A face-on Tully-Fisher relation

We construct the first face-on Tully-Fisher (TF) relation for 24 galaxies with inclinations between 16degrees and 41degrees. The enabling measurements are integral-field echelle spectroscopy from the WIYN 3.5 m telescope, which yield accurate kinematic estimates of disk inclination to similar to15degrees. Kinematic inclinations are of sufficient accuracy that our measured TF scatter of 0.42 mag is comparable to other surveys even without internal absorption corrections. Three of four galaxies with significant kinematic and photometric asymmetries also have the largest deviations from our TF relation, suggesting that asymmetries make an important contribution to TF scatter. By measuring inclinations below 40degrees, we establish a direct path to linking this scatter to the unprojected structure of disks and making nondegenerate dynamical mass decompositions of spiral galaxies.