Context. We present the CFHTLS-Archive-Research Survey (CARS). It is a virtual multi-colour survey that is based on public archive images from the Deep and Wide components of the CFHT-Legacy-Survey (CFHTLS). Our main scientific interests in the CFHTLS Wide-part of CARS are optical searches for galaxy clusters from low to high redshift and their subsequent study with photometric and weak-gravitational lensing techniques. Aims. As a first step in the CARS project, we present multi-colour catalogues from 37 sq. degrees of the CFHTLS-Wide component. Our aims are first to create astrometrically and photometrically well-calibrated co-added images from publicly available CFHTLS data. Second, we offer five-band (u * g r i z ) multi-band catalogues with an emphasis on reliable estimates for object colours. These are subsequently used for photometric redshift estimates. Methods. We consider all those CFHTLS-Wide survey pointings that were publicly available on January 2008 and that also have fiveband coverage in u * g r i z . The data were calibrated and processed with our GaBoDS/THELI image processing pipeline. The quality of the resulting images was thoroughly checked against the Sloan-Digital-Sky Survey (SDSS) and already public high-end CFHTLS data products. From the co-added images we extracted source catalogues and determined photometric redshifts using the public code Bayesian Photometric Redshifts (BPZ). Fifteen of our survey fields directly overlap with public spectra from the VIMOS VLT deep (VVDS), DEEP2 and SDSS redshift surveys, which we used for calibration and verification of our redshift estimates. Furthermore we applied a novel technique, based on studies of the angular galaxy cross-correlation function, to quantify the reliability of photo-z's. Results. With this paper we present 37 sq. degrees of homogeneous and high-quality five-colour photometric data from the CFHTLSWide survey. The median seeing of our data is better than 0. 9 in all bands and our catalogues reach a 5σ limiting magnitude of about i AB ≈ 24.5. Comparisons with the SDSS indicate that most of our survey fields are photometrically calibrated to an accuracy of 0.04 mag or better. This allows us to derive photometric redshifts of homogeneous quality over the whole survey area. The accuracy of our high-confidence photo-z sample (10−15 galaxies per sq. arcmin) is estimated with external spectroscopic data to σ Δz/(1+z) ≈ 0.04−0.05 up to i AB < 24 with typically only 1−3% outliers. In the spirit of the Legacy Survey we make our catalogues available to the astronomical community. Our products consist of multi-colour catalogues and supplementary information, such as image masks and JPEG files to visually inspect our catalogues. Interested users can obtain the data by request to the authors.

We present the weak lensing analysis of the Wide-Field Imager SZ Cluster of galaxy (WISCy) sample, a set of 12 clusters of galaxies selected for their Sunyaev-Zel'dovich (SZ) effect. After developing new and improved methods for background selection and determination of geometric lensing scaling factors from absolute multi-band photometry in cluster fields, we compare the weak lensing mass estimate with public X-ray and SZ data. We find consistency with hydrostatic X-ray masses with no significant bias, no mass dependent bias and less than 20% intrinsic scatter and constrain f gas,500c = 0.128 +0.029 −0.023 . We independently calibrate the South Pole Telescope significance-mass relation and find consistency with previous results. The comparison of weak lensing mass and Planck Compton parameters, whether extracted selfconsistently with a mass-observable relation (MOR) or using X-ray prior information on cluster size, shows significant discrepancies. The deviations from the MOR strongly correlate with cluster mass and redshift. This could be explained either by a significantly shallower than expected slope of Compton decrement versus mass and a corresponding problem in the previous X-ray based mass calibration, or a size or redshift dependent bias in SZ signal extraction.

We present results for a galaxy-galaxy lensing study based on imaging data from the Canada-France-Hawaii Telescope Legacy Survey Wide. From a 12 million object multi-colour catalogue for 124 deg 2 of photometric data in the u * g ′ r ′ i ′ z ′ filters, we compute photometric redshifts (with a scatter of σ ∆z/(1+z) = 0.033 and an outlier rate of η = 2.0 per cent for i ′ 22.5) and extract galaxy shapes down to i ′ = 24.0. We select a sample of lenses and sources with 0.05 < z d 1 and 0.05 < z s 2. We fit three different galaxy halo profiles to the lensing signal, a singular isothermal sphere (SIS), a truncated isothermal sphere (BBS) and a universal density profile (NFW). We derive velocity dispersions by fitting an SIS out to 100 h −1 kpc to the excess surface mass density ∆Σ and perform maximum likelihood analyses out to a maximum scale of 2 h −1 Mpc to obtain halo parameters and scaling relations. We find luminosity scaling relations of σ red ∝ L 0.24±0.03 for the red lens sample, σ blue ∝ L 0.23±0.03 for blue lenses and σ ∝ L 0.29±0.02 for the combined lens sample with zeropoints of σ * red = 162 ± 2 km s −1 , σ * blue = 115 ± 3 km s −1 and σ * = 135 ± 2 km s −1 at a chosen reference luminosity L * r ′ = 1.6 × 10 10 h −2 L r ′ ,⊙ . The steeper slope for the combined sample is due to the different zeropoints of the blue and red lenses and the fact that blue lenses dominate at low luminosities and red lenses at high luminosities.The mean effective redshifts for the lens samples are z red = 0.28 for red lenses, z blue = 0.35 for blue lenses and z = 0.34 for the combined lens sample. The BBS maximum likelihood analysis yields for the combined sample a velocity dispersion of σ * = 131 +2 −2 km s −1 and a truncation radius of s * = 184 +17 +14 h −1 kpc, corresponding to a total mass of M * total,BBS = 2.32 +0.28 −0.25 × 10 12 h −1 M ⊙ and a mass-to-light (M/L) ratio of M * total,BBS /L * = 178 +22 −19 h M ⊙ /L r ′ ,⊙ at L * r ′ . At a given luminosity, both velocity dispersion σ and truncation radius s are larger for red galaxies than for blue galaxies. For an NFW profile, we measure at L * r ′ a virial radius of r * 200 = 133 +3 −2 h −1 kpc and a concentration parameter of c * = 6.4 +0.9 −0.7 , implying a virial mass of M * 200 = 7.6 +0.5 −0.3 × 10 11 h −1 M ⊙ . At L * r ′ for blue galaxies the concentration parameter is slightly higher than for red galaxies and r 200 is significantly lower. For the combined sample, if described as a single power law, the M/L-ratio scales as M total,BBS /L ∝ L 0.12 +0.10 −0.11 , the concentration parameter scales as c ∝ L −0.07 +0.11 −0.11 . Analysing the M/L-scaling for red and blue galaxies separately, we find that a broken power law (with a flat slope at high luminosities) provides a more appropriate description for the red and possibly also for the blue galaxies. We measure M 200 /M star for red galaxies over 2.5 decades in stellar mass. We find a minimum for this ratio at M star ∼ 3 − 4 × 10 10 h −2 M ⊙ with a strong increase for lower stellar masses.

We present a catalog of 129 X-ray galaxy groups, covering a redshift range 0.04 < z < 1.23, selected in the ∼3 deg 2 part of the CFHTLS W1 field overlapping XMM observations performed under the XMM-LSS project. We carry out a statistical study of the redshift evolution out to redshift one of the magnitude gap between the first and the second brightest cluster galaxies of a well defined mass-selected group sample. We find that the slope of the relation between the fraction of groups and the magnitude gap steepens with redshift, indicating a larger fraction of fossil groups at lower redshifts. We find that 22.2 ± 6% of our groups at z ≤ 0.6 are fossil groups. We compare our results with the predictions of three semi-analytic models based on the Millennium simulation. The intercept of the relation between the magnitude of the brightest galaxy and the value of magnitude gap becomes brighter with increasing redshift. This trend is steeper than the model predictions which we attribute to the younger stellar age of the observed brightest cluster galaxies. This trend argues in favor of stronger evolution of the feedback from active galactic nuclei at z < 1 compared to the models. The slope of the relation between the magnitude of the brightest cluster galaxy and the value of the gap does not evolve with redshift and is well reproduced by the models, indicating that the tidal galaxy stripping, put forward as an explanation of the occurrence of the magnitude gap, is both a dominant mechanism and sufficiently well modeled.

We present a weak lensing analysis of the cluster of galaxies RXC J2248.7-4431, a massive system at z = 0.3475 with prominent strong lensing features covered by the Cluster Lensing And Supernova survey with Hubble (CLASH). Based on UBVRIZ imaging from the Wide-Field Imager camera at the MPG/ESO 2.2-m telescope, we measure photometric redshifts and shapes of background galaxies. The cluster is detected as a mass peak at 5σ significance. Its density can be parametrized as a Navarro-Frenk-White (NFW) profile with two free parameters, the mass M 200m = 33.1 +9.6 −6.8 × 10 14 M ⊙ and concentration c 200m = 2.6 +1.5 −1.0 . We discover a second cluster inside the field of view at a photometric redshift of z ≈ 0.6, with an NFW mass of M 200m = 4.0 +3.7 −2.6 ×10 14 M ⊙ .

We introduce an ordinal classification algorithm for photometric redshift estimation, which significantly improves the reconstruction of photometric redshift probability density functions (PDFs) for individual galaxies and galaxy samples. As a use case we apply our method to CFHTLS galaxies. The ordinal classification algorithm treats distinct redshift bins as ordered values, which improves the quality of photometric redshift PDFs, compared with non-ordinal classification architectures. We also propose a new single value point estimate of the galaxy redshift, that can be used to estimate the full redshift PDF of a galaxy sample. This method is competitive in terms of accuracy with contemporary algorithms, which stack the full redshift PDFs of all galaxies in the sample, but requires orders of magnitudes less storage space.The methods described in this paper greatly improve the log-likelihood of individual object redshift PDFs, when compared with a popular Neural Network code (ANNz). In our use case, this improvement reaches 50% for high redshift objects (z 0.75).We show that using these more accurate photometric redshift PDFs will lead to a reduction in the systematic biases by up to a factor of four, when compared with less accurate PDFs obtained from commonly used methods. The cosmological analyses we examine and find improvement upon are the following: gravitational lensing cluster mass estimates, modelling of angular correlation functions, and modelling of cosmic shear correlation functions.

To measure the mass of foreground objects with weak gravitational lensing, one needs to estimate the redshift distribution of lensed background sources. This is commonly done in an empirical fashion, i.e. with a reference sample of galaxies of known spectroscopic redshift, matched to the source population. In this work, we develop a simple decision tree framework that, under the ideal conditions of a large, purely magnitudelimited reference sample, allows an unbiased recovery of the source redshift probability density function p(z), as a function of magnitude and colour. We use this framework to quantify biases in empirically estimated p(z) caused by selection effects present in realistic reference and weak lensing source catalogues, namely (1) complex selection of reference objects by the targeting strategy and success rate of existing spectroscopic surveys and (2) selection of background sources by the success of object detection and shape measurement at low signal-to-noise. For intermediate-to-high redshift clusters, and for depths and filter combinations appropriate for ongoing lensing surveys, we find that (1) spectroscopic selection can cause biases above the 10 per cent level, which can be reduced to ≈ 5 per cent by optimal lensing weighting, while (2) selection effects in the shape catalogue bias mass estimates at or below the 2 per cent level. This illustrates the importance of completeness of the reference catalogues for empirical redshift estimation.

We present the results of a search for extended X-ray sources and their corresponding galaxy groups from 800-ks Chandra coverage of the All-wavelength Extended Groth Strip International Survey (AEGIS). This yields one of the largest X-ray selected galaxy group catalogs from a blind survey to date. The red-sequence technique and spectroscopic redshifts allow us to identify 100% of reliable sources, leading to a catalog of 52 galaxy groups. The groups span the redshift range z ∼ 0.066 − 1.544 and virial mass range M 200 ∼ 1.34 × 10 13 − 1.33 × 10 14 M . For the 49 extended sources which lie within DEEP2 and DEEP3 Galaxy Redshift Survey coverage, we identify spectroscopic counterparts and determine velocity dispersions. We select member galaxies by applying different cuts along the line of sight or in projected spatial coordinates. A constant cut along the line of sight can cause a large scatter in scaling relations in low-mass or high-mass systems depending on the size of cut. A velocity dispersion based virial radius can more overestimate velocity dispersion in comparison to X-ray based virial radius for low mass systems. There is no significant difference between these two radial cuts for more massive systems. Independent of radial cut, overestimation of velocity dispersion can be created in case of existence of significant substructure and also compactness in X-ray emission which mostly occur in low mass systems. We also present a comparison between X-ray galaxy groups and optical galaxy groups detected using the Voronoi-Delaunay method (VDM) for DEEP2 data in this field.