Weak-Lensing Results from the 75 Square Degree Cerro Tololo Inter-American Observatory Survey

Abstract

We measure seeing-corrected ellipticities for 2 × 10 6 galaxies with magnitude R ≤ 23 in 12 widely separated fields totaling 75 deg 2 of sky. At angular scales ≳30', ellipticity correlations are detected at high significance and exhibit nearly the pure " E mode" behavior expected of weak gravitational lensing. Even when smoothed to the full field size of 2 5, which is ≈25 h -1 Mpc at the lens distances, an rms shear variance of ⟨γ 2 ⟩ 1/2 = 0.0012 ± 0.0003 is detected. At smaller angular scales, there is significant " B -mode" power, an indication of residual uncorrected point-spread function distortions. The data at scales above 30' constrain the power spectrum of matter fluctuations on comoving scales of ≈10 h -1 Mpc to have σ 8 (Ω m /0.3) 0.57 = 0.71 (95% confidence level, ΛCDM, Γ = 0.21), where the systematic error includes statistical and calibration uncertainties, cosmic variance, and a conservative estimate of systematic contamination based upon the detected B -mode signal. This normalization of the power spectrum is lower than, but generally consistent with, previous weak-lensing results, is at the lower end of the σ 8 range from various analyses of galaxy cluster abundances, and agrees with recent determinations from cosmic microwave background and galaxy clustering. The large and dispersed sky coverage of our survey reduces random errors and cosmic variance, while the relatively shallow depth allows us to use existing redshift survey data to reduce systematic uncertainties in the N ( z ) distribution to insignificance. Reanalysis of the data with more sophisticated algorithms will hopefully reduce the systematic ( B mode) contamination and allow more precise, multidimensional constraint of cosmological parameters.