  We investigate the use of spiral arm pitch angles as a probe of disk galaxy
mass profiles. We confirm our previous result that spiral arm pitch angles (P)
are well correlated with the rate of shear (S) in disk galaxy rotation curves.
We use this correlation to argue that imaging data alone can provide a powerful
probe of galactic mass distributions out to large look-back times. We then use
a sample of 13 galaxies, with Spitzer 3.6-$\mu$m imaging data and observed
H$\alpha$ rotation curves, to demonstrate how an inferred shear rate coupled
with a bulge-disk decomposition model and a Tully-Fisher-derived velocity
normalization can be used to place constraints on a galaxy's baryon fraction
and dark matter halo profile. Finally we show that there appears to be a trend
(albeit a weak correlation) between spiral arm pitch angle and halo
concentration. We discuss implications for the suggested link between
supermassive black hole (SMBH) mass and dark halo concentration, using pitch
angle as a proxy for SMBH mass.
