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Register a new userExtragalactic Large-Scale Structures behind the Southern Milky Way. -- II. Redshifts Obtained at the SAAO in the Crux Region
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In our systematic optical galaxy search behind the southern Milky Way, 3760 (mostly unknown) galaxies with diameters D > 0.2 arcminutes were identified in the Crux region (287 < l < 318 degrees, |b| < 10 degrees, Woudt & Kraan-Korteweg 1997). Prior to this investigation, only 65 of these galaxies had known redshifts. In order to map the galaxy distribution in redshift space we obtained spectra for 226 bright (B_J < 18.0 mag) objects with the 1.9m telescope of the South African Astronomical Observatory (SAAO). Two main structures crossing the Galactic Plane in the Crux region have now become clear. A narrow, nearby filament from (l,b) = (340 deg, -25 deg) to the Centaurus cluster can be traced. This filament runs almost parallel to the extension of the Hydra-Antlia clusters found earlier and is part of what we have earlier termed the ``Centaurus Wall extending in redshift-space between 0 < v < 6000 km/s (Fairall & Paverd 1995). The main outcome of this survey however, is the recognition of another massive extended structure between 4000 < v < 8000 km/s. This broad structure, dubbed the Norma Supercluster (Woudt et al. 1997), runs nearly parallel to the Galactic Plane from Vela to ACO 3627 (its centre) from where it continues to the Pavo cluster. This massive structure is believed to be associated with the Great Attractor. The survey has furthermore revealed a set of cellular structures, similar to those seen in redshift space at higher galactic latitudes, but never before seen so clearly behind the Milky Way.
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A deep optical galaxy search behind the southern Milky Way and a subsequent redshift survey of the identified obscured galaxies traces clusters and superclusters into the deepest layers of Galactic foreground extinction (A_B <= 3^m - 5^m). In the Great Attractor region, we have identified a low-mass cluster (the Centaurus-Crux cluster) at (l, b, v, sigma) = (305.5deg, +5.5deg, 6214 km/s, 472 km/s) and found that ACO 3627 (the Norma cluster) at (l, b, v, sigma) = (325.3deg, -7.2deg, 4844 km/s, 848 km/s) is the most massive cluster in the Great Attractor region known to date. It is comparable in virial mass, richness and size to the well-known but more distant Coma cluster. The Norma cluster most likely marks the bottom of the potential well of the Great Attractor. It is located at the intersection of two main large-scale structures, the Centaurus Wall and the Norma supercluster. The flow field observed around the Great Attractor probably is caused by the confluence of these two massive structures.
About 25% of the optical extragalactic sky is obscured by the dust and stars of our Milky Way. Dynamically important structures might still lie hidden in this zone. Various approaches are presently being employed to uncover the galaxy distribution in the Zone of Avoidance (ZOA) but all suffer from (different) limitations and selection effects. We investigated the potential of using the DENIS NIR survey for studies of galaxies behind the obscuration layer of our Milky Way and for mapping the Galactic extinction. As a pilot study, we recovered DENIS I, J and K band images of heavily obscured but optically still visible galaxies. We determined the I, J and K band luminosity functions of galaxies on three DENIS strips that cross the center of the nearby, low-latitude, rich cluster Abell 3627. The extinction-corrected I-J and J-K colours of these cluster galaxies compare well with that of an unobscured cluster. We searched for and identified galaxies at latitudes where the Milky Way remains fully opaque (|b| < 5deg and A_B > 4-5mag) - in a systematic search as well as around positions of galaxies detected with the blind HI survey of the ZOA currently conducted with the Multibeam Receiver of the Parkes Radiotelescope.
A first analysis of a deep blind HI survey covering the southern Zone of Avoidance plus an extension towards the north (196 < l < 52 deg) obtained with the Multibeam receiver at the 64m Parkes telescope reveals slightly over a thousand galaxies within the latitude completeness limit of |b| < 5deg. The characteristics and the uncovered large-scale structures of this survey are described, in particular the prominence of the Norma Supercluster, the possible cluster around PKS 1343-601 (both in the Great Attractor region), as well as the Local Void and the clustering in the Puppis region. In this blind HI survey, HIZOA J0836-43, one of the most massive spiral galaxies known to date was discovered (M(HI) = 7.3 10^10 Msun M(tot) = 1.1 10^12 Msun; Ho = 75 km/s/Mpc). Although of similar mass as Malin 1-like objects, this galaxy does not share their typical low-surface brightness properties but seems an exceptionally massive but normal, high-surface brightness, star-forming galaxy.
Various dynamically important extragalactic large-scale structures in the local Universe lie behind the Milky Way. Most of these structures (predicted and unexpected) have only recently been made ``visible through dedicated deep surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and X-ray) for galaxies in the Zone of Avoidance (ZOA) will be reviewed and the uncovered and suspected large-scale structures summarised. Particular emphasis is given to the Great Attractor region where the existence of yet another cluster is suspected (Woudt 1998). Predictions from reconstructions of the density field in the ZOA are discussed and compared with observational evidence. Although no major structures are predicted out to about v < 10000 km/s for which no observational evidence exists, the comparison between reconstructed density fields and the observed galaxy distribution remain important as they allow derivations of the density and biasing parameters.
We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind the southern Galactic plane (253o < l < 356o, |b| < 1.5o), and use these data in combination with published data to probe the large-scale structure of the Milky Ways magnetic field. We show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with the locations of the Galactic spiral arms. The observed pattern in RMs requries the presence of at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the Sagittarius- Carina and Scutum-Crux spiral arms. To quantitatively compare our measurements to other recent studies, we consider all available extragalactic and pulsar RMs in the region we have surveyed, and jointly fit these data to simple models in which the large-scale field follows the spiral arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant is directed clockwise in the Sagittarius-Carina spiral arm (as viewed from the North Galactic pole), but is oriented counter- clockwise in the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed counter-clockwise in the Sagittarius- Carina arm. Also in contrast to recent pulsar RM studies, our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale magnetic field reversals are not required to account for observations.
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