Maximal Disks and the Tully-Fisher Relation

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  • 25 July 1997
Abstract
We explore Tully-Fisher (TF) residuals for luminous, high-surface brightness (HSB) spirals to show that a maximal disk fit is ruled out by the statistics of the rotation speed, v2.2, at 2.2 exponential scale lengths. For maximal disks, the size, or surface-brightness, of the disk should be a significant additional parameter in the TF relation. Using the Courteau-Faber sample of late-type spirals, we find that the fractional deviations, dlogv2.2 and dlogRexp from the mean relations, v2.2(Mr) and Rexp(Mr), are not significantly correlated. The case of dlogv2.2/dlogRexp = -0.5, expected for pure disks, is clearly ruled out. We use simple models of adiabatic infall of the luminous matter to explore what values of dlogv2.2/dlogRexp are expected in the presence of modest bulges and a variety of dark matter halos. We find that the TF relation requires that Vdisk ~= 0.5 Vtot, fairly insensitive to the details of the dark matter halo and the presence of a bulge. Our results are consistent with measurements of the stellar velocity dispersions in HSB spirals, the estimated mass density near the Sun, and constraints from swing amplification theory. Sub-maximal disks establish a natural continuity between HSB and LSB galaxies which appear to be completely dark matter dominated even in their inner regions.

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