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

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.