Resolving the mystery of the dwarf galaxy HIZSS003

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Abstract
The nearby galaxy HIZSS003 was recently discovered during a blind H i survey of the Zone of Avoidance. Follow-up Very Large Array (VLA) observations as well as optical and nearinfrared 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 H ii 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 H ii 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 H i mass of 2.6 × 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 ∼5 × 107M⊙. For the bigger galaxy HIZSS003A we derive an HI mass of 1.4 × 107M⊙. The velocity field of this galaxy is quite regular, and from its rotation curve we derive a total dynamical mass of ∼6.5 × 108M⊙.
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