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′(2.2 μm)-band photometry to trace the stellar surface mass density. Assuming the disks are coplanar, we characterize their deviation from axisymmetry by the fractional amplitudes, A_m(R)/A₀(R), and phases, φ_m(R), of the mth azimuthal Fourier components at radii R about the photometric galaxy center. We find that most disks exhibit a wealth of nonaxisymmetric structures, specifically (1) that about one-third of them are substantially lopsided (A₁/A₀ ≳ 0.20) at 2.5 disk exponential scale length, (2) that almost half of them have strong two-armed spirals with an arm/interarm surface-brightness contrast 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_-0.02. 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〉 ∼ 7 km s^-1 due to lopsided distortions and 〈v_R〉 ∼ 6 km s^-1 due to intrinsic ellipticity. These noncircular motions are expected to contribute ∼0.15 mag scatter to measurements of the Tully-Fisher relation.