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 23 ± 27. 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%.