H I in very metal-poor galaxies: the SBS 0335-052 system

Abstract

We present Giant Metrewave Radio Telescope, H I 21 cm observations of SBS 0335-052E and SBS 0335-052W, a close pair of dwarf galaxies, which are further unusual in being the most metal-poor star-forming galaxies known. We present images at several angular resolutions, ranging from ^~40 to 4 arcsec. These images show that SBS 0335-052 is a strongly interacting system, with a faint diffuse H I bridge seen at low resolution, and elongated tails seen at the higher resolutions. The overall morphology suggests that the pair represents a major (as both galaxies have similar H i masses) merger of extremely gas-rich galaxies, which is currently past the first close encounter. The low-resolution velocity field is dominated by the velocity difference between the two galaxies and the velocity gradient along the tidal features. However, for SBS 0335-052W at least, at high angular resolution, one sees a central velocity field that could be associated with the spin of the original undisturbed disc. The two galaxies have very similar H I masses, but very different optical properties and current star formation rates. A possible reason for this is the differing amounts of tidally induced star formation, because of the different spin orientations of these interacting galaxies. The highest angular resolution H i images show that the ionized superbubble, identified by Thuan, Izotov & Lipovetsky, in the Hubble Space Telescope images of SBS 0335-052E, is extended along one of the diffuse tidal features, and that there is a high-density H I clump at the other end of the superbubble. The star formation in SBS 0335-052E occurs mainly in a group of superstar clusters (SSCs) with a clear age gradient; the age decreases as one approaches the dense H I clump. We suggest that this propagating star formation is driven by the superbubble expanding into a medium with a tidally produced density gradient. The high pressures associated with the compressed material would also naturally explain why current star formation is mainly concentrated in SSCs.