HI studies of extremely metal deficient galaxies-II: Giant Metrewave Radio Telescope observations of SBS 1129 + 576

Abstract

We present Giant Metrewave Radio Telescope H i 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 H i 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 8 arcsec. 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 × 10⁸, ^~2.7 × 10⁹ and ^~2.1 × 10⁸ M_⊙ 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 H i mass of (M_HI^~1.1 × 10⁷ M_⊙) 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 H i column density and those with ongoing star formation. The two brightest H ii regions in SBS 1129+576 have (inclination-corrected) gas column densities of ^~1.6 × 10²¹ and ^~1.8 × 10²¹ atoms cm^-2, respectively. The inclination-corrected H i column density near the H ii regions in SBS 1129+577 is generally above ^~2.0 × 10²¹ atoms cm⁻². 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.