EXTRAGALACTIC LARGE-SCALE STRUCTURES BEHIND THE SOUTHERN MILKY-WAY .1. REDSHIFTS OBTAINED AT THE SAAO IN THE HYDRA ANTLIA EXTENSION

Spectroscopic observations have been carried out for galaxies in the M ilky Way with the 1.9 m telescope of the South African Astronomical Ob servatory (SAAO). The galaxies were selected from a deep optical galax y search covering 266 degrees less than or similar to e less than or s imilar to 296 degrees, \b\ less than or similar to 10 degrees (Kraan-K orteweg 1994). This is in the extension of the Hydra and Antlia cluste rs and in the approximate direction of the dipole anisotropy in the Co smic Microwave Background radiation. The galaxies in the SAAO observin g program - one of the three complementary approaches in mapping the 3 D galaxy distribution in the ZOA - were selected for high central surf ace brightness and for even distribution over the whole search area. T he majority of the galaxies have (absorbed) magnitudes in the range 14 .5(m) < B-J < 17.5(m). Good S/N redshifts were determined for 115 gala xies. One spectrum confirmed a planetary nebula, whereas another objec t with upsilon = 57 km sec(-1) most likely is galactic as well. The sp ectra of the other 25 galaxies either have low S/N or were dominated b y superimposed foreground stars. A preliminary description of the dist ribution in velocity space is given. We do find evidence for a continu ation of the Hydra/Antlia supercluster across the ZOA to b approximate to -10 degrees, making it one of the larger structures (supercluster? ) in the nearby Universe. However, the prominent overdensity unveiled in the galaxy search in Vela (l approximate to 280 degrees,b approxima te to +6 degrees) does not, contrary to what may be expected from the 2D distribution, blend with the Hydra and Antlia supercluster (upsilon approximate to 3000 km sec(-1)). The latter seems concentrated at ups ilon approximate to 6000 km sec(-1). Whether it is related to the Grea t Attractor cannot yet be assessed. Another weaker concentration is fo und at upsilon approximate to 9700 km sec(-1). Further out, only tenta tive conclusions can be made. However, a distinct overabundance of red shifts around upsilon approximate to 16000 km sec(-1) is found, corres ponding to that of the adjacent dense Shapley (Alpha) region in the no rthern galactic hemisphere and to that of the Horologium superclusters in the southern galactic hemisphere. This might indicate that these t wo overdensities are part of a single massive structure, bisected by t he Milky Way.