Measuring galaxy asymmetries in 3D

Abstract

One of the commonly used non-parametric morphometric statistics for galaxy profiles and images is the asymmetry statistic. With an eye to current and upcoming large neutral hydrogen (H i) surveys, we develop a 3D version of the asymmetry statistic that can be applied to datacubes. This statistic is more resilient to variations due to the observed geometry than 1D asymmetry measures, and can be successfully applied to lower spatial resolutions (3-4 beams across the galaxy major axis) than the 2D statistic. We have also modified the asymmetry definition from an ‘absolute difference’ version to a ‘squared difference’ version that removes much of the bias due to noise contributions for low signal-to-noise observations. Using a suite of mock asymmetric cubes we show that the background-corrected, squared difference 3D asymmetry statistic can be applied to many marginally resolved galaxies in large wide-area H i surveys such as WALLABY on the Australian SKA Pathfinder (ASKAP).