The Tully-Fisher Relation as a Measure of Luminosity Evolution: A Low-Redshift Baseline for Evolving Galaxies^*

We use optical rotation curves to investigate the R-band Tully-Fisher properties of a sample of 90 spiral galaxies in close pairs, covering a range of luminosities, morphological types, and degrees of tidal distortion. The galaxies follow the Tully-Fisher relation remarkably well, with the exception of eight distinct ∼3 σ outliers. Although most of the outliers show signs of recent star formation, gasdynamical effects are probably the dominant cause of their anomalous Tully-Fisher properties. Four outliers with small emission-line widths have very centrally concentrated line emission and truncated rotation curves; the central emission indicates recent gas infall after a close galaxy-galaxy pass. These four galaxies may be local counterparts to compact, blue galaxies at intermediate redshift. The remaining galaxies have a negligible offset from the reference Tully-Fisher relation, but a shallower slope (2.6 σ significance) and a 25% larger scatter. We argue that triggered star formation is a significant contributor to the slope difference. We characterize the nonoutlier sample with measures of distortion and star formation to search for third-parameter dependence in the residuals of the TF relation. Severe kinematic distortion is the only significant predictor of TF residuals; this distortion is not, however, responsible for the slope difference from the reference distribution. Because the outliers are easily removed by sigma clipping, we conclude that even in the presence of some tidal distortion, detection of moderate (≳0.5 mag in rest-frame R) luminosity evolution should be possible with high-redshift samples the size of this 90-galaxy study. The slope of the TF relation, although difficult to measure, is as fundamental for quantifying luminosity evolution as the zero-point offset.