An exploration of the Tully-Fisher relation for extreme late-type spiral galaxies

In an earlier paper we presented new high-precision H I velocity width measurements for a sample of 30 extreme late-type spiral galaxies. Here we explore the adherence of those galaxies, as well as 17 additional extreme late-type spirals, to the B- and V-band Tully-Fisher relations defined by a sample of local calibrators. In both bands we find the mean luminosity at a given line width for extreme late-type spirals to lie below that predicted by standard Tully-Fisher relations. While many of the extreme late-type spirals do follow the Tully-Fisher relation to within our observational uncertainties, most of the galaxies lie below the normal, linear Tully-Fisher relation, and some are underluminous by more than 2 sigma (i.e., > 1.16 mag in V). This suggests a possible downward curvature of the Tully-Fisher relation for some of the smallest and faintest rotationally supported disk galaxies. This may be a consequence of the increasing prevalence of dark matter in these systems. We find the deviation from the Tully-Fisher relation to increase with decreasing luminosity and decreasing optical linear size in our sample, implying that the physically smallest and faintest spirals may be a structurally and kinematically distinct class of objects.