The Intrinsic Ellipticity of Spiral Disks

We have measured the distribution of intrinsic ellipticities for a sample of 28 relatively face–on spiral disks. We combine Hα velocity fields and R and I-band images to determine differences between kinematic and photometric inclination and position angles, from which we estimate intrinsic ellipticities of galaxy disks. Our findings suggest disks have a lognormal distribution of ellipticities (ǫ = 0.06) and span a range from ǫ = 0 (circular) to ǫ = 0.2. We are also able to construct a tight Tully-Fisher relation for our face-on sample. We use this to assess the contribution of disk ellipticity on the observed Tully-Fisher scatter. 1. Disk Ellipticity Survey Binney (1978) showed triaxial halos could affect disks by inducing warping and twists. In particular, the axis ratio of halos lead to disks which are intrinsically elliptical (Franx & de Zeeuw 1992; Jog 2000). Hence the ellipticity of disks may plausibly be used to estimate the axis ratios of dark matter halos. However, the inability to disentangle the ellipticity from the phase angle of this distortion makes such measurements difficult (e.g. Zaritsky & Rix 1995; Schoenmakers 1999). Andersen et al. (2001) presented a method which removed this degeneracy and yielded unique solutions for the disk ellipticity of nearly face-on galaxies by comparing kinematic and photometric inclination and position angles. This method assumes differences between these angles are solely the effect of ellipticity and not some other distortion. Here we present results of a larger study to define the distribution of disk ellipticities and to establish if ellipticity is related to other physical quantities, e.g., Tully-Fisher (TF) scatter. A sample of 39 galaxies were selected from the Principal Galaxy Catalog (Paturel et al. 1997) which have (1) t-types between 1.5–8.5, (2) axis ratios close to unity, (3) apparent disk sizes commensurate with the field of view of the integral field unit, DensePak, on the WIYN 3.5m telescope (Barden, Sawyer & Honeycutt 1998), and (4) low galactic absorption. We also required galaxies in the sample to be unbarred, isolated and have constant photometric position angles at three scale lengths. R and I-band images were acquired at the WIYN 3.5m, KPNO 2.1m, McDonald Observatory 2.7m telescopes. We used these images to measure axis ratios and position angles in a way designed to be unaffected by warps or spiral