The fourth data release of the Kilo-Degree Survey: ugri imaging and nine-band optical-IR photometry over 1000 square degrees

TL;DR

KiDS-ESO-DR4 delivers a major public data release by reprocessing over 1000 square degrees with two pipelines, adding VIKING near-IR photometry to create a nine-band KiDS+VIKING catalog for ~100 million galaxies. The release employs Gaia DR2-based photometric calibration, Stellar Locus Regression, and PSF Gaussianization (GAaP) to homogenize colours, enabling improved photometric redshifts and weak-lensing analyses. DR4 introduces full co-added optical images, r-band detections from theli, and forced GAaP photometry across all bands, with depth reaching approximately u=24.23, g=25.12, r=25.02, i=23.68 (5σ, 2″ aperture) and an average r-band seeing of 0.70″. The data products are accessible via ESO, Astro-WISE, and the KiDS website, setting the stage for DR5 to complete the KiDS/VIKING footprint and deliver added weak-lensing shape measurements and calibrations.

Abstract

The Kilo-Degree Survey (KiDS) is an ongoing optical wide-field imaging survey with the OmegaCAM camera at the VLT Survey Telescope, specifically designed for measuring weak gravitational lensing by galaxies and large-scale structure. When completed it will consist of 1350 square degrees imaged in four filters (ugri). Here we present the fourth public data release which more than doubles the area of sky covered by data release 3. We also include aperture-matched ZYJHKs photometry from our partner VIKING survey on the VISTA telescope in the photometry catalogue. We illustrate the data quality and describe the catalogue content. Two dedicated pipelines are used for the production of the optical data. The Astro-WISE information system is used for the production of co-added images in the four survey bands, while a separate reduction of the r-band images using the theli pipeline is used to provide a source catalogue suitable for the core weak lensing science case. All data have been re-reduced for this data release using the latest versions of the pipelines. The VIKING photometry is obtained as forced photometry on the theli sources, using a re-reduction of the VIKING data that starts from the VISTA pawprints. Modifications to the pipelines with respect to earlier releases are described in detail. The photometry is calibrated to the Gaia DR2 G band using stellar locus regression. In this data release a total of 1006 square-degree survey tiles with stacked ugri images are made available, accompanied by weight maps, masks, and single-band source lists. We also provide a multi-band catalogue based on r-band detections, including homogenized photometry and photometric redshifts, for the whole dataset. Mean limiting magnitudes (5 sigma in a 2" aperture) are 24.23, 25.12, 25.02, 23.68 in ugri, respectively, and the mean r-band seeing is 0.70".

Figures (22)

• Figure 1: Sky distribution of survey tiles released in KiDS-ESO-DR4. Tiles shown in green are released for the first time; those in blue were included in the earlier data releases (DR1+2+3) but have been reprocessed for DR4. The full KiDS+VIKING area ($\sim1350 \deg^2$) is shown in grey. Top: KiDS-North. Bottom: KiDS-South. The single pointing at RA=$150\deg$ is centred on the COSMOS/CFHTLS D2 field.
• Figure 2: Progress of the KiDS observations at the VST, in the four survey bands. Each Observing Block (OB) produces a square-degree co-added image. The i-band data, for which data taking was significantly faster because of less competition for bright time on the telescope, had covered the originally planned 1500 square degree footprint by the time it was decided to limit the survey to the 1350 square degree area that comprise the completed VIKING area. The dashed lines indicate the cutoff dates for KiDS-ESO data releases 1 to 4.
• Figure 3: Distributions of tile-by-tile data quality parameters for the KiDS DR4 data, grouped by filter, from top to bottom u, g, r and i. The light-coloured histograms represent the subset of the data that was previously released in DR1+2+3. Left: seeing. The differences between the bands reflect the observing strategy of reserving the best-seeing dark time for r-band observations. Middle column: Average PSF ellipticity $\langle |e_{\rm psf}|\rangle$, where $e$ is defined as $1-b/a$ for major/minor axis lengths $a$ and $b$. Right: Limiting AB magnitude (5-$\sigma$ in a 2 aperture). The wider distribution of the i-band observations is a caused by variations in the moon illumination, since the i-band data were mostly taken in bright time.
• Figure 4: Schematic representation of the DR4 processing steps and content. Yellow boxes show the input data from VST and VISTA, green indicates image products, and source lists are shown in pink. The lensfit-based lensing measurements -- initially not released in DR4 -- are shown as dotted lines.
• Figure 5: Residual u-band magnitude variation when Eq. (\ref{['eq:SLRu']}) is used without a dependence on Galactic latitude $b$ (i.e., $f(b)=0$). Each point shows the average offset $\hbox{u}_{\rm KiDS}-\hbox{u}_{\rm SDSS}$ for the calibration stars in a separate KiDS-N tile. The line represents the Galactic latitude correction of Eq. (\ref{['eq:latcorr']}).