Gravitational Lensing Analysis of the Kilo Degree Survey

TL;DR

KiDS introduces a new, high-quality, multi-band weak lensing survey using VST/OmegaCAM, delivering lensing shapes from r-band data and photometric redshifts from ugri photometry with GAaP. It details a two-track data processing pipeline, meticulous PSF modelling, and a comprehensive suite of systematic tests, including multiplicative and additive bias calibrations, redshift-scaling checks, and field-level PSF diagnostics. The paper demonstrates a first cosmic shear signal with negligible B-modes and provides public datasets, including shear and photometric redshift catalogues, for community use. Together, these results establish KiDS as a competitive, well-calibrated platform for weak lensing cosmology and galaxy-mass studies, with a clear path to larger-area analyses.

Abstract

The Kilo-Degree Survey (KiDS) is a multi-band imaging survey designed for cosmological studies from weak lensing and photometric redshifts. It uses the ESO VLT Survey Telescope with its wide-field camera OmegaCAM. KiDS images are taken in four filters similar to the SDSS ugri bands. The best-seeing time is reserved for deep r-band observations that reach a median 5-sigma limiting AB magnitude of 24.9 with a median seeing that is better than 0.7arcsec. Initial KiDS observations have concentrated on the GAMA regions near the celestial equator, where extensive, highly complete redshift catalogues are available. A total of 109 survey tiles, one square degree each, form the basis of the first set of lensing analyses, which focus on measurements of halo properties of GAMA galaxies. 9 galaxies per square arcminute enter the lensing analysis, for an effective inverse shear variance of 69 per square arcminute. Accounting for the shape measurement weight, the median redshift of the sources is 0.53. KiDS data processing follows two parallel tracks, one optimized for galaxy shape measurement (for weak lensing), and one for accurate matched-aperture photometry in four bands (for photometric redshifts). This technical paper describes how the lensing and photometric redshift catalogues have been produced (including an extensive description of the Gaussian Aperture and Photometry pipeline), summarizes the data quality, and presents extensive tests for systematic errors that might affect the lensing analyses. We also provide first demonstrations of the suitability of the data for cosmological measurements, and explain how the shear catalogues were blinded to prevent confirmation bias in the scientific analyses. The KiDS shear and photometric redshift catalogues, presented in this paper, are released to the community through http://kids.strw.leidenuniv.nl .

Figures (28)

• Figure 1: Example of high-quality KiDS data obtained with VST/OmegaCAM. PSF SExtractor parameters shown are for the stacked $r$-band image of tile KIDS_132.0_-0.5. Left: direction and strength of the ellipticities of stars in the field. Right: PSF ellipticity ( top) and FWHM size ( bottom) vs. distance from the centre of the image.
• Figure 2: Distribution of mean ellipticities and standard deviations of ellipticities of co-added images in data releases 1 and 2 of KiDS. The values are based on SExtractor ellipticity measurements of the 500 brightest unsaturated stars in each tile. The grey scale indicates the number of survey tiles in each bin. Top: $r$ band only; Bottom: data from all filters.
• Figure 3: PSF ellipticity patterns caused by a non-optimal optical configuration of the telescope. The curved focal plane of the VST translates any primary mirror astigmatism into increased ellipticity in the centre of the field (top). A tilt of the secondary mirror results in increased ellipticity near one edge of the field (bottom panel).
• Figure 4: Automatic star-galaxy separation based on the second and fourth order moment radii $Q^{1/2}$ and $J^{1/4}$ of individual sources, for a typical KiDS observation. Five out of the six square panels show the distributions for the individual sub-exposures, with the objects identified as stars shown in red. As the seeing differs between the sub-exposures, the combined distribution for the observation, in the sixth square panel, reveals a series of distinct stellar peaks. The right-most panel shows the distribution of these points in the traditional radius-magnitude plane for the co-added image.
• Figure 5: Selecting the optimal fitting orders for the PSF model for a sample of representative KiDS observations. The upper panels show the residual PSF ellipticity correlation, measured at 1 arcmin separation, as a function of the average PSF ellipticity within the sub-exposure. The lower panels show the two-point residual PSF size correlation measured at 1 arcmin separation as a function of the average PSF size $R^2_{\rm PSF}$. Each data point represents a different sub-exposure, with the point style indicating the polynomial orders $(n, n_{\rm c})$ of the model. Left: results for $n= 3$; right: results for $n= 4$.