Tools for Identifying Spurious Luminosity Offsets in Tully-Fisher Studies: Application at Low Redshift and Implications for High Redshift

Studies of high-redshift galaxies usually interpret offsets from the Tully- Fisher (TF) relation as luminosity evolution. However, apparent luminosity offsets may actually reflect rotation curve anomalies such as strong asymmetries or radial truncation, due to galaxy interactions or low S/N data. Here we analyze emission-line rotation curves from two low-z samples: the Close Pairs Survey, which contains many interacting galaxies, and the Nearby Field Galaxy Survey (NFGS), which represents the general galaxy population. In the spirit of high-z samples, these surveys reflect the natural diversity of emission-line galaxies, including peculiar and early-type morphologies. We adopt objective criteria to identify severe kinematic anomalies. In the Close Pairs Survey, TF offsets associated with such anomalies collectively resemble the ``differential luminosity evolution'' claimed in some high-z studies, with larger offsets at lower luminosities. With anomalous galaxies rejected, however, the TF relations for the two surveys are quite similar. Furthermore, the two surveys follow similar relations between color and TF residuals, with the Pairs Survey relation extending to bluer colors and brighter TF residuals. Strong outliers from this relation are nearly always kinematically anomalous. Thus the color-TF residual relation can serve as a powerful tool for separating reliable luminosity offsets from offsets associated with kinematic anomalies. We reanalyze high-z TF data from the FORS Deep Field and find: (1) overall luminosity evolution of ~0.3 mag; (2) strong slope evolution driven by kinematically anomalous galaxies, which show TF offsets of up to ~2 mag at low luminosities; and (3) an additional zero-point offset of ~0.2 mag linked to anomalous galaxies.