The dark matter content of the blue compact dwarf NGC 2915

NGC 2915 is a nearby blue compact dwarf with the H i properties of a late-type spiral. Its large, rotating H i disc (extending out to R similar to 22 B-band scalelengths) and apparent lack of stars in the outer H i disc make it a useful candidate for dark matter studies. New H i synthesis observations of NGC 2915 have been obtained using the Australia Telescope Compact Array. These data are combined with high-quality 3.6-mu m imaging from the Spitzer Infrared Nearby Galaxies Survey. The central regions of the H i disc are shown to consist of two distinct H i concentrations with significantly non-Gaussian line profiles. This discredits the idea of a massive H i bar constituting the inner H i morphology. We fit a tilted ring model to the H i velocity field to derive a rotation curve. This is used as input for mass models that determine the contributions from the stellar and gas discs as well as the dark matter halo. The galaxy is dark matter dominated at nearly all radii. At the last measured point of the rotation curve the total mass to blue light ratio is M(tot)/L(B) similar to 140 M(circle dot)/L(B), making NGC 2915 one of the darkest galaxies known. We show that the stellar disc cannot account for the steeply rising portion of the observed rotation curve. The best-fitting dark matter halo is a pseudo-isothermal sphere with a core density (0) similar to 0.17 +/- 0.03 M(circle dot) pc-3 and a core radius r(c) similar to 0.9 +/- 0.1 kpc.