NGC 2915 is a blue compact dwarf galaxy with a very extended \ion H1 disk .
This disk shows a short central bar and extended spiral arms , both reaching far beyond the optical component .
We use Tremaine & Weinberg ’ s \markcite tw84 ( 1984 ) method to measure the pattern speed of the bar and spiral arms from \ion H1 radio synthesis data .
Our measurements yield a pattern speed of \Omega _ { p } = 0.21 \pm 0.06 km s ^ { -1 } arcsec ^ { -1 } ( 8.0 \pm 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 } \approx 180 \arcsec puts corotation at more than 1.7 r _ { b } .
The existence of the pattern is also problematic .
Because NGC 2915 is isolated , gravitational interactions can not account for the structure observed in the \ion H1 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 \markcite t811981 ) or bar-driven spiral arms could explain the bar and spiral pattern .

Based on the similarity of the dark matter and \ion H1 surface density profiles , we discuss the possibility of dark matter distributed in a disk and following closely the \ion H1 distribution .
This disk then becomes gravitationally unstable and can naturally form a bar and spiral pattern .
However , this explanation is 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 \ion H1 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 .