An Exploration of the Tully-Fisher Relation for Extreme Late-Type Spiral Galaxies

This paper explores the adherence of 47 extreme late-type galaxies to the B-and V-band Tully-Fisher relations defined by a sample of local calibrators. Inboth bands we find the mean luminosity at a given line width for extremelate-type spirals to lie below that predicted by standard Tully-Fisherrelations. While many of the extreme late-type spirals do follow theTully-Fisher relation to within our observational uncertainties, most of thesegalaxies lie below the normal, linear Tully-Fisher relation, and some areunderluminous by more than 2 sigma (i.e. >1.16 magnitudes in V). This suggestsa possible downward curvature of the Tully-Fisher relation for some of thesmallest and faintest rotationally supported disk galaxies. This may be aconsequence of the increasing prevalence of dark matter in these systems. Wefind the deviation from the Tully-Fisher relation to increase with decreasingluminosity and decreasing optical linear size in our sample, implying that thephysically smallest and faintest spirals may be a structurally andkinematically distinct class of objects.Comment: 32 pages, 13 figures; to appear in the November A