Physical Sources of Scatter in the Tully-Fisher Relation

We analyze residuals from the Tully-Fisher relation for the emission-linegalaxies in the Nearby Field Galaxy Survey, a broadly representative surveydesigned to fairly sample the variety of galaxy morphologies and environmentsin the local universe. For spirals brighter than M_R^i=-18, we find strongcorrelations between Tully-Fisher residuals and both B-R color and EW(Halpha).The extremes of the correlations are populated by Sa galaxies, which showconsistently red colors, and spirals with morphological peculiarities, whichare often blue. If we apply an EW(Halpha)-dependent or B-R color-dependentcorrection term to the Tully-Fisher relation, the scatter in the relation nolonger increases from R to B to U but instead drops to a nearly constant levelclose to the scatter we expect from measurement errors. We argue that theseresults probably reflect correlated offsets in luminosity and color as afunction of star formation history. Broadening the sample in morphology andluminosity, we find that most non-spirals brighter than M_R^i=-18 follow thesame correlations as do spirals, albeit with greater scatter. However, thecolor and EW(Halpha) correlations do not apply to galaxies fainter thanM_R^i=-18 or to emission-line S0 galaxies with anomalous gas kinematics. Forthe dwarf galaxy population, the parameters controlling Tully-Fisher residualsare instead related to the degree of recent disturbance: overluminous dwarfshave higher rotation curve asymmetries, brighter U-band effective surfacebrightnesses, and shorter gas consumption timescales than their underluminouscounterparts. As a result, sample selection strongly affects the measuredfaint-end slope of the Tully-Fisher relation. Passively evolving, rotationallysupported galaxies display a break toward steeper slope at low luminosities.Comment: 58 pages including 21 figures, AJ, accepte