The Intrinsic Shape of Spiral Galaxies in the 2MASS Large Galaxy Atlas

Abstract

The apparent shapes of spiral galaxies in the 2-Micron All Sky Survey Large Galaxy Atlas are used to constrain the intrinsic shape of their disks. When the distribution of apparent axis ratios is estimated using a nonparametric kernel method, the shape distribution is inconsistent with axisymmetry at the 90% confidence level in the B band and at the 99% confidence level in the K band. If spirals are subdivided by Hubble type, the late-type spirals (Sc and later) are consistent with axisymmetry, while the earlier spirals are strongly inconsistent with axisymmetry. The distribution of disk ellipticity can be fitted adequately with either a Gaussian or a lognormal distribution. The best fits for the late spirals imply a median ellipticity of epsilon = 0.07 in the B band and epsilon = 0.02 in the K band. For the earlier spirals, the best fits imply a median ellipticity of epsilon = 0.18 in the B band and epsilon = 0.30 in the K band. The observed scatter in the Tully-Fisher relation, for both late and early spirals, is consistent with the disk ellipticity measured in the B band. This indicates that excluding spirals of Hubble type earlier than Sc will reduce the intrinsic scatter in the Tully-Fisher relation used as a distance indicator.