2006MNRAS.372..853E

We present Giant Metrewave Radio Telescope HI observations of an extremely metal-deficient galaxy SBS 1129+576. SBS 1129+576 has a weighted mean oxygen abundance of 12 + log (O/H) = 7.41±0.07, or 1/18 of the solar value. Our HI observations show that the galaxy is strongly interacting with a companion (projected separation ∼27 kpc) galaxy, SBS 1129+577. HI emission from a third, smaller galaxy, SDSS J113227.68+572142.3, is also present in the data cube. We study the HI morphology and kinematics of this small group at angular resolutions ranging from ∼40 to 8arcsec. The low-resolution map shows a bridge of emission connecting the two larger galaxies and a large one-armed spiral distortion of the disc of SBS 1129+577. We measure HI masses of ∼4.2 x10⁸, ∼2.7x10⁹ and ∼2.1x10⁸M_☉for SBS 1129+576, SBS 1129+577 and the gas in the bridge, respectively. Assuming that most of the bridge gas originally came from SBS 1129+576, approximately one-third of its original gas mass has been stripped off. The third smaller galaxy has an HI mass of (M_HI∼ 1.1x10⁷M_☉) and does not show any sign of interaction with the other two galaxies. The higher-resolution maps show that SBS 1129+577 has a central bar and a ring surrounding the bar; there is also a hint of an integral-shaped warp in SBS 1129+576. All these features are very likely to have been induced by the tidal interaction. In both SBS 1129+576 and SBS 1129+577, there is, in general, a good correspondence between regions with high HI column density and those with ongoing star formation. The two brightest HII regions in SBS 1129+576 have (inclination-corrected) gas column densities of ∼1.6x10²¹ and ∼1.8x10²¹ atoms/cm2, respectively. The inclination-corrected HI column density near the HII regions in SBS 1129+577 is generally above ∼2.0x10²¹ atoms/cm2. These values are close to the threshold density for star formation observed in other blue compact galaxies. In contrast to SBS 1129+576 and SBS 1129+577 which are very gas-rich, the third member of this group, SDSS J113227.68+572142.3, is gas-poor.