2005MNRAS.359L..53B

The nearby galaxy HIZSS003 was recently discovered during a blind Hi survey of the Zone of Avoidance. Follow-up Very Large Array (VLA) observations as well as optical and near-infrared imaging and spectroscopy confirm that it is a low-metallicity dwarf irregular galaxy. However, there are two puzzling aspects of these observations: (i) current star formation, as traced by Hα emission, is confined to a small region at the edge of the VLA HI image; and (ii) the metallicity of the older red giant branch stars is higher than that of the gas in the HII region. We present high spatial and velocity resolution Giant Metrewave Radio Telescope observations which resolve these puzzles by showing that HIZSS003 is actually a galaxy pair and that the HII region lies at the centre of a much smaller companion galaxy (HIZSS003B) to the main galaxy (HIZSS003A). The HI emission from these two galaxies overlaps in projection, but can be separated in velocity space. HIZSS003B has an HI mass of 2.6 x 10⁶M_☉, and a highly disturbed velocity field. Since the velocity field is disturbed, an accurate rotation curve cannot be derived; however, the dynamical mass indicated is ∼5x10⁷M_☉. For the bigger galaxy HIZSS003A we derive an HI mass of 1.4x10⁷M_☉. The velocity field of this galaxy is quite regular, and from its rotation curve we derive a total dynamical mass of ∼6.5x10⁸M_☉.