We have measured the intrinsic disk ellipticity for 7 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 kinematic inclinations and position angles from H \alpha velocity fields , and 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 non-zero ellipticity , \epsilon .
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 .