Weak Lensing Results from the 75 Square Degree CTIO Survey

TL;DR

This study presents a 75 deg$^2$ weak-lensing survey using the CTIO BTC and Mosaic II instruments to measure ellipticities for ~2$\times10^6$ galaxies and extract E-mode lensing signals while testing for B-mode systematics. By employing an E/B decomposition and aperture-mass statistics, the authors detect coherent lensing at angular scales $\gtrsim30'$ and constrain the matter power spectrum normalization through $\sigma_8(\Omega_m/0.3)^{0.57} = 0.71^{+0.12}_{-0.16}$ (95% CL, assuming $\Gamma=0.21$), with a quantified systematic uncertainty from B-mode power. The large sky coverage reduces random errors and cosmic variance, and the use of spectroscopic redshift surveys for $N(z)$ minimizes depth-related systematics, though a residual B-mode signals remaining at small scales motivates future improvements in PSF modeling. Overall, the results are broadly aligned with CMB and galaxy-clustering constraints, support a low-to-mid $\Omega_m$ universe with $\sigma_8$ in a compatible range, and highlight the need to mitigate PSF-induced systematics for tighter cosmological inferences.

Abstract

We measure seeing-corrected ellipticities for 2 x 10^6 galaxies with magnitude R<23 in 12 widely separated fields totalling 75 deg^2 of sky. At angular scales >30\arcmin, 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 degrees, which is ~25h^-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 PSF distortions. The >30\arcmin data constrain the power spectrum of matter fluctuations on comoving scales of ~10h^-1 Mpc to have σ_8 (Ω_m/0.3)^{0.57} = 0.71^{+0.12}_{-0.16} (95% CL, \LambdaCDM, 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 previous weak-lensing results but generally consistent them, is at the lower end of the σ_8 range from various analyses of galaxy cluster abundances, and agrees with recent determinations from CMB 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.

Figures (12)

• Figure 1: Whisker plots of star ellipticities before and after processing for one of our BTC exposures. The length of each "whisker" is proportional to the magnitude of the ellipticity, and the orientation corresponds to the direction of the ellipticity. The whisker in the center corresponds to a 1% ellipticity. The remaining 1-2% ellipticity values after processing are seen to be essentially uncorrelated and are primarily due to measurement noise.
• Figure 2: Post-processing star shapes binned according to chip position for each of the BTC (left) and Mosaic (right) chips. The whiskers indicate the magnitude and orientation of this average shape. The central whisker in each plot corresponds to 1% ellipticity. The slight residuals which remain are well below the 1% level.
• Figure 3: Final post-processing shapes of the stars binned according to their initial observed shape for each of the two components of the ellipticity. The fitted slope is of order 1/300, leaving an rms residual effect of 0.03% and a maximum effect of less than 0.1%, which is well below the strength of the lensing signal.
• Figure 4: Post-processing galaxy shapes binned according to chip position for the BTC (left) and Mosaic (right) chips. For each shape observation, we subtract off the mean shape for that object as calculated from several observations from widely separated chip positions before binning. This removes the effect of shape noise. The central whisker in each plot corresponds to 1% ellipticity.
• Figure 5: Final post-processing galaxy shapes binned according to the PSF where they were observed. Clearly, there is still some bias relative to the PSF with a slope of order 0.015, which corresponds to a maximum bias of 0.4% for our worst PSFs, and a 0.2% rms effect. Note the greatly expanded vertical scale.