A measurement of disk ellipticity in nearby spiral galaxies

We have measured the intrinsic disk ellipticity for seven nearby, nearly fa ce-on spiral galaxies by combining Densepak integral-field spectroscopy wit h I-band imaging from the WIYN telescope. Initially assuming an axisymmetri c model, we determine the kinematic inclinations and position angles from H alpha velocity fields and the photometric axis ratios and position angles from imaging data. We interpret the observed disparities between kinematic and photometric disk parameters in terms of an intrinsic nonzero ellipticit y epsilon. The mean ellipticity of our sample is 0.05. If the majority of d isk galaxies have such intrinsic axis ratios, this would account for roughl y 50% of the scatter in the Tully-Fisher relation. This result, in turn, pl aces tighter constraints on other sources of scatter in this relation, the most astrophysically compelling of which is galaxy mass-to-light ratios.