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.