What is now considered to be the mechanism for the production of the double lobes of radio emission that appear on either side of many galaxies?

The mechanism is known as radiogalaxis and refers to two oppositely directed jets of relativistic particles spiraling around magnetic fields.

Explanation:

Radiogalaxes usually have a double structure, with two emitting lobes located on either side of the galaxy, whose brightness occurs because of synchrotron radiation. Produced by the movement of electrons in the galaxy's magnetic field at speeds very close to the speed of light ie relativistic electrons. This is the mechanism that accelerates electrons, and is responsible for the origin of radio emission from the radio galaxy. Another feature of some radiogalaxes is the presence of a jet of matter coming from a central source in the galaxy's nucleus, charged moving in a magnetic field.

Because of all this movement, radiogalaxes can be considered very active anomalous galaxies (as well as quasars and seyfert galaxies), emitting a colossal amount of radio waves.

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me