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8 Remarks on the Great Attractor

Our deep optical search for partially obscured galaxies in the Crux and Great Attractor region has revealed several overdensities which belong to the Great Attractor. These surveys show that a fair fraction of the mass of the Great Attractor was indeed previously hidden behind the Milky Way. In this section, we will summarise our results of the Great Attractor and discuss the possibility that other rich clusters of galaxies might still be hidden behind the remaining optical Zone of Avoidance ( $A_B \ge 3^{\rm m}$).

8.1 Unveiled structures

Following our galaxy search, the below listed structures unveiled (and recognised) at low Galactic latitude can be associated with the Great Attractor (see also Fig. 16):

A broad, extended overdensity north of the Galactic Plane at $305{^\circ}\le \ell \le 320{^\circ}$;
The low-mass Centaurus-Crux galaxy cluster at $(\ell, b, v) \approx (305.5{^\circ}, +5.5{^\circ},
6214$ kms-1) (Fairall et al. 1998);
The rich and nearby Norma cluster at $(\ell, b, v) \approx (325.3{^\circ}, -7.2{^\circ}, 4844$ km s-1) (Kraan-Korteweg et al. 1996; Woudt et al. 1999).
Tentative evidence from the magnitude distribution of the galaxies in the GA region, that the Great Attractor survey region is dominated by a rich cluster at the distance of the Great Attractor (see Fig. 12 and the discussion in Sect. 6.4) is confirmed beyond doubt by our follow-up redshift surveys (Kraan-Korteweg et al. 1996; Woudt et al. 1999). With a velocity dispersion of 896 kms-1, the Norma cluster has a mass of the same order as the Coma cluster. This is confirmed by ROSAT PSPC observations of this cluster (Böhringer et al. 1996). Simulations have furthermore shown that the well-known Coma cluster would appear the same as the Norma cluster if Coma were located behind the Milky Way at the location of the Norma cluster (Woudt 1998).

The Norma cluster is the most massive cluster known to date in the Great Attractor overdensity. Its optical appearance, however, is not very prominent, due to the obscuring effects of the Galactic foreground extinction. It is the most likely candidate to mark the Great Attractor's hitherto unseen core (Kraan-Korteweg et al. 1996).

8.1.1 The Norma cluster

Irrespective of its position within the Great Attractor overdensity, the Norma cluster is an important cluster in its own right. The cluster is comparable to the Coma cluster, but is located closer (Woudt 1998), i.e., it is the nearest rich cluster in the local Universe. It is X-ray bright (Böhringer et al. 1996), HI deficient (Vollmer et al. 2001), reveals signs of an ongoing merger close to the centre of the cluster (Böhringer et al. 1996; Woudt 1998), harbours an infalling spiral-rich subgroup at a distance of 2-3  h50-1 Mpc from the centre (Woudt 1998), contains 2 central cD galaxies (like the Coma cluster) and has two strong radio continuum sources, namely PKS 1610-608 (the central Wide-Angle-Tail source) and PKS 1610-605 (an extended Head-Tail source) (Jones & McAdam 1992). Due to the proximity of the Norma cluster, it is a good laboratory to study the interaction of galaxies in a rich cluster with the Intracluster Medium.

Within the Abell radius of the Norma cluster there are 603 galaxies in our ZOA catalogue. We have obtained redshifts for 266 galaxies, 219 of which are bona fide cluster members. More redshifts were obtained in 2001 with 2dF observations of this cluster. A full dynamical analysis of the Norma cluster will be presented elsewhere (Woudt et al. in prep.). We have furthermore obtained deep R-band images with the ESO/MPG 2.2-m telescope (with the Wide Field Imager) covering the entire Abell radius of the Norma cluster. These data will allow a good determination of the luminosity function of this nearby and rich cluster. In combination with recently obtained pointed K' observations for $\sim$50 elliptical galaxies in the Norma cluster, a distance to the Norma cluster can be derived through a Fundamental Plane analysis. The observed (R-K') colours of the elliptical galaxies in this sample will provide additional information on the Galactic foreground extinction (see discussion in Sect. 5.2).

A reliable distance to the Norma cluster (with an uncertainty of $\lesssim$100 kms-1) is essential in distinguishing between the various existing models of the GA; a nearby GA (Tonry et al. 2000), a more distant GA (Kolatt et al. 1995), or a GA which partakes in a cosmic flow to even larger distances.

\par\includegraphics[width=9cm,clip]{}\end{figure} Figure 16: An equal area projection of galaxies with $D^0 \ge 1\hbox {$.\mkern -4mu^\prime $ }3$ and $A_B \le 3^{\rm m}$ centred on the Great Attractor at $(\ell , b) = (320{^\circ }, 0{^\circ })$ within a radius of 70${^\circ }$. The galaxies are taken from the ESO Uppsala Catalogue (Lauberts 1982), the Uppsala General Catalogue UGC (Nilson 1973), the Morphological Catalogue of Galaxies MCG (Vorontsov-Velyaminov & Archipova 1963-74), and our Zone of Avoidance catalogues which significantly reduce the optical ZOA. Over the extent of our survey region, we have marked the $A_B = 3^{\rm m}$ extinction-contour. Our search areas in progress (the Scorpius region to the left, and the Vela region to the right) are indicated by the solid line. Prominent overdensities are marked and labelled. The galaxies are diameter-coded as in Fig. 1.

8.1.2 The galaxy distribution

Figure 16 shows the most complete view of the optical galaxy distribution in the Great Attractor region to date. This equal area projection is centred on the Great Attractor at $(\ell , b) = (320{^\circ }, 0{^\circ })$. All galaxies with extinction-corrected diameters larger than $D^0 \ge 1\hbox {$.\mkern -4mu^\prime $ }3$ limited by $A_B \le 3^{\rm m}$ are shown. Galaxies were taken from the Lauberts (1982) catalogue, the Uppsala General Catalogue UGC (Nilson 1973), the Morphological Catalogue of Galaxies MGC (Vorontsov-Velyaminov & Archipova 1963-74), and our Zone of Avoidance Catalogues (Paper I, and this paper). Details on diameter corrections for galaxies in the Lauberts, UGC and MCG catalogues are given by Kraan-Korteweg (2000b).

The resulting galaxy distribution shows a strong concentration of galaxies towards the Galactic Plane on either side of the centre of the Great Attractor. This concentration of galaxies, previously unnoticed (compare with Fig. 1), most likely is part of the Centaurus Wall and the Norma Supercluster (see also Fairall et al. 1998) and hence associated with the Great Attractor. Comparing Fig. 16 with Fig. 1 of Paper I, shows that we have greatly reduced the optical Zone of Avoidance (by more than 50%) and have unveiled a large fraction of the Great Attractor overdensity, previously unseen, but also that the Great Attractor region is not entirely unveiled yet.

The remaining ZOA ( $A_B \ge 3^{\rm m}$) is best surveyed at higher wavelengths. This is done already in the near infrared by DENIS (Schröder et al. 1999) and 2MASS (Jarrett et al. 2000), at 21 cm by the Parkes HI ZOA Sky Survey (Juraszek et al. 2000; Henning et al. 2000) and in the X-rays (Böhringer et al. 2000; Ebeling et al. 2000).

8.2 What remains hidden?

One can, for instance, not exclude that another Norma-like cluster has remained hidden behind the reduced optical Zone of Avoidance, i.e., at $A_B \ge 3^{\rm m}$. The ZOA survey greatly enhanced the optical appearance of the Norma cluster within the extinction interval $A_B = 1^{\rm m} {-} 2^{\rm m}$. At $A_B \ge 3^{\rm m}$ we become increasingly incomplete, however, and at $A_B \ge 5^{\rm m}$ we stop finding galaxies altogether in the optical. Even a rich, nearby cluster cannot be detected by optical surveys at the highest extinction levels.

Rich clusters can, however, be detected through a fair bit of extinction by X-ray surveys (Böhringer et al. 2000; Ebeling et al. 2000) although even these searches are limited by the current survey material. Alternatively, a strong central radio source, such as PKS 1610-608 in the Norma cluster, could point to unidentified clusters.

Exactly such a source lies in the deepest layers of the Galactic foreground extinction ( $A_B = 12^{\rm m}$) at $(\ell, b, v) = (309.7{^\circ}, +1.7{^\circ}, 3872$ kms-1), right in the Great Attractor overdensity (Kraan-Korteweg & Woudt 1999). We are currently involved in a deep infrared survey ( $J, H, K\hbox{$^\prime$ }$) with the 1.4-m Infrared Survey Facility of Nagoya University at the South African Astronomical Observatory, to see if this strong radio-source (PKS 1343-601) is the central source of a highly obscured, rich cluster (Nagayama et al., in prep.). If there is indeed another rich cluster in the Great Attractor region, this would have serious implications for our understanding of the formation of this nearby massive overdensity.

Part of this survey was performed at the Kapteyn Astronomical Institute of the University of Groningen. Their support is greatfully acknowledged. PAW is supported by strategic funds made available by the University of Cape Town. He further acknowledges financial support from the National Research Foundation. RCKK thanks CONACyT for their support (research grant 27602E). The authors acknowledge the Referee, Dr. R. Peletier, for the useful comments and suggestions. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.

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