Extragalactic Large-Scale Structures behind the Southern Milky Way. - I. Redshifts Obtained at the SAAO in the Hydra/Antlia Extension

Spectroscopic observations have been carried out for galaxies in the Milky Way with the 1.9 m telescope of the South African Astronomical Observatory (SAAO). The galaxies were selected from a deep optical galaxy search covering 266<l<296 |b|<10deg. This is in the extension of the Hydra and Antlia clusters and in the approximate direction of the dipole anisotropy in the Cosmic Microwave Back- ground radiation. The galaxies in the SAAO observing program -- one of the three complementary approaches in mapping the 3D galaxy distribution in the ZOA -- were selected for high central SB and for even distribution over the search area. The galaxies have magnitudes in the range 14.5<B_J<17.5. Good S/N redshifts were determined for 115 galaxies. One spectrum confirmed a PN, and another object (v=57km\s) most likely is galactic as well. The other 25 spectra have low S/N or were dominated by superimposed foreground stars. We do find evidence for a continuation of the Hydra/Antlia supercluster across the ZOA to b=-10, making it one of the larger structures (supercluster?) in the nearby Universe. However, the prominent overdensity unveiled in the galaxy search in Vela (l~280, b~+6) does not, contrary to what may be expected from the 2D distribution, blend with the Hydra and Antlia supercluster (v~3000km/s). The latter seems concentrated at v~6000km/s$. Whether it is related to the Great Attractor cannot yet be assessed. Another weaker concentration is found at v~9700k/ms. A distinct overabundance of redshifts around v~16000km/s is found, corresponding to that of the adjacent dense Shapley (Alpha) region in the northern galactic hemisphere andto that of the Horologium superclusters in the southern galactic hemisphere. This might indicate that these two overdensi- ties are part of a single massive structure, bisected by the Milky Way.