K. Kuijken

CFHTLenS: the Canada–France–Hawaii Telescope Lensing Survey

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.

Evidence of the accelerated expansion of the Universe from weak lensing tomography with COSMOS

We present a comprehensive analysis of weak gravitational lensing by large-scale structure in the Hubble Space Telescope Cosmic Evolution Survey (COSMOS), in which we combine space-based galaxy shape measurements with ground-based photometric redshifts to study the redshift dependence of the lensing signal and constrain cosmological parameters. After applying our weak lensing-optimized data reduction, principal-component interpolation for the spatially, and temporally varying ACS point-spread function, and improved modelling of charge-transfer inefficiency, we measured a lensing signal that is consistent with pure gravitational modes and no significant shape systematics. We carefully estimated the statistical uncertainty from simulated COSMOS-like fields obtained from ray-tracing through the Millennium Simulation, including the full non-Gaussian sampling variance. We tested our lensing pipeline on simulated space-based data, recalibrated non-linear power spectrum corrections using the ray-tracing analysis, employed photometric redshift information to reduce potential contamination by intrinsic galaxy alignments, and marginalized over systematic uncertainties. We find that the weak lensing signal scales with redshift as expected from general relativity for a concordance ACDM cosmology, including the full cross-correlations between different redshift bins. Assuming a flat ACDM cosmology, we measure σ 8 (Ω m /0.3) 0.51 = 0.75 ± 0.08 from lensing, in perfect agreement with WMAP-5, yielding joint constraints Ω m = 0.266 +0.025 -0.023 σ 8 = 0.802 +0.028 -0.029 (all 68.3% conf.). Dropping the assumption of flatness and using priors from the HST Key Project and Big-Bang nucleosynthesis only, we find a negative deceleration parameter q 0 at 94.3% confidence from the tomographic lensing analysis, providing independent evidence of the accelerated expansion of the Universe. For a flat ωCDM cosmology and prior ω ∈ [-2, 0], we obtain ω < -0.41 (90% conf.). Our dark energy constraints are still relatively weak solely due to the limited area of COSMOS. However, they provide an important demonstration of the usefulness of tomographic weak lensing measurements from space.

CFHTLenS: the Canada-France-Hawaii Telescope Lensing Survey - imaging data and catalogue products

We present data products from the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS). CFHTLenS is based on the Wide component of the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS). It encompasses 154 deg^2 of deep, optical, high-quality, sub-arcsecond imaging data in the five optical filters u*g′r′i′z′. The scientific aims of the CFHTLenS team are weak gravitational lensing studies supported by photometric redshift estimates for the galaxies. This paper presents our data processing of the complete CFHTLenS data set. We were able to obtain a data set with very good image quality and high-quality astrometric and photometric calibration. Our external astrometric accuracy is between 60 and 70 mas with respect to Sloan Digital Sky Survey (SDSS) data, and the internal alignment in all filters is around 30 mas. Our average photometric calibration shows a dispersion of the order of 0.01–0.03 mag for g′r′i′z′ and about 0.04 mag for u* with respect to SDSS sources down to i_(SDSS) ≤ 21. We demonstrate in accompanying papers that our data meet necessary requirements to fully exploit the survey for weak gravitational lensing analyses in connection with photometric redshift studies. In the spirit of the CFHTLS, all our data products are released to the astronomical community via the Canadian Astronomy Data Centre at http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/community/CFHTLens/query.html. We give a description and how-to manuals of the public products which include image pixel data, source catalogues with photometric redshift estimates and all relevant quantities to perform weak lensing studies.

Bayesian galaxy shape measurement for weak lensing surveys - III. Application to the Canada-France-Hawaii Telescope Lensing Survey

A likelihood-based method for measuring weak gravitational lensing shear in deep galaxy surveys is described and applied to the Canada–France–Hawaii Telescope (CFHT) Lensing Survey (CFHTLenS). CFHTLenS comprises 154 deg^2 of multi-colour optical data from the CFHT Legacy Survey, with lensing measurements being made in the i′ band to a depth i′_(AB) < 24.7, for galaxies with signal-to-noise ratio ν_(SN) ≳ 10. The method is based on the lensfit algorithm described in earlier papers, but here we describe a full analysis pipeline that takes into account the properties of real surveys. The method creates pixel-based models of the varying point spread function (PSF) in individual image exposures. It fits PSF-convolved two-component (disc plus bulge) models to measure the ellipticity of each galaxy, with Bayesian marginalization over model nuisance parameters of galaxy position, size, brightness and bulge fraction. The method allows optimal joint measurement of multiple, dithered image exposures, taking into account imaging distortion and the alignment of the multiple measurements. We discuss the effects of noise bias on the likelihood distribution of galaxy ellipticity. Two sets of image simulations that mirror the observed properties of CFHTLenS have been created to establish the methods accuracy and to derive an empirical correction for the effects of noise bias.

CFHTLenS: Improving the quality of photometric redshifts with precision photometry

Here we present the results of various approaches to measure accurate colours and photometric redshifts (photo-z’s) from wide-field imaging data. We use data from the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS) which have been re- processed by the CFHT 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’s arising from the different Point Spread Functions (PSF) 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’s to large spectroscopic redshift (spec-z) data sets. Careful homogenisation of the PSF between bands leads to increased overall accuracy of photo-z’s. The gain is particularly pronounced at fainter magnitudes where galaxies are smaller and flux measurements are affected more by PSF-effects. We also study possible re- calibrations of the photometric zeropoints (ZPs) with the help of galaxies with known spec-z’s. 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 KiDS, DES, LSST, or Euclid are mixed. On the one hand, ZP re-calibrations with spec-z’s might not be as accurate as previously thought. On the other hand, careful PSF homogenisation 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. (abridged)

Kinematic properties of early-type galaxy haloes using planetary nebulae★

We present new planetary nebulae (PNe) positions, radial velocities and magnitudes for six early-type galaxies obtained with the Planetary Nebulae Spectrograph (PNS), along with derived two-dimensional velocity and velocity dispersion fields, and the α parameters (i.e. the number of PNe per unit luminosity). We also present new deep absorption-line long-slit kinematics for three galaxies in the sample, obtained with the FOcal Reducer and low dispersion Spectrograph (FORS2) at the Very Large Telescope (VLT). We extend this study to include additional 10 early-type galaxies with PNe radial velocity measurements available from the literature, including previous PNS studies, in order to obtain a broader description of the outer-halo kinematics in early-type galaxies. These data extend the information derived from stellar absorption-line kinematics to typically several and up to 8 effective radii. The combination of photometry, absorption-line and PNe kinematics shows (i) a good agreement between the PNe number density distribution and the stellar surface brightness in the region where the two data sets overlap; (ii) a good agreement between PNe and absorption-line kinematics; (iii) that the mean rms velocity profiles fall into two groups, with part of the galaxies characterized by slowly decreasing profiles and the remainder having steeply falling profiles; (iv) a larger variety of velocity dispersion radial profiles; (v) that twists and misalignments in the velocity fields are more frequent at large radii, including some fast rotator galaxies; (vi) that outer haloes are characterized by more complex radial profiles of the specific angular momentum-related λ_R parameter than observed within 1 R_e; (vii) that many objects are more rotationally dominated at large radii than in their central parts and (viii) that the halo kinematics are correlated with other galaxy properties, such as total B band and X-ray luminosity, isophotal shape, total stellar mass, V/σ and α parameter, with a clear separation between fast and slow rotators. Based in part on observations made with the William Herschel Telescope operated by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos on the island of La Palma, of the Instituto de Astrofisica de Canarias, and on observations collected at the European Southern Observatory, Chile, Program: 76.B-0788(A). E-mail: lcoccato@mpe.mpg.de

Galaxy and Mass Assembly (GAMA): The GAMA Galaxy Group Catalogue (G3Cv1)

Using the complete Galaxy and Mass Assembly I (GAMA-I) survey covering ∼142 deg2 to rAB= 19.4, of which ∼47 deg2 is to rAB= 19.8, we create the GAMA-I galaxy group catalogue (G3Cv1), generated using a friends-of-friends (FoF) based grouping algorithm. Our algorithm has been tested extensively on one family of mock GAMA lightcones, constructed from Λ cold dark matter N-body simulations populated with semi-analytic galaxies. Recovered group properties are robust to the effects of interlopers and are median unbiased in the most important respects. G3Cv1 contains 14 388 galaxy groups (with multiplicity ≥2), including 44 186 galaxies out of a possible 110 192 galaxies, implying ∼40 per cent of all galaxies are assigned to a group. The similarities of the mock group catalogues and G3Cv1 are multiple: global characteristics are in general well recovered. However, we do find a noticeable deficit in the number of high multiplicity groups in GAMA compared to the mocks. Additionally, despite exceptionally good local spatial completeness, G3Cv1 contains significantly fewer compact groups with five or more members, this effect becoming most evident for high multiplicity systems. These two differences are most likely due to limitations in the physics included of the current GAMA lightcone mock. Further studies using a variety of galaxy formation models are required to confirm their exact origin. The G3Cv1 catalogue will be made publicly available as and when the relevant GAMA redshifts are made available at http://www.gama-survey.org.

Discovery of Three z > 6.5 Quasars in the VISTA Kilo-Degree Infrared Galaxy (VIKING) Survey

Studying quasars at the highest redshifts can constrain models of galaxy and black hole formation, and it also probes the intergalactic medium in the early universe. Optical surveys have to date discovered more than 60 quasars up to z 6.4, a limit set by the use of the z-band and CCD detectors. Only one z 6.4 quasar has been discovered, namely the z = 7.08 quasar ULAS J1120+0641, using near-infrared imaging. Here we report the discovery of three new z 6.4 quasars in 332 deg2 of the Visible and Infrared Survey Telescope for Astronomy Kilo-degree Infrared Galaxy (VIKING) survey, thus extending the number from 1 to 4. The newly discovered quasars have redshifts of z = 6.60, 6.75, and 6.89. The absolute magnitudes are between –26.0 and –25.5, 0.6-1.1 mag fainter than ULAS J1120+0641. Near-infrared spectroscopy revealed the Mg II emission line in all three objects. The quasars are powered by black holes with masses of ~(1-2) × 109 M ☉. In our probed redshift range of 6.44 109 M ☉) > 1.1 × 10–9 Mpc–3. The discovery of three quasars in our survey area is consistent with the z = 6 quasar luminosity function when extrapolated to z ~ 7. We do not find evidence for a steeper decline in the space density of quasars with increasing redshift from z = 6 to z = 7.

Galaxy and Mass Assembly (GAMA): the star formation rate dependence of the stellar initial mass function

The stellar initial mass function (IMF) describes the distribution in stellar masses produced from a burst of star formation. For more than 50 yr, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the high-mass IMF slope for a sample of low-to-moderate redshift galaxies from the Galaxy and Mass Assembly survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMF–star formation rate dependency is discovered, which shows that highly star-forming galaxies form proportionally more massive stars (they have IMFs with flatter power-law slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF.

Galaxy And Mass Assembly (GAMA): spectroscopic analysis

The Galaxy And Mass Assembly (GAMA) survey is a multiwavelength photometric and spectroscopic survey, using the AAOmega spectrograph on the Anglo-Australian Telescope to obtain spectra for up to ∼300 000 galaxies over 280 deg2, to a limiting magnitude of rpet < 19.8 mag. The target galaxies are distributed over 0 < z ≲ 0.5 with a median redshift of z ≈ 0.2, although the redshift distribution includes a small number of systems, primarily quasars, at higher redshifts, up to and beyond z = 1. The redshift accuracy ranges from σv ≈ 50 km s−1 to σv ≈ 100 km s−1 depending on the signal-to-noise ratio of the spectrum. Here we describe the GAMA spectroscopic reduction and analysis pipeline. We present the steps involved in taking the raw two-dimensional spectroscopic images through to flux-calibrated one-dimensional spectra. The resulting GAMA spectra cover an observed wavelength range of 3750 ≲ λ ≲ 8850 A at a resolution of R ≈ 1300. The final flux calibration is typically accurate to 10–20 per cent, although the reliability is worse at the extreme wavelength ends, and poorer in the blue than the red. We present details of the measurement of emission and absorption features in the GAMA spectra. These measurements are characterized through a variety of quality control analyses detailing the robustness and reliability of the measurements. We illustrate the quality of the measurements with a brief exploration of elementary emission line properties of the galaxies in the GAMA sample. We demonstrate the luminosity dependence of the Balmer decrement, consistent with previously published results, and explore further how Balmer decrement varies with galaxy mass and redshift. We also investigate the mass and redshift dependencies of the [N II]/Hα versus [O III]/Hβ spectral diagnostic diagram, commonly used to discriminate between star forming and nuclear activity in galaxies.