Optical Images of the Radio Galaxy Cygnus A

When the radio sky was first mapped, the brightest sources were named after the constellation in which they were found. For example, several famous radio sources are Perseus A, Centaurus A, and Cygnus A, where the "A" designation means this is the brightest radio source that was found in that particular constellation. The galaxy featured here -- Cygnus A -- is not only the brightest source in Cygnus, but it was identified in the early days of radio astronomy with a very powerful and active radio galaxy. Two beautiful jets of radio radiation shoot out from the galaxy's nuclear region and form a classical "double radio source" with bright lobes symmetrically placed on either side of the galaxy.

The origin of the radio radiation is believed to be a collapsed object, most likely a black hole, buried in the galaxy nucleus. Material falling into the nucleus of the galaxy gets captured by the black hole, and this provides the fuel to power the outflow of radio radiation. Exactly how material gets forced into the nuclear region of an active radio galaxy is sometimes difficult to explain. However, for Cygnus A the material is likely being driven into the nucleus by a collision or merger of two nuclei, as the following images show.

The three frames shown immediately below are actually taken from the same image which is a 20 minute exposure through a V filter. Each frame looks different because "cuts" were made at different intensity levels (and transferred to the computer display) in order to show the important features of this radio galaxy. In the right hand frame the overall shape of the large elliptical galaxy can be seen with the stars symmetrically distributed around the galaxy core. The center image shows the galaxy's central regions at an intermediate brightness level. Clearly the light has broken up into two lobes. The image on the left shows the very inner details of central nucleus where two bright cores -- and a tidal tail or streamer -- can be seen as evidence of two merging nuclei. As the nuclei merge, stray gas is likely to stream into the nuclear region where the central engine captures it as fuel for the radio galaxy core. An enlarged image of the central core is shown at the very bottom. Of course, these frames are displayed a negative images, so the sky looks white and the galaxy looks black.

The images shown here were taken at the University of Hawaii 2.2-m Telescope on Mauna Kea in 1983 and were published in the Astrophysical Journal Letters, vol. 279, page L47).

Inner Core Intermediate Region Full Galaxy Extent

Enlarged Inner Core Image