Radio Images of Galaxies
The galaxy M81 is a spiral galaxy about 11 milliion light-years from Earth. It is about 50,000 light-years across. This VLA image was made using data taken during three of the VLA's four standard configurations for a total of more than 60 hours of observing time. The spiral structure is clearly shown in this image, which shows the relative intensity of emission from neutral atomic hydrogen gas. In this pseudocolor image, red indicates strong radio emission and blue weaker emission.
This pair of images illustrates the need to study celestial objects at different wavelengths in order to get "the whole picture" of what is happening with those objects. At left, you see a visible-light image of the M81 Group of galaxies. This image largely shows light coming from stars in the galaxies. At right, a radio image, made with the VLA, shows the hydrogen gas, including streamers of gas connecting the galaxies. From the radio image, it becomes apparent that this is an interacting group of galaxies, not isolated objects.
The radio source Cygnus A is produced in a galaxy some 600 million light-years away. The radio waves are coming from electrons propelled at nearly the speed of light through a long, thin "jet" at the core of the galaxy and deposited in giant "radio lobes." It is here where the speeding electrons are trapped by the magnetic field around the galaxy to produce radio waves much like the Van Allen radiation belts around the Earth. Where did all the electrons come from? From the bright, small radio component in the center of the galaxy -- the location of a black hole.