The Shape and Figure Rotation of the Dark Halo of NGC 2915

NGC 2915 is a blue compact dwarf galaxy with a very extended HI disk showinga short central bar and extended spiral arms, both reaching far beyond theoptical component. We use Tremaine & Weinberg (1984) method to measure thepattern speed of the bar from HI radio synthesis data. Our measurements yield apattern speed of 0.21+/-0.06 km/s/arcsec (8.0+/-2.4 km/s/kpc for D=5.3 Mpc), indisagreement with the general view that corotation in barred disks lies justoutside the end of the bar, but consistent with recent models of barredgalaxies with dense dark matter halos. Our adopted bar semi-length putscorotation at more than 1.7 bar radii. The existence of the pattern is alsoproblematic. Because NGC 2915 is isolated, interactions cannot account for thestructure observed in the HI disk. We also demonstrate that the low observeddisk surface density and the location of the pseudo-rings make it unlikely thatswing amplification or bar-driven spiral arms could explain the bar and spiralpattern. Based on the similarity of the dark matter and HI surface density profiles,we discuss the possibility of dark matter distributed in a disk and followingclosely the HI distribution. The disk then becomes unstable and can naturallyform a bar and spiral pattern. However, this explanation is hard to reconcilewith some properties of NGC 2915. We also consider the effect of a massive andextended triaxial dark matter halo with a rotating figure. The existence ofsuch halos is supported by CDM simulations showing strongly triaxial dark haloswith slow figure rotation. The observed structure of the HI disk can then arisethrough forcing by the rotating triaxial figure. We associate the measuredpattern speed in NGC 2915 with the figure rotation of its dark halo.