What is so special about radio galaxies?

Radio galaxies are important because they help us understand how galaxies have changed throughout the cosmic history of the universe. Most galaxies, like our own Milky Way, have a supermassive black hole at the centre. It can be millions of times the mass of our Sun. In radio galaxies, ‘stuff’ is falling into the supermassive black hole. It’s gorging itself on gas and dust and generally wreaking havoc. As this happens, enormous amounts of energy are released, and sometimes huge jets of plasma grow out of either side of the galaxy. Think of a water jet from the blowhole of a whale. It comes out in a narrow column, then turns into a cloudy plume. That’s more or less what the plasma jets look like. The plasma jets glow not in optical light, which we can see with our own eyes, but in radio light, which can be seen with radio telescopes. This is why these systems are called radio galaxies. The plasma jets can heat up the gas in the rest of the galaxy, or completely blow it out of the galaxy. This essentially ‘kills’ the galaxy, because without cold gas it cannot form new stars. Studying the way the jets change the galaxy helps us understand why galaxies look the way they do today and why they looked different in the past.

Jacinta Delhaize, lecturer in astronomy at the University of Cape Town, South Africa

Composite image of Cygnus A, our closest powerful radio galaxy. X-ray data is presented in blue and radio emissions in red Artist’s concept of the possible rings surrounding Rhea An artist’s impression of a distant hypothetical solar system that is a similar age to our own