A Face–On Tully-Fisher Relation

We construct the first “face-on” Tully-Fisher (TF) relation for 24 galaxies with inclinations between 16◦ and 41◦. The enabling measurements are integralfield, echelle spectroscopy from the WIYN 3.5m telescope, which yield accurate kinematic estimates of disk inclination to ∼ 15◦. 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◦, 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. Subject headings: galaxies: fundamental parameters — galaxies: structure — galaxies: kinematics and dynamics — galaxies: spiral