Radio galaxies blow lobes of plasma far beyond their visible stars
Seen in ordinary light, a radio galaxy looks like a large elliptical galaxy. Tuned to radio wavelengths, it sprouts two vast lobes stretching well past its stars, inflated over tens to hundreds of millions of years by jets shot out from the black hole at its core.
The glow comes from synchrotron radiation, produced when electrons moving near light speed spiral through magnetic fields. Astronomers recognise it from its smooth spectrum across many frequencies and its strong polarisation. The same electrons can knock ambient photons up to X-ray energies through inverse-Compton scattering, and detecting that emission lets researchers estimate how energy divides between particles and magnetic fields; many powerful sources turn out to sit close to the minimum energy possible for their brightness. Synchrotron light sometimes appears in optical or even X-ray bands, which demands electrons above a trillion electron volts. Fermi acceleration is one likely way such particles get their energy.
Radio maps reveal assorted shapes. Most common are paired, roughly ellipsoidal lobes on either side of the nucleus; fainter sources often show long plumes instead. Some display narrow jets running from the core toward the lobes, most famously the giant galaxy M87 in the Virgo Cluster. Since the 1970s the favoured explanation has been that beams of energetic particles and magnetic field, visible as jets, feed these structures.
In 1974 Bernard Fanaroff and Julia Riley sorted sources into two classes. Class I objects are brightest near the centre and relatively faint; Class II objects are brightest at their outer edges and far more luminous. The split reflects how energy travels: Class II jets stay relativistic, at least half light speed, all the way out, ending in bright hotspots where they slam to a halt in shocks, while Class I jets slow down within their host galaxy.
Other shapes carry their own labels. Head-tail sources look swept back by ram pressure as their host galaxy ploughs through a cluster, and fat doubles have diffuse lobes with neither jets nor hotspots, perhaps relics whose engine has switched off. Systems reaching about 0.7 megaparsecs or more are called giant radio galaxies, and because they can be spotted at great distances they serve as useful probes for cosmology.
Source: Radio galaxy
