4\ensuremathσ detection of an anisotropic shear signal from the matter that connects them. Adopting a filament density profile, motivated from N-body simulations, the average density at the centre of these filamentary structures was found to be 15 \ensuremath\pm 4 times the critical density.

- A&AKiDS+VIKING-450: Cosmic shear tomography with optical and infrared dataH. Hildebrandt, F. Köhlinger, J. L. van den Busch, and 25 more authors
*Astronomy and Astrophysics*, Jan 2020We present a tomographic cosmic shear analysis of the Kilo-Degree Survey (KiDS) combined with the VISTA Kilo-Degree Infrared Galaxy Survey. This is the first time that a full optical to near- infrared data set has been used for a wide-field cosmological weak lensing experiment. This unprecedented data, spanning 450 deg^2, allows us to significantly improve the estimation of photometric redshifts, such that we are able to include robustly higher-redshift sources for the lensing measurement, and - most importantly - to solidify our knowledge of the redshift distributions of the sources. Based on a flat \ensuremathΛCDM model we find S_8 \ensuremath≡ \ensuremathσ_8 \ensuremathΩ_m/0.3 = 0.737^+0.040_-0.036 in a blind analysis from cosmic shear alone. The tension between KiDS cosmic shear and the Planck-Legacy CMB measurements remains in this systematically more robust analysis, with S_8 differing by 2.3\ensuremathσ. This result is insensitive to changes in the priors on nuisance parameters for intrinsic alignment, baryon feedback, and neutrino mass. KiDS shear measurements are calibrated with a new, more realistic set of image simulations and no significant B-modes are detected in the survey, indicating that systematic errors are under control. When calibrating our redshift distributions by assuming the 30-band COSMOS-2015 photometric redshifts are correct (following the Dark Energy Survey and the Hyper Suprime-Cam Survey), we find the tension with Planck is alleviated. The robust determination of source redshift distributions remains one of the most challenging aspects for future cosmic shear surveys. \\textbackslashData products from this analysis are available at the CDS via anonymous ftp to <A href=“http://cdsar c.u-strasbg.fr/”>http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href=“http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/633/A69”>http://cdsarc.u-strasbg.fr/viz- bin/cat/J/A+A/633/A69</A> and at <A href=“http://kids.strw.leid enuniv.nl”>http://kids.strw.leidenuniv.nl</A>

## 2019

- VizieR Online Data Catalog: KiDS-VIKING-450 cosmic shear (Hildebrandt+, 2020)H. Hildebrandt, F. Koehlinger, J. L. van den Busch, and 25 more authors
*VizieR Online Data Catalog*, Nov 2019Here we provide: \\textbackslash1) The data vector (tomographic two-point correlation function) 2) The covariance matrix 3) The redshift distributions 4) The fiducial output chains of the likelihood analysis \\(8 data files).

- MNRASLuminous red galaxies in the Kilo-Degree Survey: selection with broad-band photometry and weak lensing measurementsMohammadjavad Vakili, Maciej Bilicki, Henk Hoekstra, and 6 more authors
*Monthly Notices of the Royal Astronomical Society*, Aug 2019We use the overlap between multiband photometry of the Kilo-Degree Survey (KiDS) and spectroscopic data based on the Sloan Digital Sky Survey and Galaxy And Mass Assembly 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 \ensuremathσ_z \ensuremath∼ 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 \ensuremath∼ 0.7.

- A&AThe dependence of intrinsic alignment of galaxies on wavelength using KiDS and GAMAChristos Georgiou, Harry Johnston, Henk Hoekstra, and 8 more authors
*Astronomy and Astrophysics*, Feb 2019The 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.

## 2018

- MNRASCosmological simulations for combined-probe analyses: covariance and neighbour-exclusion biasJ. Harnois-Déraps, A. Amon, A. Choi, and 12 more authors
*Monthly Notices of the Royal Astronomical Society*, Nov 2018We present a public suite of weak-lensing mock data, extending the Scinet Light Cone Simulations (SLICS) to simulate cross- correlation analyses with different cosmological probes. These mocks include Kilo Degree Survey (KiDS)-450- and LSST-like lensing data, cosmic microwave background lensing maps and simulated spectroscopic surveys that emulate the Galaxy And Mass Assembly, BOSS, and 2-degree Field Lensing galaxy surveys. With 844 independent realizations, our mocks are optimized for combined-probe covariance estimation, which we illustrate for the case of a joint measurement involving cosmic shear, galaxy- galaxy lensing, and galaxy clustering from KiDS-450 and BOSS data. With their high spatial resolution, the SLICS are also optimal for predicting the signal for novel lensing estimators, for the validation of analysis pipelines, and for testing a range of systematic effects such as the impact of neighbour- exclusion bias on the measured tomographic cosmic shear signal. For surveys like KiDS and Dark Energy Survey, where the rejection of neighbouring galaxies occurs within \ensuremath∼2 arcsec, we show that the measured cosmic shear signal will be biased low, but by less than a per cent on the angular scales that are typically used in cosmic shear analyses. The amplitude of the neighbour-exclusion bias doubles in deeper, LSST-like data. The simulation products described in this paper are made available at http://slics.roe.ac.uk/.

- MNRASKiDS-i-800: comparing weak gravitational lensing measurements from same-sky surveysA. Amon, C. Heymans, D. Klaes, and 21 more authors
*Monthly Notices of the Royal Astronomical Society*, Jul 2018We present a weak gravitational lensing analysis of 815 deg^2 of i-band imaging from the Kilo-Degree Survey (KiDS-i-800). In contrast to the deep r-band observations, which take priority during excellent seeing conditions and form the primary KiDS data set (KiDS-r-450), the complementary yet shallower KiDS-i-800 spans a wide range of observing conditions. The overlapping KiDS-i-800 and KiDS-r-450 imaging therefore provides a unique opportunity to assess the robustness of weak lensing measurements. In our analysis we introduce two new ‘null’ tests. The ‘nulled’ two-point shear correlation function uses a matched catalogue to show that the calibrated KiDS-i-800 and KiDS-r-450 shear measurements agree at the level of 1 \ensuremath\pm 4 per cent. We use five galaxy lens samples to determine a ‘nulled’ galaxy-galaxy lensing signal from the full KiDS-i-800 and KiDS-r-450 surveys and find that the measurements agree to 7 \ensuremath\pm 5 per cent when the KiDS-i-800 source redshift distribution is calibrated using either spectroscopic redshifts, or the 30-band photometric redshifts from the COSMOS survey.

## 2016

- The Effect of Detector Nonlinearity on WFIRST PSF Profiles for Weak Gravitational Lensing MeasurementsA. A. Plazas, C. Shapiro, A. Kannawadi, and 3 more authors
*Publications of the ASP*, Oct 2016Weak gravitational lensing (WL) is one of the most powerful techniques to learn about the dark sector of the universe. To extract the WL signal from astronomical observations, galaxy shapes must be measured and corrected for the point-spread function (PSF) of the imaging system with extreme accuracy. Future WL missions—such as NASA’s Wide-Field Infrared Survey Telescope (WFIRST)—will use a family of hybrid near-infrared complementary metal-oxide-semiconductor detectors (HAWAII-4RG) that are untested for accurate WL measurements. Like all image sensors, these devices are subject to conversion gain nonlinearities (voltage response to collected photo-charge) that bias the shape and size of bright objects such as reference stars that are used in PSF determination. We study this type of detector nonlinearity (NL) and show how to derive requirements on it from WFIRST PSF size and ellipticity requirements. We simulate the PSF optical profiles expected for WFIRST and measure the fractional error in the PSF size (\ensuremath∆R/R) and the absolute error in the PSF ellipticity (\ensuremath∆e) as a function of star magnitude and the NL model. For our nominal NL model (a quadratic correction), we find that, uncalibrated, NL can induce an error of \ensuremath∆R/R = 1 \texttimes 10^-2 and \ensuremath∆e _2 = 1.75 \texttimes 10^-3 in the H158 bandpass for the brightest unsaturated stars in WFIRST. In addition, our simulations show that to limit the bias of \ensuremath∆R/R and \ensuremath∆e in the H158 band to \ensuremath∼10% of the estimated WFIRST error budget, the quadratic NL model parameter \ensuremathβ must be calibrated to \ensuremath∼1% and \ensuremath∼2.4%, respectively. We present a fitting formula that can be used to estimate WFIRST detector NL requirements once a true PSF error budget is established.

## 2014

- ApJSThe Third Gravitational Lensing Accuracy Testing (GREAT3) Challenge HandbookRachel Mandelbaum, Barnaby Rowe, James Bosch, and 28 more authors
*Astrophysical Journal, Supplement*, May 2014The GRavitational lEnsing Accuracy Testing 3 (GREAT3) challenge is the third in a series of image analysis challenges, with a goal of testing and facilitating the development of methods for analyzing astronomical images that will be used to measure weak gravitational lensing. This measurement requires extremely precise estimation of very small galaxy shape distortions, in the presence of far larger intrinsic galaxy shapes and distortions due to the blurring kernel caused by the atmosphere, telescope optics, and instrumental effects. The GREAT3 challenge is posed to the astronomy, machine learning, and statistics communities, and includes tests of three specific effects that are of immediate relevance to upcoming weak lensing surveys, two of which have never been tested in a community challenge before. These effects include many novel aspects including realistically complex galaxy models based on high-resolution imaging from space; a spatially varying, physically motivated blurring kernel; and a combination of multiple different exposures. To facilitate entry by people new to the field, and for use as a diagnostic tool, the simulation software for the challenge is publicly available, though the exact parameters used for the challenge are blinded. Sample scripts to analyze the challenge data using existing methods will also be provided. See <A href=‘ ‘http://great3challenge.info”>http://great3challenge.info</A> and <A href=“http://great3.projects.phys.ucl.ac.uk/leaderboard/ ”>http://great3.projects.phys.ucl.ac.uk/leaderboard/</A> for more information.