Australia Telescope National Facility

Mapping the Hidden Universe:
The Galaxy Distribution in the Zone of Avoidance
Ren\'ee C. Kraan-Korteweg , Sebastian Juraszek, PASA, 17 (1), 6.

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Introduction

In 1988, Lynden-Bell & Lahav for the first time prepared a ``whole-sky'' distribution of the extragalactic light to map the density enhancements in the local Universe, to compare them to cosmic flow fields and to determine the gravity field on the Local Group. Assuming that light traces mass, this could be realized through a diameter-coded distribution of galaxies larger than

${\rm D} \,\raisebox{-0.4ex}{$\stackrel{>}{\scriptstyle\sim}$}\, 1\hbox{$.\mkern-4mu^\prime$}0$ taken from the following galaxy catalogs: the Uppsala General Catalog UGC (Nilson 1973) for the north (

$\delta \ge -2\hbox{$.\!\!^\circ$}5$ ), the ESO Uppsala Catalog (Lauberts 1982) for the south (

$\delta \le -17\hbox{$.\!\!^\circ$}5$ ), while the missing strip (

$-17\hbox{$.\!\!^\circ$}5 < \delta < -2\hbox{$.\!\!^\circ$}5$ ) was filled with data from the Morphological Catalog of Galaxies MCG (Vorontsov-Velyaminov & Archipova 1963-74).

Because these optical galaxy catalogs are limited to the largest galaxies they become increasingly incomplete close to the Galactic equator where the dust thickens, reducing the apparent diameters of the galaxies. Added to this are the growing number of foreground stars which fully or partially block the view of galaxy images. This is clearly demonstrated in Fig. 1, where such a light distribution is presented in an Aitoff equal-area projection centered on the Galactic plane. The same corrections as advocated in Lahav (1987) have been applied to homogenize the data of the three different galaxy catalogs, i.e.

${\rm D_{25} = 1.15 \cdot D_{UGC}, D_{25} = 0.96 \cdot D_{ESO}}$ and

${\rm D_{25} = 1.29 \cdot D_{MCG}}$ . A cut-off at

${\rm D} =1\hbox{$.\mkern-4mu^\prime$}3$ was imposed - the completeness limit of the respective catalogs according to Hudson & Lynden-Bell (1991).

**Figure 1:** Aitoff equal-area projection in Galactic coordinates of galaxies with
${\rm D}\ge 1\hbox {$.\mkern -4mu^\prime $}$ 3. The galaxies are diameter-coded: small circles represent galaxies with

$1{\hbox {$.\mkern -4mu^\prime $}}3 \le {\rm D} < 2\hbox {$^\prime $}$ , larger circles

$2\hbox {$^\prime $}\le {\rm D} < 3\hbox {$^\prime $}$ , and big circles

${\rm D} \ge 3\hbox {$^\prime $}$ . The contour marks absorption in the blue of

${\rm A_B} = 1\hbox{$.\!\!^{\rm m}$}0$ as determined from the Schlegel etal. (1998) dust extinction maps. The displayed contour surrounds the area where the galaxy distribution becomes incomplete (the ZOA) remarkably well.
$\begin{figure} \begin{center} \hfil \psfig{file=figait.ps,width=16cm}\hfil \end{center}\end{figure}$

Fig. 1 clearly displays the irregularity in the distribution of galaxies in the nearby Universe with its dynamically important density enhancements such as the Local Supercluster visible as a circle (the Supergalactic Plane) centered on the Virgo cluster at

$\ell=284{^\circ}, b=74{^\circ}$ , the Perseus-Pisces chain bending into the ZOA at

$\ell=95{^\circ}$ and

$\ell=165{^\circ}$ , the general overdensity in the Cosmic Microwave Background dipole direction (

$\ell=280{^\circ},b=27{^\circ}$ , Kogut etal. 1993) and the Great Attractor region centered on

$\ell=320{^\circ}, b=0{^\circ}$ (Kolatt, Dekel & Lahav 1995) with the Hydra (

$270{^\circ},27{^\circ}$ ), Antlia (

$273{^\circ},19{^\circ}$ ), Centaurus (

$302{^\circ},22{^\circ}$ ) and Pavo (

$332{^\circ},-24{^\circ}$ ) clusters.

Most conspicuous in this distribution is, however, the very broad, nearly empty band of about 20 ${^\circ}$ : the Zone of Avoidance. Comparing this band with the 100 $\mu$ m dust extinction maps of the DIRBE experiment (Schlegel, Finkbeiner & Davis 1998) we found that the ZOA - the area where our galaxy counts are severely incomplete - is described almost perfectly by the extinction contour

${\rm A_B} = 1\hbox{$.\!\!^{\rm m}$}0$ (where

${\rm A_B} = 4.14 \cdot {\rm E(B-V)}$ , Cardelli, Clayton & Mathis 1989).

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