SIMBAD references

NGC 2915 is a blue compact dwarf galaxy embedded in an extended, low surface brightness H I disk exhibiting a two-armed spiral structure and a central barlike component. Commonly accepted mechanisms are unable to explain the existence of these patterns, and Bureau et al. proposed disk dark matter (scaling with the H I distribution) or a rotating triaxial dark halo as alternative solutions. In an attempt to explore these mechanisms, hydrodynamic simulations were run for each case and compared to observations using customized column density and kinematic constraints. The spiral structure can be accounted for by both 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 λ is much higher than predicted by current cold dark matter 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. The surface density remains under Kennicutt's star formation threshold for a gaseous disk, and no stars are expected to form, as required by observations.