2 The galaxy search

The search area is divided into five separate regions as illustrated in Fig. 1: the "Hydra/Antlia region'' (Paper I), the "Crux region'' (this paper), the "Great Attractor region'' (this paper), the "Scorpius region'' (Kraan-Korteweg & Fairall, in prep.) and the "Vela region'' (Kraan-Korteweg & Salem, in prep.).

Figure 2: The distribution in Galactic coordinates of galaxies in the Crux and Great Attractor regions. The search areas are marked. The 8182 unveiled galaxy candidates (with $D\gtrsim 0$ $.\mkern -4mu^\prime$2) are shown as small dots. The encircled dots are Lauberts (1982) galaxies. The contours are lines of equal Galactic foreground extinction, taken from the Galactic reddening maps of Schlegel et al. (1998). The contours correspond to $A_B = 1\hbox{$.\!\!^{\rm m}$ }0$, $3\hbox{$.\!\!^{\rm m}$ }0$ (thick line), and $5\hbox{$.\!\!^{\rm m}$ }0$.

An extensive description of the galaxy search is given in Paper I. The IIIaJ film copies of the SRC sky survey have been examined systematically by eye in a darkened room, using a proto-type blinking machine (on semi-permanent loan from the Astronomisches Institut der Universität Basel) with a 50 times magnification. An area of 3 $\hbox{$.\mkern-4mu^\prime$ }5 \times 4 \hbox{$.\mkern-4mu^\prime$ }$0 is projected on a screen.

The selection of the IIIaJ film copies of the SRC sky survey as our plate material was made after careful tests (Paper I). Even though the effects of the Galactic foreground extinction are stronger in the blue, the hypersensitized and fine grained emulsion of the IIIaJ films go deeper and show more resolution, compared to their red counterparts.

We imposed a diameter limit of $D \gtrsim 0.2$ arcmin for our search. For every galaxy we recorded the major and minor diameter. In addition, we made an estimate of the average surface brightness and the morphological type of the galaxy. The magnitude of each galaxy was derived using the diameters and the estimated average surface brightness. Finally, the positions of all the galaxies were measured with the Optronics machine at ESO in Garching.

The Crux and Great Attractor regions together cover approximately 850 square degrees, ranging in Galactic longitude from $\ell \approx 289{^\circ}$ to $\ell \approx 338{^\circ}$ and limited at $\vert b\vert \approx 10{^\circ}$. It contains 37 fields of the ESO/SRC sky survey, namely F62-67, F94-100, F130-138, F171-180 and F221-225. These film copies were systematically examined by eye by the first author (PAW) with the magnifying viewer.

In total, 8182 galaxy candidates have been identified: 3759 in the Crux region and 4423 in the Great Attractor region, respectively.

2.1 The distribution of galaxies in the Crux region

The resulting distribution of the 3759 galaxies in the Crux region is shown in Galactic coordinates in Fig. 2. The search area is outlined. Next to the expected dependence of the number density on Galactic latitude, strong variations with Galactic longitude are evident. Folding the galaxy distribution with the Galactic reddening maps of Schlegel et al. (1998), indicates that these density fluctuations are indeed extragalactic of origin, even though dark clouds do cause holes in the galaxy distribution (see for instance the galaxy distribution at ( $\ell, b) \approx (301{^\circ}, -9{^\circ}$). Most noticable in this respect are the dark clouds around $\ell \approx 316{^\circ}{-} 318{^\circ}$ and $b \approx +5{^\circ}{-} +6{^\circ}$. However, the observed overdensities and filamentary structures in the galaxy distribution can, overall, not be explained by extreme transparent regions.

Three filamentary structures are seen in the Crux region. Two of them are located above the Galactic Plane (GP), around $\ell \approx 305{^\circ}$ and $\ell \approx 313{^\circ}$. These filaments are a likely continuation of the Centaurus Wall (Fairall et al. 1998) into the ZOA. The filament around $\ell \approx 305{^\circ}$ could, however, also be a part of a larger overdensity extending to lower Galactic longitudes, incorporating the overdensity at ( $\ell, b) \approx (297{^\circ}, +9{^\circ}$), since dark clouds (Feitzinger & Stüwe 1984) possibly obscure part of this overdensity. One filament below the GP at $\ell \approx 315{^\circ}$ has, however, no visible counterpart in the galaxy distribution beyond the ZOA. Given the relative small diameters of the galaxies in this filament, it most likely is at larger distances and not connected with the Centaurus-Pavo connection, i.e., the Centaurus Wall.

Number density contours also reveal an overdensity around $(\ell, b) \approx (305.5{^\circ}, +5.5{^\circ})$ which could mark a possible cluster of galaxies. Redshifts taken at the South African Astronomical Observatory seem to confirm this cluster, given the small finger-of-god seen at this position (see Fig. 6 of Fairall et al. 1998). This low-mass cluster (the Centaurus-Crux cluster) is part of the Great Attractor overdensity at a mean redshift-distance of 6214 kms^-1, albeit on the far side of the Great Attractor.

2.2 The distribution of galaxies in the Great Attractor region

Also shown in Fig. 2 is the distribution in Galactic coordinates of the 4423 galaxies that were found in the GA region. In addition, we have displayed in Fig. 3 the galaxy density contours in the GA region. Examination of both figures reveals three distinct overdensities: one above the Galactic Plane at $(\ell, b) = (321{^\circ}, +9{^\circ})$, and two at negative Galactic latitudes, at $(\ell, b) = (329{^\circ}, -9{^\circ})$and $(\ell, b) \approx (325{^\circ}, -7{^\circ})$, respectively. The latter is by far the most prominent overdensity of galaxies in the southern Zone of Avoidance (including the surveyed Hydra/Antlia and Crux region). This overdensity is a factor f = 8-10 more dense compared to regions at similar Galactic latitude. It is associated with ACO 3627 (Abell et al. 1989), hereafter the Norma cluster. No other Abell cluster is located so close to the Galactic Plane (Andernach 1991).

The overdensity at $(\ell, b) \approx (329{^\circ}, -9{^\circ})$ is located not far from the Norma cluster. It is, however, unrelated to the Norma cluster. The overdensity is more distant at $v \approx 15\,000$ kms^-1 (Woudt et al. 1999) and most likely connected to the X-ray bright Triangulum Australis cluster at $(\ell, b, v) = (324{^\circ}, -12{^\circ}, 15\,300$ kms^-1) (McHardy et al. 1981).

Figure 3: The galaxy density distribution in the GA region. The contours mark 0.5, 5 (dotted line), 10, 25 (thick solid line) and 50 (thick dashed line) galaxies per square degree, respectively.

Comparing the extinction contours in Fig. 2 with the galaxy density contours in Fig. 3, shows that the DIRBE/IRAS reddening map in general is a reliable tracer of extinction down to very low Galactic latitude. The lowest galaxy density contour (0.5 galaxies per square degree) compares well with the $A_B = 5^{\rm m}$ extinction contour. At higher extinction values hardly any galaxies are visible.