  NGC 2915 is a blue compact dwarf galaxy embedded in an extended, low surface
brightness HI disk exhibiting a two-armed spiral structure and a central
bar-like component. Commonly accepted mechanisms are unable to explain the
existence of these patterns and Bureau et al. proposed disk dark matter
(scaling with the HI distribution) or a rotating triaxial dark halo as
alternative solutions. In an attempt to explore these mechanisms,
hydrodynamical simulations were run for each case and compared to observations
using customized column density and kinematic constraints. The spiral structure
can be accounted for both by an unseen bar or triaxial halo, the former fitting
the observations slightly better. However, the large bar mass or halo pattern
frequency required make it unlikely that the spiral wave is driven by an
external perturber. In particular, the spin parameter is much higher than
predicted by current cold dark matter (CDM) structure formation scenarios. The
massive disk models show that when the observed gas surface density is scaled
up by a factor about 10, the disk develops a spiral structure resembling
closely the observed one, in perturbed density as well as perturbed velocity.
This is consistent with more limited studies in other galaxies and suggests
that the disk of NGC 2915 contains much more mass than is visible, tightly
linked to the neutral hydrogen. A classic (quasi-)spherical halo is
nevertheless still required, as increasing the disk mass further to fit the
circular velocity curve would make the disk violently unstable. Scaling the
observed surface density profile by an order of magnitude brings the disk and
halo masses to comparable values within the disk radius.
