Unbiased Cluster Lens Reconstruction

Abstract

Weak lensing observations measure the shear field and hence the gradient of the dimensionless surface density $\kappa$. We present several new algorithms to recover $\kappa$ {}from shear estimates on a finite region and compare how they perform with realistically noisy data. The reconstruction methods studied here are divided into 2 classes: direct reconstruction and regularized inversion. Direct reconstruction techniques express $\kappa$ as a 2D integral of the shear field. This yields an estimator for $\kappa$ as a discrete sum over background galaxy ellipticities which is straightforward to implement, and allows a simple estimate of the noise. We study 3 types of direct reconstruction methods: 1) $\kappa$-estimators that measure the surface density at any given target point relative to the mean value in a reference region 2) a method that explicitly attempts to minimize the rotational part of $\nabla \kappa$ that is due to noise and 3) a novel, exact Fourier-space inverse gradient operator. We also develop two `regularized maximum likelihood' methods, one of which employs the conventional discrete Laplacian operator as a regularizer and the other uses regularization of all components in Fourier space. We compare the performance of all the estimators by means of simulations and noise power analysis. A general feature of these unbiased methods is an enhancement of the low frequency power which, for some of the methods, can be quite severe. We find the best performance is provided by the maximum likelihood method with Fourier space regularization although some of the other methods perform almost as well.