Cosmic Shear with Keck: Systematic Effects

Cosmic shear probes the distribution of dark matter via gravitational lensing of distant, background galaxies. We describe our cosmic shear survey consisting of deep blank fields observed with the Keck II telescope. We have found biases in the standard weak lensing analysis, which are enhanced by the elongated geometry of the Keck fields. We show how these biases can be diagnosed and corrected by masking edges and chip defects. Images of background galaxies become weakly distorted by gravitational lensing as light travels through intervening large-scale structure to the observer. These distortions provide a direct measurement of the mass distribution in the universe, without assumptions about the nature, state or luminosity of the dark matter. This “cosmic shear” effect is small, with the axis ratio of a typical galaxy being elongated by only about 1%. However, adjacent lines of sight have passed through the same large-scale structures and close (∼1’) pairs of background galaxies receive coherent distortions. After removing systematic shape measurement biases, we may average over many galaxy pairs to detect this signal. We present a survey covering 0.6 square degrees to R ∼ 26, scattered randomly around the sky in 173 pointings of the Echelle Spectrograph and Imager on Keck II. After cuts, our final catalogue contains 63,000 galaxies. Shear Measurement and Elimination of Systematic Biases Measurement of such a small effect demands careful data reduction and elimination of systematic errors. Bias subtraction and flat fielding were optimised using the background level and the overscan regions at the edge of individual exposures. Three slightly dithered exposures were then astrometrically realigned and co-added to produce each survey field. As a by-product, this allowed the monitoring of instrumental distortion at different telescope positions. Keck proves ideal for cosmic shear, with negligible <0.3% internal distortions even at the edges of the 2’×8’ field of view. The PSF shape was measured from stars across each reduced field. The isotropic and anisotropic contributions of the PSF to galaxy shapes was then corrected using the KSB method (Kaiser, Squires, & Broadhurst 1995), which is well tested and calibrated (Bacon et al. 2001; Erben et al. 2001).