Abstract. We report a measurement of cosmic shear correlations using an effective area of 6.5 deg 2 of the VIRMOS deep imaging survey in progress at the Canada-France-Hawaii Telescope. We measured various shear correlation functions, the aperture mass statistic and the top-hat smoothed variance of the shear with a detection significance exceeding 12 σ. We present results on angular scales from 3 arcsec to half a degree. The lensing origin of the signal is confirmed through tests that rely on the scalar nature of the gravitational potential. The different statistical measures give consistent results over the full range of angular scales. These important tests of the measurements demonstrate that the measured correlations could provide accurate constraints on cosmological parameters, subject to the systematic uncertainty in the source redshift distribution. The measurement over more than two decades of scale allows one to evaluate the effect of the shape of the power spectrum on cosmological parameter estimation. The degeneracy on σ8 − Ω0 can be broken if priors on the shape of the linear power spectrum (parameterized by Γ) are assumed. For instance, with Γ = 0.21 and at the 95% confidence level, we obtain 0.65 < σ8 < 1.2 and 0.22 < Ω0 < 0.55 for open models, and σ8 > 0.7 and Ω0 < 0.4 for flat (Λ-CDM) models. We discuss how these results would scale if the assumed source redshift distribution needed to be modified with forthcoming measurements of photometric redshifts. From the tangential/radial mode decomposition we can set an upper limit on the intrinsic shape alignment, which has recently been suggested as a possible contribution to the lensing signal. Within the error bars, there is no detection of intrinsic shape alignment for scales larger than 1 .

We combine weak lensing measurements from the Red-Sequence Cluster Survey (RCS) and the VIRMOS-DESCART survey, and present the first direct measurements of the bias parameter b and the galaxy-mass cross-correlation coefficient r on scales ranging from 0.2 to 9.3 h −1 50 Mpc (which correspond to aperture radii of 1.5 ′ − 45 ′ ) at a lens redshift z ≃ 0.35. We find strong evidence that both b and r change with scale for our sample of lens galaxies (19.5 < R C < 21), which have luminosities around L * . For the currently favored cosmology (Ω m = 0.3, Ω Λ = 0.7), we find b = 0.71 +0.06 −0.04 (68% confidence) on a scale of 1 − 2h −1 50 Mpc, increasing to ∼ 1 on larger scales. The value of r has only minimal dependence on the assumed cosmology. The variation of r with scale is very similar to that of b, and reaches a minimum value of r ∼ 0.57 +0.08 −0.07 (at 1h −1 50 Mpc; 68% confidence). This suggests significant stochastic biasing and/or non-linear biasing. On scales larger than ∼ 4h −1 50 Mpc the value of r is consistent with a value of r = 1. In addition we use RCS data alone to measure the ratio b/r on scale ranging from 0.15 to 12.5 h −1 50 Mpc (1 ′ − 60 ′ ) and find that the ratio varies somewhat with scale. We obtain an average value of b/r = 1.090 ± 0.035, in good agreement with previous estimates. A (future) careful comparison of our results with models of galaxy formation can provide unique constraints, as r is linked intimately to the details of galaxy formation.

No abstract

Abstract. We present a gravitational lensing analysis of the cluster of galaxies MS 1008-1224 (z = 0.31), based on very deep observations obtained using the VLT with FORS1 and ISAAC during the science verification phase. Two different mass reconstruction algorithms were applied to the B-, V -, R-and I-band data to obtain similar projected mass distributions in all the bands. The FORS1 (BVRI) and ISAAC (JK) data were combined to determine the photometric redshift distribution of galaxies within the ISAAC field and to estimate the mass. We inferred from weak shear a minimum mass of 2.3 × 10 14 h −1 M on large scales (within ∼700 h −1 kpc, diameter) which agrees well with the X-ray mass mass estimate. The Mass-to-light ratios are also in excellent agreement. The observed mass profile is consistent with Pseudo-Isothermal Sphere models as well as a Navarro, Frenk and White model. In the inner regions the lensing mass is about twice as high as the X-ray mass which supports the long-held view that complex physical processes occuring in the innermost parts of lensing-clusters are mainly responsible for the X-ray-lensing mass discrepancy. We found that the central part of the cluster comprises two mass peaks whose center of mass is located 10-20 arcsec north of the cD galaxy. A similar offset between the cD and the peak of the X-ray distribution has been reported before. The optical, X-ray and the mass distributions show that MS 1008-1224 is composed of several subsystems which are probably undergoing a merger. It is likely that the gas is not in equilibrium in the innermost regions which vitiates the X-ray mass estimate there. We discovered that MS 1008-1224 shows a remarkable case of cluster-cluster lensing. The photometric redshifts show an excess of galaxies located 30 arcsec south-west of the cD galaxy at a redshift of ∼0.9. This distant cluster is therefore also lensed by MS 1008-1224, which, if confirmed with spectroscopic data, would make this the first known case of magnification of a distant cluster by another one.