The galaxy octopole moment as a probe of weak-lensing shear fields

In this paper, we introduce the octopole moment of the light distribution in galaxies as a probe of the weak-lensing shear field. While traditional ellipticity estimates of the local shear derived from the quadrupole moment are limited by the width of the intrinsic ellipticity distribution of background galaxies, the dispersion in the intrinsic octopole distribution is expected to be much smaller, implying that the signal from this higher order moment is ultimately limited by measurement noise, and not by intrinsic scatter. We present the computation of the octopole moment and show that current observations are at the regime in which the octopole estimates will soon be able to contribute to the overall accuracy of the estimates of local shear fields. Therefore, the prospects for this estimator from future data sets such as the Advanced Camera for Survey and the Next Generation Space Telescope are very promising.