H I and dark matter in the windy starburst dwarf galaxy NGC 1705

We present 21-cm H I line observations of the blue compact dwarf galaxy NGC 1705. Previous optical observations show a strong outflow powered by an ongoing starburst dominating the H II morphology and kinematics. In contrast, most of the Hr lies in a rotating disc. An extraplanar H I spur accounts for similar to 8 per cent of the total H I mass, and is possibly associated with the H II outflow. The inferred mass loss rate out of the core of the galaxy is significant, similar to 0.2-2 M-. yr(-1), but does not dominate the HI dynamics. Mass model fits to the rotation curve show that the dark matter (DM) halo is dominant at nearly all radii and has a central density rho(0) approximate to 0.1 M-. pc(-3): ten times higher than typically found in dwarf irregular galaxies, but similar to the only other mass-modelled blue compact dwarf, NGC 2915. This large difference strongly indicates that there is little evolution between dwarf irregular and blue compact dwarf types. Instead, dominant DM haloes may regulate the morphology of dwarf galaxies by setting the critical surface density for disc star formation. Neither our data nor catalogue searches reveal any likely external trigger to the starburst in NGC 1705.