Extragalactic large-scale structures behind the southern Milky Way. - III. Redshifts obtained at the SAAO in the Great Attractor region

Abstract

(abbreviated) In the third of a series of papers on large-scale structures behind the southern Milky Way, we report here on redshifts obtained at the South African Astronomical Observatory (SAAO) in the Great Attractor region (318deg < l < 340deg, |b| <= 10deg, Woudt 1998). This region encompasses the peak in the reconstructed mass density field, associated with the Great Attractor (Kolatt et al. 1995, Dekel et al. 1998) and covers the crossing of the Supergalactic Plane with the Galactic Plane. We have obtained reliable redshifts for 309 galaxies in the Great Attractor region with the ``Unit'' spectrograph (first with a Reticon, then with a CCD detector) at the 1.9-m telescope of the SAAO. We realise here, that the Great Attractor region is dominated by ACO 3627 (hereafter referred to as the Norma cluster), a highly obscured, nearby and massive cluster of galaxies close to the plane of the Milky Way (l,b,v) = (325.3deg, -7.2deg, 4844 km/s) (Kraan-Korteweg et al. 1996, Woudt 1998). Previous redshift surveys in the GA region have failed to gauge the significance of the Norma cluster, primarily due to the diminishing effects of the Galactic foreground extinction on the partially obscured galaxies. In the absence of the obscuring effects of the Milky Way, the Norma cluster would have appeared as prominent as the well-known Coma cluster, but nearer in redshift-space. This cluster most likely marks the bottom of the potential well of the Great Attractor (Woudt 1998).