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. The median 5-σ limiting AB magnitude is 24.9 and the median seeing is below 0.7 . 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 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 weak lensing measurement and one for accurate matched-aperture photometry (for photometric redshifts). This technical paper describes the lensing and photometric redshift measurements (including a detailed 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 describe our blinding procedure for preventing confirmation bias in the scientific analyses. The KiDS catalogues presented in this paper are released to the community through http://kids.strw.leidenuniv.nl.

Context. The Kilo-Degree Survey (KiDS) is an optical wide-field imaging survey carried out with the VLT Survey Telescope and the OmegaCAM camera. KiDS will image 1500 square degrees in four filters (ugri), and together with its near-infrared counterpart VIKING will produce deep photometry in nine bands. Designed for weak lensing shape and photometric redshift measurements, its core science driver is mapping the large-scale matter distribution in the Universe back to a redshift of ∼0.5. Secondary science cases include galaxy evolution, Milky Way structure, and the detection of high-redshift clusters and quasars. Aims. KiDS is an ESO Public Survey and dedicated to serving the astronomical community with high-quality data products derived from the survey data. Public data releases, the first two of which are presented here, are crucial for enabling independent confirmation of the survey's scientific value. The achieved data quality and initial scientific utilization are reviewed in order to validate the survey data. Methods. A dedicated pipeline and data management system based on A-WISE, combined with newly developed masking and source classification tools, is used for the production of the data products described here. Science projects based on these data products and preliminary results are outlined. Results. For 148 survey tiles (≈160 sq.deg.) stacked ugri images have been released, accompanied by weight maps, masks, source lists, and a multi-band source catalogue. Limiting magnitudes are typically 24.3, 25.1, 24.9, 23.8 (5σ in a 2 aperture) in ugri, respectively, and the typical r-band PSF size is less than 0.7 . The photometry prior to global homogenization is stable at the ∼2% (4%) level in gri (u) with some outliers due to non-photometric conditions, while the astrometry shows a typical 2D rms of 0.03 . Early scientific results include the detection of nine high-z QSOs, fifteen candidate strong gravitational lenses, high-quality photometric redshifts and structural parameters for hundreds of thousands of galaxies.

Cosmological galaxy surveys aim at mapping the largest volumes to test models with techniques such as cluster abundance, cosmic shear correlations or baryon acoustic oscillations (BAO), which are designed to be independent of galaxy bias. Here, we explore an alternative route to constrain cosmology: sampling more moderate volumes with the cross‐correlation of photometric and spectroscopic surveys. We consider the angular galaxy–galaxy auto‐correlation in narrow redshift bins and its combination with different probes of weak gravitational lensing (WL) and redshift space distortions (RSD). Including the cross‐correlation of these surveys improves by factors of a few the constraints on both the dark energy equation of state w(z) and the cosmic growth history, parametrized by γ. The additional information comes from using many narrow redshift bins and from measurement of galaxy bias with both WL and RSD, breaking degeneracies that are present when using each method separately. We show forecasts for a joint w(z) and γ figure of merit (FoM) using linear scales over a deep (iAB < 24) photometric survey and a brighter (iAB < 22.5) spectroscopic or very accurate (0.3 per cent) photometric redshift survey. Magnification or shear in the photometric sample produce FoM that are of the same order of magnitude of those of RSD or BAO over the spectroscopic sample. However, the cross‐correlation of these probes over the same area yields a FoM that is up to a factor of 100 times larger. Magnification alone, without shape measurements, can also be used for these cross‐correlations and can produce better results than using shear alone. For a spectroscopic follow‐up survey strategy, measuring the spectra of the foreground lenses to perform this cross‐correlation provides five times better FoM than targeting the higher redshift tail of the galaxy distribution to study BAO over a 2.5 times larger volume.

The PAU Survey (PAUS) is an innovative photometric survey with 40 narrow bands at the William Herschel Telescope (WHT). The narrow bands are spaced at 100Å intervals covering the range 4500Å to 8500Å and, in combination with standard broad bands, enable excellent redshift precision. This paper describes the technique, galaxy templates and additional photometric calibration used to determine early photometric redshifts from PAUS. Using bcnz2, a new photometric redshift code developed for this purpose, we characterise the photometric redshift performance using PAUS data on the COSMOS field. Comparison to secure spectra from zCOSMOS DR3 shows that PAUS achieves σ 68 /(1+z) = 0.0037 to i AB < 22.5 when selecting the best 50% of the sources based on a photometric redshift quality cut. Furthermore, a higher photo-z precision (σ 68 /(1 + z) ∼ 0.001) is obtained for a bright and high quality selection, which is driven by the identification of emission lines. We conclude that PAUS meets its design goals, opening up a hitherto uncharted regime of deep, wide, and dense galaxy survey with precise redshifts that will provide unique insights into the formation, evolution and clustering of galaxies, as well as their intrinsic alignments.

The Physics of the Accelerating Universe (PAU) survey at the William Herschel Telescope (WHT) will use a new optical camera (PAUCam) with a large set of narrow-band filters to perform a photometric galaxy survey with a quasi-spectroscopic redshift precision of σ(z)/(1 + z) ∼ 0.0035 and map the large-scale structure of the universe in three dimensions up to i AB < 22.5-23.0. In this paper we present a detailed photo-z performance study using photometric simulations for 40 equally-spaced 12.5-nm-wide (FWHM) filters with a ∼25% overlap and spanning the wavelength range from 450 nm to 850 nm, together with a ugrizY broad-band filter system. We then present the migration matrix r ij , containing the probability that a galaxy in a true redshift bin j is measured in a photo-z bin i, and study its effect on the determination of galaxy autoand cross-correlations. Finally, we also study the impact on the photo-z performance of small variations of the filter set in terms of width, wavelength coverage, etc., and find a broad region where slightly modified filter sets provide similar results, with the original set being close to optimal.

We present – and make publicly available – accurate and precise photometric redshifts in the ACS footprint from the COSMOS field for objects with iAB ≤ 23. The redshifts are computed using a combination of narrow-band photometry from PAUS, a survey with 40 narrow bands spaced at $100\,\mathring{\rm A}$ intervals covering the range from 4500 to $8500\,\mathring{\rm A}$, and 26 broad, intermediate, and narrow bands covering the UV, visible and near-infrared spectrum from the COSMOS2015 catalogue. We introduce a new method that models the spectral energy distributions as a linear combination of continuum and emission-line templates and computes its Bayes evidence, integrating over the linear combinations. The correlation between the UV luminosity and the O ii line is measured using the 66 available bands with the zCOSMOS spectroscopic sample, and used as a prior which constrains the relative flux between continuum and emission-line templates. The flux ratios between the O ii line and Hα, Hβ and $\mathrm{O\,{\small III}}$ are similarly measured and used to generate the emission-line templates. Comparing to public spectroscopic surveys via the quantity Δz ≡ (zphoto − zspec)/(1 + zspec), we find the photometric redshifts to be more precise than previous estimates, with σ68(Δz) ≈ (0.003, 0.009) for galaxies at magnitude iAB ∼ 18 and iAB ∼ 23, respectively, which is three times and 1.66 times tighter than COSMOS2015. Additionally, we find the redshifts to be very accurate on average, yielding a median of the Δz distribution compatible with |median(Δz)| ≤ 0.001 at all redshifts and magnitudes considered. Both the added PAUS data and new methodology contribute significantly to the improved results. The catalogue produced with the technique presented here is expected to provide a robust redshift calibration for current and future lensing surveys, and allows one to probe galaxy formation physics in an unexplored luminosity-redshift regime, thanks to its combination of depth, completeness, and excellent redshift precision and accuracy.

The PAU (Physics of the Accelerating Universe) Survey goal is to obtain photometric redshifts (photo-z) and Spectral Energy Distribution (SED) of astronomical objects with a resolution roughly one order of magnitude better than current broad band photometric surveys. To accomplish this, a new large field of view camera (PAUCam) has been designed, built, commissioned and is now operated at the William Herschel Telescope (WHT). With the current WHT Prime Focus corrector, the camera covers ∼1 degree diameter Field of View (FoV), of which, only the inner ∼40 arcmin diameter are unvignetted. The focal plane consists of a mosaic of 18 2k×4k Hamamatsu fully depleted CCDs, with high quantum efficiency up to 1 µm in wavelength. To maximize the detector coverage within the FoV, filters are placed in front of the CCDs inside the camera cryostat (made out of carbon fiber) using a challenging movable tray system. The camera uses a set of 40 narrow band filters ranging from ∼4500 to ∼8500 Angstroms complemented with six standard broad-band filters, ugrizY . The PAU Survey aims to cover roughly 100 square degrees over fields with existing deep photometry and galaxy shapes to obtain accurate photometric redshifts for galaxies down to i AB ∼ 22.5, detecting also galaxies down to i AB ∼ 24 with less precision in redshift. With this data set we will be able to measure intrinsic alignments, galaxy clustering and perform galaxy evolution studies in a new range of densities and redshifts. Here, we describe the PAU camera, its first commissioning results and performance.

Upcoming and ongoing large area weak lensing surveys will also discover large samples of galaxy clusters. Accurate and precise masses of galaxy clusters are of major importance for cosmology, for example, in establishing well calibrated observational halo mass functions for comparison with cosmological predictions. We investigate the level of statistical uncertainties and sources of systematic errors expected for weak lensing mass estimates. Future surveys that will cover large areas on the sky, such as Euclid or LSST and to lesser extent DES, will provide the largest weak lensing cluster samples with the lowest level of statistical noise regarding ensembles of galaxy clusters. However, the expected low level of statistical uncertainties requires us to scrutinize various sources of systematic errors. In particular, we investigate the bias due to cluster member galaxies which are erroneously treated as background source galaxies due to wrongly assigned photometric redshifts. We find that this effect is significant when referring to stacks of galaxy clusters. Finally, we study the bias due to miscentring, i.e., the displacement between any observationally defined cluster centre and the true minimum of its gravitational potential. The impact of this bias might be significant with respect to the statistical uncertainties. However, complementary future missions such as eROSITA will allow us to define stringent priors on miscentring parameters which will mitigate this bias significantly.