Constraining Dark Matter Halo Profiles and Galaxy Formation Models Using Spiral Arm Morphology. I. Method Outline

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 by using a much larger sample (51 galaxies) than used previously (17 galaxies). 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. In contrast to previous work, we show that observed spiral arm pitch angles are similar when measured in the optical (at 0.4 μm) and the near-infrared (at 2.1 μm) with a mean difference of 2 3 ± 2 7. This is then used to strengthen the known correlation between P and S using B -band images. We then use two example galaxies 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. We show that ESO 582-G12, a galaxy with a high shear rate (slightly declining rotation curve) at ~10 kpc, favors an adiabatically contracted halo, with high initial NFW concentration ( c vir > 16) and a high fraction of halo baryons in the form of stars (~15%-40%). In contrast, IC 2522 has a low shear rate (rising rotation curve) at ~10 kpc and favors nonadiabatically contracted models with low NFW concentrations ( c vir ≃ 2-8) and a low stellar baryon fraction <10%.