1 The Great Attractor and the Zone of Avoidance

Dust and stars in the plane of the Milky Way obscure $\sim$20% of the optical extragalactic sky and 10% of the IRAS extragalactic sky. As a result, existing optical galaxy catalogues are severely incomplete close to the Galactic Equator leading to a "Zone of Avoidance'' (ZOA) in the distribution of galaxies. For example, the main optical galaxy catalogue of the southern sky (Lauberts 1982) is complete for galaxies with an observed diameter $D \ge 1$ $.\mkern -4mu^\prime$3 (Hudson & Lynden-Bell 1991) down to extinction-levels of $A_B \le 1^{\rm m}$ (see Fig. 1 of Kraan-Korteweg & Lahav 2000). At higher extinction-levels, galaxies with an intrinsic diameter of 1 $.\mkern -4mu^\prime$3 fail to meet the selection criteria (Lauberts' 1982 selection criterion is $D_{\rm obs} \ge 1'$) and only the intrinsically largest and brightest galaxies are detected near the Galactic Plane.

This incompleteness limits our understanding of the origin of the peculiar motion of the Local Group with respect to the Cosmic Microwave Background, and the origin of velocity flow fields in the local Universe.

In our previous Zone of Avoidance (ZOA) catalogue paper (Kraan-Korteweg 2000a, hereafter Paper I), a detailed motivation for our deep optical galaxy search behind the southern Milky Way was given. The main arguments for embarking on a survey of this nature are briefly reiterated here.

Figure 1: The distribution in Galactic coordinates of Lauberts (1982) galaxies with extinction-corrected diameters $D^0 \ge 1$ $.\mkern -4mu^\prime$3 and $A_B \le 3^{\rm m}$, supplemented with galaxies from the deep galaxy search in the Hydra/Antlia region (Kraan-Korteweg 2000a) with the same limits. The contour is a line of equal Galactic foreground extinction, taken from the Galactic reddening maps of Schlegel et al. (1998), and corresponds to $A_B = 3^{\rm m}$. The galaxies are diameter-coded: the galaxies with 1 $.\mkern -4mu^\prime$ $3 \le D^0 \le 2'$ are displayed as small circles, the galaxies with $2' \le D^0 \le 3'$ as medium-sized circles and the galaxies with $D^0 \ge 3'$ as large circles. The thick solid line outlines the Crux (right) and Great Attractor (left) search areas. The dotted line marks the other search areas: the Scorpius region (adjacent to the Great Attractor region) and the Vela region (to the right). The Centaurus, Pavo II, Centaurus-Crux and ACO 3627 clusters are labelled, as is the peak of the reconstructed mass density field associated with the Great Attractor.

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To improve the determination of the optical galaxy density field across the sky through the reduction of the ZOA. By directly observing the galaxy distribution in the ZOA - contrary to inferring the galaxy density field from the velocity flow field (e.g., Kolatt et al. 1995), or interpolating the galaxy density field outside the ZOA into the ZOA (Yahil et al. 1991) - and comparing this observed distribution with the velocity flow field, fundamental cosmological parameters (such as $\beta = \Omega^{0.6}/b$, where b is the linear biasing parameter) can be derived (Strauss & Willick 1995).

In Paper I we have already noted that the deep optical galaxy searches lead to a complete optical galaxy distribution for galaxies with extinction-corrected diameters $D^0 \ge 1\hbox {$.\mkern -4mu^\prime $ }3$ for $A_B \le 3^{\rm m}$, resulting in a reduction of the optical ZOA of over 50% (see also Fig. 4 of Kraan-Korteweg & Lahav 2000).

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To unveil the full extent of the Great Attractor. The Great Attractor (GA) is seen primarily in the peculiar velocity field of galaxies in the local Universe (Dressler et al. 1987; Lynden-Bell et al. 1988; Kolatt et al. 1995; Tonry et al. 2000). There is no doubt that this overdensity exists. There is, however, still some ambiguity about the true nature and extent of the Great Attractor (e.g., Staveley-Smith et al. 2000). This is primarily caused by its unfortunate location right behind the southern Milky Way at ( $\ell, b, v$) $\approx$ ( $320{^\circ}, 0{^\circ}, 4000$ kms^-1) (Kolatt et al. 1995). It is very likely that dust and stars in our Galaxy have greatly diminished the optical appearance of the GA, and that a significant fraction (of the mass) of the GA overdensity lies behind the Milky Way.

Figure 1 shows the complete diameter-limited southern sky distribution of galaxies, down to a diameter-limit of $D^0 \ge 1$ $.\mkern -4mu^\prime$3. Only galaxies for which the foreground extinction is less or equal than $A_B \le 3^{\rm m}$ are shown. The diameters have been corrected for the diminishing effects of the Galactic foreground extinction (Cameron 1990). For the extinction correction we have used the Galactic reddening maps of Schlegel et al. (1998). The results of our survey in the Hydra/Antlia region (Paper I) have been included in this graph.

The regions under investigation in this paper, the Crux region ( $\ell \approx 289^\circ{-} 318^\circ$) and the Great Attractor region ( $\ell \approx 316^\circ{-} 338^\circ$), are demarcated by the thick solid line. They lie in between the Hydra/Antlia region (Paper I) and the Scorpius region. The Crux and Great Attractor regions are of particular interest due to their proximity to the Great Attractor. If a large fraction of the mass associated with the Great Attractor has remained hidden behind the Milky Way, this survey should reveal that.

In Sect. 2 we briefly describe the galaxy search. The catalogues are presented in Sect. 3, and in Sect. 4 we discuss the characteristics of the magnitudes and diameters of the galaxies in our survey. In Sect. 5 we discuss the Galactic foreground extinction and in Sect. 6 we assess the completeness of our survey. The performance of the IRAS Point Source Catalogue for studies of large-scale structures at low Galactic latitudes is discussed in Sect. 7. In Sect. 8, we assess the impact of our survey on the current understanding of the Great Attractor.