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A Face-On Tully-Fisher Relation

We construct the first "face-on" Tully-Fisher (TF) relation for 24 galaxies with inclinations between 16 degrees and 41 degrees. The enabling measurements are integral-field, echelle spectroscopy from the WIYN 3.5m telescope, which yield accurate kinematic estimates of disk inclination to 15 degrees. 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 40 degrees, we establish a direct path to linking this scatter to the unprojected structure of disks and making non-degenerate dynamical mass-decompositions of spiral galaxies.