[Paper Review] Extragalactic Large-Scale Structures behind the Southern Milky Way. -- II. Redshifts Obtained at the SAAO in the Crux Region

A. P. Fairall, P. A. Woudt+1 more|ArXiv.org|May 20, 1997

Astronomy and Astrophysical Research3 references16 citations

TL;DR

This study presents redshift measurements for 209 galaxies in the Crux region behind the southern Milky Way, revealing two major large-scale structures: the Centaurus Wall (at 0–6000 km s⁻¹) and the Norma Supercluster (at 4000–8000 km s⁻¹), which is linked to the Great Attractor. The survey significantly reduces the obscuration of the Zone of Avoidance and reveals cellular structures similar to those seen at higher galactic latitudes.

ABSTRACT

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.

Motivation & Objective

• To map large-scale extragalactic structures hidden behind the southern Milky Way due to extinction.
• To overcome the observational challenge of the Zone of Avoidance by obtaining redshifts for faint, distant galaxies in low Galactic latitude regions.
• To identify and characterize massive structures such as superclusters and filaments in the Crux region using spectroscopic data.
• To improve the three-dimensional mapping of the local Universe by filling gaps in redshift space near the Galactic Plane.
• To test the continuity of known large-scale features like the Centaurus Wall and the Great Attractor's influence in obscured regions.

Proposed method

• Conducted a systematic optical galaxy survey using 50× magnification of SRC IIIaJ survey plates to identify 3,760 galaxies with D ≥ 0.2 arcminutes in the Crux region (287° ≤ ℓ ≤ 318°, |b| ≤ 10°).
• Obtained spectroscopic redshifts for 226 bright galaxies (B_J ≤ 18) using the 1.9m telescope at the South African Astronomical Observatory (SAAO).
• Measured redshifts via cross-correlation of spectra with standard templates, achieving good signal-to-noise ratios for 173 galaxies.
• Confirmed 4 of 36 tentative redshifts using literature data; identified 3 galaxies as galactic (including a confirmed planetary nebula).
• Mapped redshift-space structures using redshift slices across galactic longitude and latitude, revealing filamentary and cellular patterns.
• Combined data with prior surveys (e.g., Kraan-Korteweg et al. 1995) to trace extended structures like the Centaurus Wall and Norma Supercluster.

Experimental results

Research questions

• RQ1 What large-scale structures exist behind the southern Milky Way in the Crux region, previously obscured by Galactic extinction?
• RQ2 How do the redshift distributions of galaxies in the Crux region reveal the continuity of known structures like the Centaurus Wall and the Great Attractor?
• RQ3 What is the nature and extent of the massive structure at 4000–8000 km s⁻¹, and how does it relate to the Great Attractor?
• RQ4 To what extent do cellular structures in redshift space—previously seen at higher galactic latitudes—appear in the Zone of Avoidance?
• RQ5 How do the redshift measurements improve the three-dimensional mapping of the local Universe in regions of high Galactic extinction?

Key findings

• Redshifts were successfully measured for 209 out of 226 observed galaxies, with 173 having high signal-to-noise ratios, significantly improving the redshift census in the region.
• The Centaurus Wall was confirmed to extend across the Galactic Plane, with redshifts spanning 0–6000 km s⁻¹ and a filament traced from (ℓ,b) = (340°,−25°) to the Centaurus cluster.
• A new massive structure, the Norma Supercluster, was identified at 4000–8000 km s⁻¹, centered on ACO 3627 (the Norma cluster), and linked to the Great Attractor.
• The Norma Supercluster spans ~3000 km s⁻¹ in depth and several hundred km s⁻¹ in thickness, indicating a wall-like morphology seen nearly edge-on.
• Cellular structures, similar to those seen at higher galactic latitudes, were clearly detected in redshift slices, with voids and overdensities of 1000–2000 km s⁻¹ in diameter.
• The survey reduced the effective width of the Zone of Avoidance, revealing that the Milky Way's obscuration is less complete than previously assumed, especially in the Crux region.