The Influence of Triaxial Halos on Collisionless Galactic Disks

We investigate the effect of rotating, triaxial halos on disk galaxies through an extensive set of numerical N -body simulations. Our simulations use a rigid potential field for the halos and bulges and collisionless particles for the disks. The triaxiality and the rotation rate of the halo are varied, as well as the masses of all three galaxy components. We analyze both the bar stability and the spiral response of the disks under these conditions. We characterize most of our models by the mass ratio of the disk to the halo at 2.3 disk scale lengths, (Md/Mh)R⊙ . For models with a mass ratio greater than 0.8, a halo pattern speed Ωh = 6.7 km s −1 kpc−1, and a intermediate-to-major axis ratio qb = 0.85, a strong bar will develop within 3 Gyr, even for models with a bulge mass Mb = 0.3Md. Models in which the bulge mass is reduced by half develop bars earlier and with lower Ωh. We create an artificial Hubble sequence of disk galaxies by varying the bulge-to-disk ratio of our models from 0 to 2.5. The torque induced by a rotating, non-axisymmetric halo creates bisymmetric spiral structure in the disk. We find that the pitch angle of the spiral arms in these models follows the same general trend found in observations of spiral galaxies, namely that later type galaxies have higher pitch angles. Our simulations follow closely the observational relation of spiral pitch angle with maximum rotational velocity of the disk, where galaxies with faster rotation have more tightly wound spiral arms. This relation is followed in our simulations regardless of whether the dominating mass component of the galaxy is the disk, the halo, or the bulge. Subject headings: galaxies: spiral – galaxies: stability – methods: numerical