1 Introduction

Due to the foreground extinction of the Milky Way, galaxies become increasingly fainter, smaller and are of lower surface brightness as the dust extinction increases. Although most of them are not intrinsically faint or small, galaxies close to the Galactic Plane fail to meet the criteria for inclusion in magnitude or diameter-limited catalogs and only few galaxies are known below Galactic latitudes of . Added to this are the enormous numbers of foreground stars that frequently fall on the galaxy images and crowd the field of view. Because of this, most extragalactic studies are done in regions which are free of the effects of this "foreground pollution'' - they avoid the so-called Zone of Avoidance (ZOA). Various questions with regard to the dynamics in the local Universe, however, require knowledge of the galaxy distribution in the ZOA:

1) To explain the peculiar velocity of the Local Group (LG) with the irregular mass distribution in the local Universe and compare this motion to the dipole in the Cosmic Microwave Background (CMB) requires "whole-sky'' coverage. Kolatt et al. (1995) have shown that the gravitational acceleration of the LG is strongly affected by the mass distribution in the ZOA. The dipole direction determined from the visible mass distribution changes significantly (by $31{^\circ}$) if the mass distribution in the ZOA is not accounted for.

2) For our understanding of velocity flow fields - such as the flow induced by the Great Attractor centered in the Galactic Plane - we need to know the galaxy distribution in the ZOA. The comparison of the actual galaxy distribution with dynamically implicated mass excesses will tell us whether galaxies are fair tracers of mass. 3) This not only concerns large-scale structures such as clusters, voids and walls. Hidden nearby massive galaxies will influence the internal dynamics of the LG, its mass derivation and the present density determination of the Universe from timing arguments (Peebles 1994). Moreover, the gravitational attraction of the nearest galaxies ( $v<300{\rm\,km\ s^{-1}}$) generates 20% of the total dipole moment (Kraan-Korteweg 1989), and six of the nine apparent brightest galaxies are located in the ZOA. Others might still remain uncovered.

For these reasons, various groups have initiated projects in recent years to unveil the galaxy distribution behind our Milky Way such as galaxy searches on optical sky surveys, near-infrared surveys (e.g., DENIS and 2MASS), far-infrared surveys (e.g., IRAS), and systematic blind HI searches. Although all are subjected to different limitations and selection effects, they have the advantage that they are complementary in the galaxies they unveil and the latitude ranges they are optimal for (see Kraan-Korteweg & Woudt 1999, for a review).

1.1 Optical galaxy searches in the southern Milky Way

We here report on the first of a series of five deep optical galaxy searches. One of the methods to reduce the width of the ZOA is to identify galaxies on sky surveys to fainter magnitude limits and lower diameter limits compared to existing catalogs. Here, examination by eye is still the best technique. A separation of galaxy and star images cannot as yet be done by automated measuring machines such as COSMOS or APM on a viable basis below (see Sect. 3.4) though surveys by eye are clearly both very trying and time consuming, and maybe not as objective.

Using the IIIaJ film copies of the ESO/SRC sky survey, we have pursued this approach and systematically surveyed five contiguous areas in the southern Zone of Avoidance (see Fig. 1). The southern Milky Way is especially exciting as many suspected large-scale structures are bisected or hidden by the Milky Way: we see traces of the nearby Puppis filament, a possible extension of the Hydra/Antlia clusters across the ZOA, the crossing of the Supergalactic Plane, and it hides a large fraction of the Great Attractor overdensity which is centered in the ZOA at $(\ell,b,v)\sim(320{^\circ},0{^\circ}, 4500{\rm\,km\ s^{-1}}$, Kolatt et al. 1995).

Figure 1: Equal area projection of all Lauberts galaxies ( $D\ge 1\hbox {$.\mkern -4mu^\prime $ }0$) in the southern sky ( $\delta \le -17\hbox{$.\!\!^\circ$ }5$) in equatorial coordinates. The galaxies are diameter-coded: the galaxies are displayed as points for $1{\hbox {$.\mkern -4mu^\prime $ }}0 \le D < 1\hbox {$.\mkern -4mu^\prime $ }3$, as small circles for $1{\hbox {$.\mkern -4mu^\prime $ }}3 \le D < 2\hbox {$^\prime $ }$, as larger circles for $2\hbox {$^\prime $ }\le D < 3\hbox {$^\prime $ }$, and as big circles for $D \ge 3\hbox {$^\prime $ }$. The most important clusters are labelled, as well as the CMB-dipole direction and the center of the Great Attractor. Within the ZOA only few galaxies are catalogued. This region has been covered by our five deep optical surveys (outlined by the solid lines). The thick contour marks the search area discussed in this paper. The dashed areas mark optical surveys performed by other groups

Figure 1 shows a diameter-coded distribution of all galaxies with $D\ge 1\hbox {$.\mkern -4mu^\prime $ }0$ in the southern sky for declinations $\delta \le -17\hbox{$.\!\!^\circ$ }5$ as taken from the Lauberts Catalog of Galaxies (Lauberts 1982). Part of the above discussed suspected features can be identified in this distribution. The most important clusters are indicated as well as the dipole direction of the CMB ( $\ell = 280{^\circ}$, $b = 27{^\circ}$, Kogut et al. 1993) and the predicted center of the Great Attractor region. Most conspicuous in this distribution is the very broad, nearly empty band of about 20${^\circ}$wide that stretches across this equal area projection i.e., the Zone of Avoidance. This is the area in which we performed our galaxy searches. The five survey regions are marked with solid lines.

The thick contour marks the search area discussed in this paper. It lies in the extension of the Hydra/Antlia clusters towards the Galactic Plane and covers about 400  $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi% ^{\circ}$ on the sky from between ). This area was chosen because of the suspicion that the Hydra and Antlia clusters are part of a much larger structure, a possible supercluster that stretches across the Milky Way. At a mean redshift distance of $v \sim 2500{\rm\,km\ s^{-1}}$ this could contribute significantly to the peculiar motion of the LG, and - seen its vicinity to the CMB dipole direction - bring the gravitationally determined peculiar motion vector in better agreement with the observed anisotropy in the CMB radiation.

The subsequent regions are the Crux region ( , , Woudt 1998; Woudt & Kraan-Korteweg, in preparation) and the Great Attractor region ( , , Woudt 1998; Woudt & Kraan-Korteweg in preparation), Papers II and III of these series. The extensions from the Hydra/Antlia region to Puppis, as well as the Great Attractor region towards the Galactic Bulge, i.e., the Scorpius region, have already been surveyed and the respective catalogs are in preparation (Salem & Kraan-Korteweg, respectively, Fairall & Kraan-Korteweg).

The five survey regions will connect to optical searches done by other groups. The other optical surveys performed in the southern sky are the Puppis region ( , ) by Saito et al. (1991) on the right hand side of our survey (dashed line in Fig. 1), the Ophiuchus Supercluster Region by Wakamatsu et al. (1994) and its extension (Wakamatsu et al., in preparation) as well as the Sagittarius region below the Galactic Center ( , ) by Roman & Saito (1997) and the Aquila/Sagittarius survey ( , ) by Roman et al. (1996) on the lefthand side of Fig. 1.

We have allowed for a small overlap of our search regions with the surveys connecting to our search areas. The interleaving of the Ophiuchus area with our Scorpius region (the narrow gap visible in Fig. 1) is currently being done by Wakamatsu et al. This will allow a homogenisation of the data given in the different catalogs into one coherent catalog of galaxies in the southern ZOA, complete to well-defined identification criteria.

The galaxy catalogs build the basis for various distinct redshift and photometric observational follow-up programs. Optical spectroscopy (individual) of all the brightest (extinction-corrected) galaxies with high central surface brightness has been obtained at the 1.9 m telescope of the SAAO for the here presented galaxy catalog (Kraan-Korteweg et al. 1995), for the Crux region (Fairall et al. 1998), and for the GA region (Woudt et al. 1999). Low-surface brightness galaxies have been observed in HI with the 64 m radio telescope at Parkes (see Kraan-Korteweg et al. 1997), and multifiber spectroscopy has been applied to high-density areas at the 3.6 m telescope of ESO with Optopus and Mefos. The latter data have been reduced and will soon be submitted for publication. Some of the results have been presented earlier (Felenbok et al. 1997; Woudt 1998).

In this first paper of the catalog series of deep optical galaxy searches in the southern ZOA, a description of the search method is given in Sect. 2 including a discussion on the uncovered galaxy distribution. This is followed by the catalog of the 3279 galaxy candidates in Sect. 3, and a detailed discussion of the quality of the listed galaxy properties in Sect. 4, including cross-identifications in the IRAS Point Source Catalog (Joint IRAS Science Working Group 1988, IRAS PSC). In the last section, the completeness of the galaxy catalog is analysed as a function of extinction, leading to a new, complete diameter-limited southern sky distribution of extinction-corrected galaxies, with the gap in the ZOA in the Hydra-Antlia region filled in to its completeness level at A_B = 3 mag.