The Shape and Figure Rotation of NGC 2915 ’ s Dark Halo

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 (1984) method to measure the pattern speed of the bar and spiral arms from H I radio synthesis data. Our measurements yield a pattern speed of Ω p = 0.21 ± 0.06 km s −1 arcsec −1 (8.0 ± 2.4 km s −1 kpc −1 for D = 5.3 Mpc), in disagreement with the general view that corotation in barred disks lies just outside the end of the bar, but consistent with recent models of barred galaxies with dense dark matter halos. Our adopted bar semi-length r b ≈ 180 ′′ puts corotation at more than 1.7 r b. The existence of the pattern is also problematic. Because NGC 2915 is isolated, gravitational interactions cannot account for the structure observed in the H I disk. We also demonstrate that the low surface density observed in the disk and the location of the pseudo-rings make it unlikely that swing amplification (Toomre 1981) or bar-driven spiral 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 possibility of dark matter distributed in a disk and following closely the H I distribution. This disk then becomes gravitationally unstable and can naturally form a bar and spiral pattern. However, this explanation is – 2 – difficult to reconcile with some properties of NGC 2915. Finally, we consider the effect of a massive and extended triaxial dark matter halo with a rotating figure. The existence of such halos is supported by CDM simulations showing strongly triaxial dark halos with slow figure rotation. The observed structure of the H I disk can then arise through forcing by the rotating triaxial figure. We associate the measured pattern speed in NGC 2915 with the figure rotation of its dark halo.