What Is a Radio Galaxy? Jets Larger Than the Galaxy Producing Them

The radio energy is synchrotron radiation, produced by electrons moving at nearly the speed of light spiralling in magnetic fields, which is a process identifiable from the shape of the spectrum and from the polarisation of the signal. Those electrons are accelerated in jets launched from the region immediately around a supermassive black hole at the galaxy's centre, where material falling inward forms a disc and a fraction of it is instead flung outward along the rotation axis at relativistic speed. The jets travel out of the galaxy entirely and terminate where they meet the surrounding intergalactic medium, where the material piles up into lobes that can extend for millions of light years, far larger than the galaxy itself. Some sources show bright compact spots at the ends of the jets, marking where the flow is stopped. The whole structure is invisible optically, which is why these objects were unknown until radio astronomy developed after the Second World War and why several of the brightest radio sources in the sky correspond to faint or unremarkable optical galaxies.

Several apparently distinct classes of active galaxy turned out to be the same thing viewed from different angles:

• •Radio galaxies, where the jets are seen from the side and the central region is obscured by a torus of dust and gas
• •Quasars, extremely luminous points where the central region is seen directly and outshines the host galaxy entirely
• •Blazars, where one jet points nearly at the observer, producing extreme variability and apparent superluminal motion caused by the geometry
• •Seyfert galaxies, lower-luminosity versions with bright compact nuclei, mostly in spiral hosts
• •The unified model, which explains much of this variety as orientation plus a range of power rather than as fundamentally different objects, and which is supported by evidence including polarised light revealing hidden broad emission lines
• •Radio-loud and radio-quiet distinction, which remains less well explained and appears to involve black hole spin and the host galaxy's history

These objects are not curiosities but a central component of how galaxies evolve. Simulations of galaxy formation that include only gravity and gas cooling produce galaxies far more massive than any observed, because gas cools and forms stars without limit, and something must be preventing that. Jets from central black holes are the leading candidate, since they inject enormous energy into the surrounding gas, heating it and preventing it from cooling and collapsing into stars, which limits how large a galaxy can grow. That process, called feedback, is now built into essentially all galaxy formation models and is what allows them to reproduce the observed distribution of galaxy masses. Direct evidence includes cavities in the hot gas of galaxy clusters, visible in X-rays, which correspond spatially to radio lobes and which represent bubbles blown into the cluster medium by the jets, allowing the energy input to be measured rather than assumed.

The field began with an accidental detection of radio emission from the Milky Way in the 1930s and developed rapidly after the war using surplus equipment and expertise. Early surveys catalogued sources with no optical counterparts, which was puzzling, and identifying them required accurately measuring their positions, which drove the development of interferometry, where signals from separated antennas are combined to achieve the resolution of a much larger instrument. Lunar occultation, timing when the moon passed in front of a source, gave a precise position for one object in 1962 and identified it with a faint star-like object whose spectrum made no sense until it was recognised as enormously redshifted, which established that these are distant and extraordinarily luminous. Modern instruments combine antennas across continents, and arrays under construction will survey the radio sky at unprecedented sensitivity, which is expected to find enormous numbers of these objects and to use them to trace how galaxies and their black holes grew together.

Electrons spiralling near light speed in magnetic fields emit the radio energy, and they are accelerated in jets launched from a supermassive black hole and terminating in lobes millions of light years across, far outside the galaxy. Radio galaxies, quasars and blazars are largely the same objects seen at different angles. The jets heat surrounding gas and prevent it forming stars, which is why simulations need them to avoid building galaxies that are far too large.