The shape and figure rotation of the dark halo of NGC 2915

NGC 2915 is a blue compact dwarf galaxy with a very extended H I disk. This disk shows a short central bar and extended spiral arms, both reaching far beyond the optical component. We use Tremaine & Weinberg's method to measu re the pattern speed of the bar and spiral arms from H I radio synthesis da ta. Our measurements yield a pattern speed Omega(p) = 0.21 +/- 0.06 km s(-1 ) arcsec(-1) (8.0 +/- 2.4 km s(-1) kpc(-1) for D = 5.3 kpc), in disagreemen t with the general view that corotation in barred disks lies just outside t he end of the bar, but consistent with recent models of barred galaxies wit h dense dark matter halos. Our adopted bar semilength, r(b) approximate to 180 ", puts corotation at more than 1.7r(b). The existence of the pattern i s also problematic. Because NGC 2915 is isolated, gravitational interaction s cannot account for the structure observed in the H I disk. We also demons trate that the low surface density observed in the disk and the location of the pseudorings make it unlikely that swing amplification or bar-driven sp iral arms could explain the bar and spiral pattern. Based on the similarity of the dark matter and H I surface density profiles, we discuss the possib ility of dark matter distributed in a disk and following closely the H I di stribution. This disk then becomes gravitationally unstable and can natural ly form a bar and spiral pattern. However, this explanation is difficult to reconcile with some properties of NGC 2915. We also consider the effect of a massive and extended triaxial dark matter halo with a rotating figure. T he existence of such halos is supported by cold dark matter simulations sho wing strongly triaxial dark halos with slow figure rotation. The observed s tructure of the H I disk can then arise through forcing by the rotating tri axial figure. We associate the measured pattern speed in NGC 2915 with the figure rotation of its dark halo.