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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Completeness of Optical Galaxy Searches

In order to merge the various deep optical ZOA surveys with existing galaxy catalogs, Kraan-Korteweg (1999) and Woudt (1998) have analyzed the completeness of their ZOA galaxy catalogs - the Hy/Ant [D₂], Crux [D₃] and GA [D₄] region - as a function of the foreground extinction.

By studying the apparent diameter distribution as a function of the extinction E(B-V) (Schlegel etal. 1998) as well as the location of the flattening in the slope of the cumulative diameter curves $\log {\rm D}$ - $\log{\rm N}$ for various extinction intervals (cf. Fig. 5 and 6 in Kraan-Korteweg 1999), we conclude that our optical ZOA surveys are complete to an apparent diameter of

${\rm D} = 14\hbox{$^{\prime\prime}$}$ - where the diameters correspond to an isophote of 24.5 mag/arcsec² (Kraan-Korteweg 1999) - for extinction levels less than

${\rm A_B} = 3\hbox{$.\!\!^{\rm m}$}0$ .

How about the intrinsic diameters, i.e. the diameters galaxies would have if they were unobscured? A spiral galaxy seen through an extinction of

${\rm A_B} = 1\hbox{$.\!\!^{\rm m}$}0$ will, for example, be reduced to $\sim 80\%$ of its unobscured size. Only $\sim 22\%$ of a (spiral) galaxy's original dimension is seen when it is observed through

${\rm A_B} = 3\hbox{$.\!\!^{\rm m}$}0$ . In 1990, Cameron derived analytical descriptions to correct for the obscuration effects by artificially absorbing the intensity profiles of unobscured galaxies. These corrections depend quite strongly on morphological type due to the difference in surface brightness profiles and mean surface brightness between early-type galaxies and spiral galaxies. Applying these corrections, we find that at

${\rm A_B} = 3\hbox{$.\!\!^{\rm m}$}0$ , an obscured spiral or elliptical galaxy at our apparent completeness limit of

${\rm D} = 14\hbox{$^{\prime\prime}$}$ would have an intrinsic diameter of

${\rm D^o} \approx 60\hbox{$^{\prime\prime}$}$ or

${\rm D^o} \approx 50\hbox{$^{\prime\prime}$}$ , respectively. At extinction levels higher than

${\rm A_B} = 3\hbox{$.\!\!^{\rm m}$}0$ , an elliptical galaxy with

${\rm D^o} = 60\hbox{$^{\prime\prime}$}$ would appear smaller than the completeness limit

${\rm D} = 14\hbox{$^{\prime\prime}$}$ and might have gone unnoticed. The here discussed optical galaxy catalog should therefore be complete to

${\rm D^o} \ge 60\hbox{$^{\prime\prime}$}$ for galaxies of all morphological types down to extinction levels of

${\rm A_B} \le 3\hbox{$.\!\!^{\rm m}$}0$ with the possible exception of extremely low-surface brightness galaxies. Only intrinsically very large and bright galaxies - particularly galaxies with high surface brightness - will be recovered in deeper extinction layers. This completeness limit could be confirmed by independently analyzing the diameter vs. extinction and the cumulative diameter diagrams for extinction-corrected diameters.

We can thus supplement the ESO, UGC and MCG catalogs - which are complete to

${\rm D} =1\hbox{$.\mkern-4mu^\prime$}3$ - with galaxies from optical ZOA galaxy searches that have

${\rm D^o} \ge 1\hbox {$.\mkern -4mu^\prime $}3$ and

${\rm A_B} \le 3\hbox{$.\!\!^{\rm m}$}0$ . As our completeness limit lies well above the ESO, UGC and MCG catalogs, we can assume that the other similarly performed optical galaxy searches in the ZOA should also be complete to

${\rm D^o} = 1\hbox{$.\mkern-4mu^\prime$}3$ for extinction levels of

${\rm A_B} \le 3\hbox{$.\!\!^{\rm m}$}0$ .

In Fig. 3 we have then taken the first step in arriving at an improved whole-sky galaxy distribution with a reduced ZOA. In this Aitoff projection we have plotted all the UGC, ESO, MCG galaxies that have extinction-corrected diameters

${\rm D^o} \ge 1\hbox {$.\mkern -4mu^\prime $}3$ (remember that galaxies adjacent to the optical galaxy search regions are also affected by absorption though to a lesser extent:

${\rm A_B} \le 1\hbox{$.\!\!^{\rm m}$}0$ ), and added the galaxies from the various optical surveys with

${\rm D^o} = 1\hbox{$.\mkern-4mu^\prime$}3$ and

${\rm A_B} \le 3\hbox{$.\!\!^{\rm m}$}0$ for which positions and diameters were available. The regions for which these data are not yet available are marked in Fig. 3. As some searches were performed on older generation POSS I plates, which are less deep compared to the second generation POSS II and ESO/SERC plates, an additional correction was applied to those diameters, i.e. the same correction as for the UGC galaxies which also are based on POSS I survey material (

${\rm D_{25} = 1.15 \cdot D_{POSS I}}$ ).

**Figure 3:** Aitoff equal-area distribution of ESO, UGC, MCG galaxies with extinction-corrected diameters
${\rm D^o} \ge 1\hbox {$.\mkern -4mu^\prime $}3$ , including galaxies identified in the optical ZOA galaxy searches for extinction-levels of

${\rm A_B} \le 3\hbox{$.\!\!^{\rm m}$}0$ (contour). The diameters are coded as in Fig. 1. With the exception of the areas for which either the positions of the galaxies or their diameters are not yet available (demarcated areas), the ZOA could be reduced considerably compared to Fig. 1.
$\begin{figure} \begin{center} \hfil \psfig{file=figaitc.ps,width=16cm}\hfil \end{center}\end{figure}$

A comparison of Fig. 1 with Fig. 3 demonstrates convincingly how the deep optical galaxy searches realize a considerable reduction of the ZOA; we can now trace the large-scale structures in the nearby Universe to extinction levels of

${\rm A_B} = 3\hbox{$.\!\!^{\rm m}$}0$ . Inspection of Fig. 3 reveals that the galaxy density enhancement in the GA region is even more pronounced and a connection of the Perseus-Pisces chain across the Milky Way at

$\ell=165{^\circ}$ more likely. Hence, these supplemented whole-sky maps certainly should improve our understanding of the velocity flow fields and the total gravitational attraction on the Local Group.

Redshift follow-ups of well-defined samples of ZOA galaxies will be important in analyzing the large-scale structures in redshift-space. Systematic redshift surveys have been performed for various ZOA regions and revealed a number of dynamically important structures such as

- the rich, massive (

$\sim 2-5 \cdot 10^{15}\mbox{${\rm M}_\odot$}$ ) cluster A3627 at (

$\ell,b,v)\sim(325{^\circ},-7{^\circ},4882~$ km ${\rm s}^{-1}$ ) which seems to constitute the previously unrecognized but predicted density peak at the bottom of the potential well of the Great Attractor (Kraan-Korteweg etal. 1996)
- the 3C129 cluster at (

$\ell,b,v)\sim(160{^\circ},0{^\circ},5500$ km ${\rm s}^{-1}$ ) connecting Perseus-Pisces and A569 across the Galactic Plane (Chamaraux etal. 1990, Pantoja etal. 1997)
- and the Ophiuchus supercluster at (

$\ell,b,v)\sim(0{^\circ},8{^\circ},8500$ km ${\rm s}^{-1}$ ) behind the Galactic Center (Wakamatsu etal. 1994, Hasegawa etal. 1999).

Optical galaxy searches, however, fail in the most opaque part of the Milky Way, the region encompassed by the

${\rm A_B} = 3\hbox{$.\!\!^{\rm m}$}0$ contour in Fig. 3 - a sufficiently large region to hide further dynamically important galaxy densities. Here, systematic HI surveys have proven very powerful as the Galaxy is transparent to the 21-cm line radiation of neutral hydrogen and HI-rich galaxies can readily be found through detection of their redshifted 21-cm emission.

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Completeness of Optical Galaxy Searches