Zone of Avoidance

Characteristics

The obscuration is caused mainly by clouds of interstellar dust that absorb and scatter optical photons, together with confusion from the high density of foreground stars. The effect is strongest close to the galactic plane, producing a band several degrees wide where conventional optical surveys miss many galaxies. At visible wavelengths the Zone of Avoidance removes roughly a fifth of the sky from straightforward study.

History and observational response

The term arose as astronomers noticed gaps in extragalactic catalogues aligned with the Milky Way. As survey techniques developed, researchers adopted wavelengths that penetrate the dust. Observations outside the visible — especially infrared and radio — have steadily reduced the apparent extent of the zone and revealed structures that were once hidden.

How astronomers see through the zone

Multiple approaches are used to recover objects behind the Milky Way:

• Near- and mid-infrared imaging, which is less affected by dust and can detect stellar light from galaxies.
• Radio spectroscopy of the 21 cm hydrogen line, which finds gas-rich galaxies regardless of dust extinction.
• X-ray and other high-energy surveys, which can reveal active galactic nuclei and clusters that are otherwise obscured.
• Combined multiwavelength catalogues and statistical reconstructions to infer large-scale structure across the masked region.

Importance and notable facts

Filling in the Zone of Avoidance is essential for mapping the universe on the largest scales, understanding galaxy flows and mass concentrations (for example those associated with the so-called Great Attractor), and producing complete sky catalogues. The width of the obscured band depends strongly on wavelength: a severe gap in optical catalogues becomes much narrower in the infrared and is almost absent for certain radio surveys. Progress continues as new all-sky and targeted surveys push deeper behind our own galaxy.

For further technical summaries and survey data see resources and survey pages: Milky Way overview, galactic structure, interstellar dust, stellar foregrounds, galactic plane and observing at different wavelengths.