Mapping the Hidden Universe: The Galaxy Distribution in the Zone of Avoidance

Abstract

Due to the foreground extinction of the Milky Way, galaxies become increasingly faint as they approach the Galactic Equator creating a 'zone of avoidance' (ZOA) in the distribution of optically visible galaxies of about 25%. A 'whole-sky' map of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity of the Local Group with respect to the Cosmic Microwave Background and velocity flow fields such as in the Great Attractor (GA) region. The current status of deep optical galaxy searches behind the Milky Way and their completeness as a function of foreground extinction will be reviewed. It has been shown that these surveys -- which in the mean time cover the whole ZOA (Fig. 2) -- result in a considerable reduction of the ZOA from extinction levels of A_B = 1.m0 (Fig. 1) to A_B = 3.m0 (Fig. 3). In the remaining, optically opaque ZOA, systematic HI surveys are powerful in uncovering galaxies, as is demonstrated for the GA region with data from the full sensitivity Parkes Multibeam HI survey (300° <= l <= 332°, |b| <= 5.°5, Fig. 4).