2 The galaxy search

The tools for this galaxy search are simple. They comprise a viewer with the ability to magnify 50 times (a proto-type blinking machine on semi-permanent loan from the Astronomisches Institut der Universität Basel) and the IIIaJ film copies of the ESO/SRC sky survey. The viewer projects an area of 3 $\hbox{$.\mkern-4mu^\prime$ }$5 $\times$ 4 $\hbox{$.\mkern-4mu^\prime$ }$0 on a screen which is viewed in a darkened room making the visual systematic scanning of the plates straightforward and comfortable.

The ZOA is the only part of the sky where it remains more efficient to scan the sky surveys by eye rather than by using modern plate measuring machines or sophisticated galaxy identification algorithms, as for instance with COSMOS, SUPERCOSMOS and MAMA. Automatic searches all fail close to the Galactic Plane, due to the crowding effects when blended stars are mistakingly identified as galaxies or when star subtraction of superimposed stars break the galaxies up in various "small'' galaxies (see Sect. 3.4).

Even though Galactic extinction effects are stronger in the blue, the IIIaJ films are chosen over their red counterparts. A careful inspection between the various surveys demonstrated that the hypersensitized and fine grained emulsion of the IIIaJ films go deeper and show more resolution. Even in the deepest extinction layers of the ZOA, the red films were found to have no advantage over the IIIaJ films.

The success of optically identifying extragalactic objects at very low latitudes is proven by the fact that less than 3% of the over 10% spectroscopically observed galaxy candidates of the catalog (Kraan-Korteweg et al. 1994; Felenbok et al. 1997) have a star-like signature, hence are either foreground stars or are overshadowed by foreground stars, and only a few were found to be Galactic nebulae.

We imposed a diameter limit of arcminutes for our search. Below this diameter the refraction crosses of the stars disappear, making it hard to differentiate consistently between stars, blended stars and faint round galaxies. In a few cases of clear clustering, smaller galaxies - mainly early-type galaxies - are retained in the list.

For every galaxy, we recorded the major and minor diameter, an estimate of the average surface brightness and the morphological type of the galaxy. From the diameters and the average surface brightness a magnitude estimate is derived. The reliability of the recorded diameters and the apparent magnitude are discussed in detail in Sect. 3. A surprisingly good relation is found for the estimated magnitudes, with no deviations from linearity even for the faintest galaxies, and a scatter of only $\sigma = 0\hbox{$.\!\!^{\rm m}$ }5$.

The positions of all the galaxies were subsequently measured with the Optronics machine at ESO in Garching. The accuracy of the positions is about 1 $\hbox{$^{\prime\prime}$ }$.

Due to the locally varying extinction it is difficult to give a homegeneous galaxy classification. The distinction between, for instance, the bulge of a spiral galaxy and an early-type galaxy remains ambiguous in very obscured regions. The details of the morphological classification thus depends on the identifiable details on the enlarged survey image.

In this way, 3279 galaxy candidates have been discovered in the Hydra/Antlia search region, a region of approximately 400  $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi% ^{\circ}$ that encompasses 18 fields of the ESO/SRC survey (F91-F93, F125-F129, F165-F170, F211-F214) within Galactic latitudes of and longitudes of . Of these 3279 galaxy candidates, 2818 are certain galaxies and 453 are likely galaxy candidates. 8 unlikely candidates were retained in the catalog as well. Among the 3279 identifications, only 97 galaxies were previously recorded by Lauberts (1982) - of which 4 Lauberts objects turned out to be two close galaxies. Eleven further galaxies were listed in other catalogs, leading to 3167 (96.6%) newly identified galaxies.

Figure 2: Distribution of galaxies in the Hydra/Antlia extension. The outlined area marks the search region in the ZOA where the dashed lines indicate the adjacent areas covered by us. The 3279 unveiled galaxy candidates ( ) are shown as small dots. In the surrounding area the Lauberts galaxies are displayed (large dots, $D\ge1\hbox{$.\mkern-4mu^\prime$ }0)$. The contours mark the dust extinction as determined from the 100 $\mu$m DIRBE maps (Schlegel et al. 1998) at the levels $A_{B} = 1\hbox{$.\!\!^{\rm m}$ }0$, $3\hbox{$.\!\!^{\rm m}$ }0$ (thick line) and $5\hbox{$.\!\!^{\rm m}$ }0$

The galaxies found in our search are entered as small dots in Fig. 2, the larger dots identify the known Lauberts galaxies in the illustrated region. It is obvious from the distribution of the newly identified galaxies that this method is quite succesful: the Zone of Avoidance has been narrowed down to .

The asymmetry of the galaxy distribution with respect to the Galactic equator reflects the asymmetry of the dustlayer. The latter can be traced by the contours in Fig. 2 which mark absorption levels of $A_{B} = 1\hbox{$.\!\!^{\rm m}$ }0, 3\hbox{$.\!\!^{\rm m}$ }0$ (thick line) and 5 $\hbox{$.\!\!^{\rm m}$ }$0. These values are based on the DIRBE/IRAS extinction maps by Schlegel et al. (1998). The offset to the south was already established by Kerr & Westerhout (1965) for the longitude range $\ell = 200{^\circ}- 330{^\circ}$ from the hydrogen column densities.

Galaxies remain visible through obscuration layers of 3 magnitudes of extinction; a few galaxies still are recognisable up to extinction levels of ${A}_{ B} = 5\hbox{$.\!\!^{\rm m}$ }0$. Overall, the mean number density follows the dust distribution remarkably well at low Galactic latitudes. The contour level of ${A}_{ B} = 5\hbox{$.\!\!^{\rm m}$ }0$, for instance, is nearly indistinguishable from the galaxy density contour at 0.5 galaxies per square degree. At intermediate extinction levels, distinct under- and overdensities are noticeable in the unveiled galaxy distribution that are uncorrelated with the foreground obscuration. They must be the signature of large-scale structures. Strong clustering is evident around $\ell=280{^\circ}, b=+6{^\circ}$ (the Vela overdensity), $\ell=275{^\circ}, b=-9{^\circ}$ and $\ell=292{^\circ}, b=+8{^\circ}$. A conspicous underdensity in the longitude range $285{^\circ}- 290{^\circ}$ above the Galactic Plane remains unexplained by the dust distribution, as well the distinct decrease in galaxy density below the Galactic Plane from the right-hand side to the left.

Although a handful of very small galaxy candidates have been found at high extinction levels, these galaxies most likely indicate holes in the dust layer. Overall, the Milky Way remains optically opaque for extinction levels above . This intransparent part of the ZOA regions is currently being filled in through the systematic blind HI searches the Dwingeloo Obscured Galaxy Survey (DOGS) in the north (Henning et al. 1998; Rivers et al. 1999), and the Multibeam ZOA-survey in the south (Kraan-Korteweg et al. 1999; Henning et al. 1999).

A discussion on structures in redshift-space based on the galaxies identified in the here presented search area can be found in Kraan-Korteweg (1992), Kraan-Korteweg & Woudt (1993, 1994), Kraan-Korteweg et al. (1994) and Kraan-Korteweg et al. (1996). Our redshift follow-up programs have proven that the prominent overdensity visible at $(\ell,b) = (280{^\circ},\ +6{^\circ})$ is actually a superposition of three structures: a filamentary thin structure that can indeed be traced from the Hydra and Antlia clusters to $(\ell,b) = (280{^\circ},-7{^\circ})$ to the oposite side of the ZOA at a mean recession velocity of $2500{\rm\,km\ s^{-1}}$, a shallow but very extended supercluster, the Vela SCL (Kraan-Korteweg & Woudt 1993) centered at $(\ell, b, v) = (280{^\circ}, +6{^\circ}, \sim6000{\rm\,km\ s^{-1}})$ as well as a number of clusters at about $16000 {\rm\,km\ s^{-1}}$. The overdense clumps below the Plane at $\ell = (268{^\circ}-286{^\circ})$ are due to clusters in the same high redshift range. This gave rise to the suspicion that these clusters mark a possible connection between the Horologium clusters below the ZOA and the Shapley clusters above the ZOA. Given its approximate distance ( $\sim 16000{\rm\,km\ s^{-1}}$) and its extent on the sky (roughly $100^\circ$), this would imply the largest known structures to date.