Zone of avoidance
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The Zone of Avoidance is the area of the night sky that is obscured by our own galaxy, the Milky Way.
Dust and stars in the plane of the Milky Way (the galactic plane) obstruct our view of around 20% of the extragalactic sky at optical wavelengths. As a result, optical galaxy catalogues are usually very incomplete close to the galactic plane. This area of incompleteness is known as the Zone of Avoidance: other galaxies seem to avoid this area (actually, this is almost certainly a selection effect) and historically astronomers have avoided doing extragalactic astronomy in this area as there are easier targets elsewhere.
In recent years, many projects have attempted to bridge the gap in our knowledge caused by the Zone of Avoidance. The stars, dust and gas in the Milky Way causes significant confusion and extinction at optical wavelengths, but the effect of extinction drops at longer wavelengths, such as the infrared, and the Milky Way is effectively transparent at radio wavelengths. Surveys in the infrared, such as IRAS and 2MASS, have given us a more complete picture of the extragalatic sky. Indeed, two very large nearby galaxies, Maffei I and Maffei II, were discovered in the Zone of Avoidance by Paulo Maffei by their infrared emission in 1968. Even so, approximately 10% of the sky remains difficult to survey as extragalactic objects can be confused with stars in the Milky Way.
Projects to survey the Zone of Avoidance at radio wavelengths, particularly using the 21 cm spin-flip emission line of neutral atomic hydrogen (known in astronomical parlance as HI), have detected many galaxies that could not be detected in the infrared. Examples of galaxies detected from their HI emission include Dwingeloo 1 and Dwingeloo 2.