Nonaxisymmetric Structures in the Stellar Disks of Galaxies

We study the azimuthal structure of the stellar disks of 18 face-on spiral galaxies, using K-band photometry to trace the stellar surface mass density. Assuming the disks are co-planar, we characterize their deviation from axisymmetry by the fractional amplitudes, A_i and phases of the azimuthal Fourier components at radii R about the photometric galaxy center. We find that most disks exhibit a wealth of non-axisymmetric structures, specifically: (1) that about one third of them are substantially lopsided (A_1/A_0>= 0.20) at 2.5 disk exponential scale length, (2) that almost one half of them have strong two-armed spirals with an arm/interarm surface-brightness contrasts of order unity, and (3) that typical disks have some intrinsic ellipticity. We estimate that in the disk plane the characteristic ellipticity of the underlying potential is 0.045+-0.03. However, the spiral pattern couples significantly to the estimate of the intrinsic ellipticity, and our measurement may represent an upper limit on the ``true" potential triaxiality. We estimate the radial streaming motions of the disk stars, $v_R$, which are produced by these distortions. By averaging over our sample of galaxies and all azimuthal angles, we find v_R=7km/s due to lopsided distortions and 6km/s due to intrinsic ellipticity. These non-circular motions are expected to contribute about 0.15mags scatter to measurements of the Tully-Fisher relation.Comment: to appear in the Astrophysical Journal, July 1, 1995. 29 pages, uuencoded. A file with the figures can be obtained via anonymous ftp from ftp://ibm-1.mpa-garching.mpg.de/pub/rix/nonaxisymm.figures.u