HI in very metal-poor galaxies: the SBS 0335-052 system
  We present Giant Metrewave Radio Telescope (GMRT), HI 21cm 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 HI bridge seen at low resolution, and elongated tails seen at the
higher resolutions. The overall morphology suggests that the pair represents a
major merger of extremely gas-rich galaxies. 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 highest
angular resolution HI images show that the ionized superbubble, identified by
Thuan, Izotov & Lipovetsky (1997), in the Hubble Space Telescope (HST) images
of SBS 0335-052E, is extended along one of the diffuse tidal features, and that
there is a high-density HI 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
HI 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 superstar
clusters.
