Extended H [CSC]i[/CSC] Spiral Structure and the Figure Rotation of Triaxial Dark Halos

The HI disk of the blue compact dwarf (BCD) galaxy NGC 2915 extends to 22 optical scalelengths and shows spiral arms reaching far beyond the opticalcomponent. None of the previous theories for spiral structure provide likelyexplanations for these very extended spiral arms. Our numerical simulationsfirst demonstrate that such large spiral arms can form in an extended gas diskembedded in a massive triaxial dark matter halo with slow figure rotation,through the strong gravitational torque of the rotating halo. We then show thatthe detailed morphological properties of the developed spirals and rings dependstrongly on the pattern speed of the figure rotation, the shape of the triaxialhalo, and the inclination of the disk with respect to the plane including thetriaxial halo's long and middle axes. These results strongly suggest that thedark matter halo of NGC 2915 is triaxial and has figure rotation. Based onthese results, we also suggest that dynamical effects of triaxial halos withfigure rotation are important in various aspect of galaxy formation andevolution, such as formation of polar ring galaxies, excitation ofnon-axisymmetric structures in low surface-brightness galaxies, and gas fuelingto the central starburst regions of BCDs.Comment: 13 pages 2 figures (fig.2 = jpg format), accepted by ApJ