zone of avoidance

The zone of avoidance (ZOA) is the region of the sky within the visible Milky Way, in which the galaxy's dust obscures more distant objects (extinction), such as other galaxies. Some surveys aim at investigating objects hidden by the ZOA through observation using portions of the electromagnetic radiation spectrum that are unaffected by dust, such as radio. The terms northern zone of avoidance and southern zone of avoidance have been used to indicate roughly the portions of the ZOA with positive and negative declinations, respectively, i.e., north and south of Earth's equator. The lack of galaxies in the region of the sky was first pointed out in 1869 by astronomer Richard Proctor who termed it the zone of few nebulae.

The designator ZOAG (for "ZOA galaxy") stems from a 1990s 21-cm line survey to identify galaxies obscured by the Milky Way, within or near the Perseus-Pisces Supercluster.