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

The apparent shapes of spiral galaxies in the 2-Micron All Sky Survey LargeGalaxy Atlas are used to constrain the intrinsic shape of their disks. When thedistribution of apparent axis ratios is estimated using a nonparametric kernelmethod, 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 stronglyinconsistent with axisymmetry. The distribution of disk ellipticity can befitted adequately with either a Gaussian or a lognormal distribution. The bestfits for the late spirals imply a median ellipticity of epsilon = 0.07 in the Bband and epsilon = 0.02 in the K band. For the earlier spirals, the best fitsimply a median ellipticity of epsilon = 0.18 in the B band and epsilon = 0.30in the K band. The observed scatter in the Tully-Fisher relation, for both lateand early spirals, is consistent with the disk ellipticity measured in the Bband. This indicates that excluding spirals of Hubble type earlier than Sc willreduce the intrinsic scatter in the Tully-Fisher relation used as a distanceindicator.Comment: 30 pages, 9 figures, to appear in Ap