On the Spiral Structure of NGC 2915 and Dark Matter

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 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 H I 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 λ 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 Send offprint requests to F. S. Masset: [email protected] Hubble Fellow