A Measurement of Disk Ellipticity in Nearby Spiral Galaxies

We have measured the intrinsic disk ellipticity for seven nearby, nearly face-on spiral galaxies by combining Densepak integral-field spectroscopy with I-band imaging from the WIYN telescope. Initially assuming an axisymmetric model, we determine the kinematic inclinations and position angles from Hα 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 ellipticity ϵ. The mean ellipticity of our sample is 0.05. If the majority of disk galaxies have such intrinsic axis ratios, this would account for roughly 50% of the scatter in the Tully-Fisher relation. This result, in turn, places tighter constraints on other sources of scatter in this relation, the most astrophysically compelling of which is galaxy mass-to-light ratios.