DISK GALAXY SCALING RELATIONS IN THE SFI++: INTRINSIC SCATTER AND APPLICATIONS

We study the scaling relations between the luminosities, sizes, and rotation velocities of disk galaxies in the SFI++, with a focus on the size-luminosity (RL) and size-rotation velocity (RV) relations. Using isophotal radii instead of disk scale lengths as a size indicator, we find relations that are significantly tighter than previously reported: the correlation coefficients of the template RL and RV relations are r = 0.97 and r = 0.85, respectively, which rival that of the more widely studied LV (Tully-Fisher) relation. The scatter in the SFI++ RL relation is 2.5-4 times smaller than previously reported for various samples, which we attribute to the reliability of isophotal radii relative to disk scale lengths. After carefully accounting for all measurement errors, our scaling relation error budgets are consistent with a constant intrinsic scatter in the LV and RV relations for velocity widths log W greater than or similar to 2.4, with evidence for increasing intrinsic scatter below this threshold. The scatter in the RL relation is consistent with constant intrinsic scatter that is biased by incompleteness at the low-L end. Possible applications of the unprecedentedly tight SFI++ RV and RL relations are investigated. Just like the Tully-Fisher relation, the RV relation can be used as a distance indicator: we derive distances to galaxies with primary Cepheid distances that are accurate to 25%, and reverse the problem to measure a Hubble constant H-0 = 72 +/- 7 km s (1) Mpc(-1). Combining the small intrinsic scatter of our RL relation (epsilon(int) = 0.034 +/- 0.001 loginverted right perpendicularh(-1) kpcinverted left perpendicular) with a simple model for disk galaxy formation, we find an upper limit in the range of disk spin parameters that is a factor of similar to 7 smaller than that of the halo spin parameters predicted by cosmological simulations. This likely implies that the halos hosting Sc galaxies have a much narrower distribution of spin parameters than previously thought.