Resolving the mystery of the dwarf galaxy HIZSS003

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 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 H I 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 H I emission from these two galaxies overlaps in projection, but can be separated in velocity space. HIZSS003B has an H I mass of 2.6 × 10⁶ M_?, 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 × 10⁷ M_⊙. For the bigger galaxy HIZSS003A we derive an H I mass of 1.4 × 10⁷ M_⊙. The velocity field of this galaxy is quite regular, and from its rotation curve we derive a total dynamical mass of ^~6.5 × 10⁸ M_⊙.