Showing papers by "Hendrik Hildebrandt published in 2012"

CFHTLenS: the Canada-France-Hawaii Telescope Lensing Survey

Abstract: We present the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) that accurately determines a weak gravitational lensing signal from the full 154 deg^2 of deep multicolour data obtained by the CFHT Legacy Survey. Weak gravitational lensing by large-scale structure is widely recognized as one of the most powerful but technically challenging probes of cosmology. We outline the CFHTLenS analysis pipeline, describing how and why every step of the chain from the raw pixel data to the lensing shear and photometric redshift measurement has been revised and improved compared to previous analyses of a subset of the same data. We present a novel method to identify data which contributes a non-negligible contamination to our sample and quantify the required level of calibration for the survey. Through a series of cosmology-insensitive tests we demonstrate the robustness of the resulting cosmic shear signal, presenting a science-ready shear and photometric redshift catalogue for future exploitation.

The next generation virgo cluster survey (ngvs). i. introduction to the survey

Abstract: The Next Generation Virgo Cluster Survey (NGVS) is a program that uses the 1 deg2 MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical imaging survey of the Virgo cluster, from its core to its virial radius—covering a total area of 104 deg2—in the u*griz bandpasses. Thanks to a dedicated data acquisition strategy and processing pipeline, the NGVS reaches a point-source depth of g ≈ 25.9 mag (10σ) and a surface brightness limit of μ g ~ 29 mag arcsec–2 (2σ above the mean sky level), thus superseding all previous optical studies of this benchmark galaxy cluster. In this paper, we give an overview of the technical aspects of the survey, such as areal coverage, field placement, choice of filters, limiting magnitudes, observing strategies, data processing and calibration pipelines, survey timeline, and data products. We also describe the primary scientific topics of the NGVS, which include: the galaxy luminosity and mass functions; the color-magnitude relation; galaxy scaling relations; compact stellar systems; galactic nuclei; the extragalactic distance scale; the large-scale environment of the cluster and its relationship to the Local Supercluster; diffuse light and the intracluster medium; galaxy interactions and evolutionary processes; and extragalactic star clusters. In addition, we describe a number of ancillary programs dealing with "foreground" and "background" science topics, including the study of high-inclination trans-Neptunian objects; the structure of the Galactic halo in the direction of the Virgo Overdensity and Sagittarius Stream; the measurement of cosmic shear, galaxy-galaxy, and cluster lensing; and the identification of distant galaxy clusters, and strong-lensing events.

CFHTLenS: Improving the quality of photometric redshifts with precision photometry

Abstract: Here we present the results of various approaches to measure accurate colours and photometric redshifts (photo-z) from wide-field imaging data. We use data from the Canada–France–Hawaii Telescope Legacy Survey which have been re-processed by the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS) team in order to carry out a number of weak gravitational lensing studies. An emphasis is put on the correction of systematic effects in the photo-z arising from the different point spread functions (PSFs) in the five optical bands. Different ways of correcting these effects are discussed and the resulting photo-z accuracies are quantified by comparing the photo-z to large spectroscopic redshift (spec-z) data sets. Careful homogenization of the PSF between bands leads to increased overall accuracy of photo-z. The gain is particularly pronounced at fainter magnitudes where galaxies are smaller and flux measurements are affected more by PSF effects. We discuss ways of defining more secure subsamples of galaxies as well as a shape- and colour-based star–galaxy separation method, and we present redshift distributions for different magnitude limits. We also study possible re-calibrations of the photometric zero-points (ZPs) with the help of galaxies with known spec-z. We find that if PSF effects are properly taken into account, a re-calibration of the ZPs becomes much less important suggesting that previous such re-calibrations described in the literature could in fact be mostly corrections for PSF effects rather than corrections for real inaccuracies in the ZPs. The implications of this finding for future surveys like the Kilo Degree Survey (KiDS), Dark Energy Survey (DES), Large Synoptic Survey Telescope or Euclid are mixed. On the one hand, ZP re-calibrations with spec-z values might not be as accurate as previously thought. On the other hand, careful PSF homogenization might provide a way out and yield accurate, homogeneous photometry without the need for full spectroscopic coverage. This is the first paper in a series describing the technical aspects of CFHTLenS.

CFHTLenS: The Canada-France-Hawaii Telescope Lensing Survey

Abstract: We present the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) that accurately determines a weak gravitational lensing signal from the full 154 square degrees of deep multi-colour data obtained by the CFHT Legacy Survey. Weak gravitational lensing by large-scale structure is widely recognised as one of the most powerful but technically challenging probes of cosmology. We outline the CFHTLenS analysis pipeline, describing how and why every step of the chain from the raw pixel data to the lensing shear and photometric redshift measurement has been revised and improved compared to previous analyses of a subset of the same data. We present a novel method to identify data which contributes a non-negligible contamination to our sample and quantify the required level of calibration for the survey. Through a series of cosmology-insensitive tests we demonstrate the robustness of the resulting cosmic shear signal, presenting a science-ready shear and photometric redshift catalogue for future exploitation.

Magnification by Galaxy Group Dark Matter Halos

Abstract: We report on the detection of gravitational lensing magnification by a population of galaxy groups, at a significance level of 4.9σ. Using X-ray-selected groups in the COSMOS 1.64 deg^2 field, and high-redshift Lyman break galaxies as sources, we measure a lensing-induced angular cross-correlation between the samples. After satisfying consistency checks that demonstrate we have indeed detected a magnification signal, and are not suffering from contamination by physical overlap of samples, we proceed to implement an optimally weighted cross-correlation function to further boost the signal to noise of the measurement. Interpreting this optimally weighted measurement allows us to study properties of the lensing groups. We model the full distribution of group masses using a composite-halo approach, considering both the singular isothermal sphere and Navarro-Frenk-White profiles, and find our best-fit values to be consistent with those recovered using the weak-lensing shear technique. We argue that future weak-lensing studies will need to incorporate magnification along with shear, both to reduce residual systematics and to make full use of all available source information, in an effort to maximize scientific yield of the observations.

The Cosmic Web and galaxy evolution around the most luminous X-ray cluster: RX J1347.5−1145

Abstract: In this paper we study large-scale structures and their galaxy content around the most X-ray luminous cluster known, RX J1347.5−1145 at z= 0.45. We make use of u*g′r′i′z′ Canada–France–Hawaii Telescope (CFHT) MegaCam photometry together with VIsible MultiObject Spectrograph (VIMOS) Very Large Telescope (VLT) spectroscopy to identify structures around RX J1347 on a scale of ∼20 × 20 Mpc2. We construct maps of the galaxy distribution and the fraction of blue galaxies. We study the photometric galaxy properties as a function of the environment traced by the galaxy density. We identify group candidates based on galaxy overdensities and study their galaxy content. We also use available Galaxy Evolution Explorer (GALEX) near-ultraviolet imaging to identify strong unobscured star-forming galaxies. We find that the large-scale structure around RX J1347 extends in the north-east–south-west direction for at least 20 Mpc, over a region in which most of the group candidates are located, some of which show X-ray emission in archival XMM–Newton observations. As in other studies, we find that the fraction of blue galaxies (Fblue) is a function of galaxy number density, but the bulk of the trend is due to galaxies belonging to massive systems. The fraction of ultraviolet- (UV-) bright galaxies is also a function of environment, but their relative number compared with the blue population seems to be constant regardless of the environment. These UV emitters also have similar properties at all galaxy densities, indicating that the transition between galaxy types occurs over short time-scales. Candidate galaxy groups show a large variation in their galaxy content and Fblue in those groups displays little dependence on galaxy number density. This may indicate possible differences in their evolutionary status, or suggest that the processes acting in groups are different from those in clusters. The large-scale structure around rich clusters provides a dynamic environment for galaxy evolution. In the case of RX J1347, the transformation may start within infalling groups and finish with the removal of the cold gas once galaxies are accreted into massive systems.