Radio galaxy
Galaxies with giant radio lobes powered by jets from active nuclei.
A radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure, powered by jets from its active galactic nucleus. These objects have luminosities up to 10^39 W at radio wavelengths between 10 MHz and 100 GHz, and their radio emission is due to the synchrotron process. The host galaxies are almost exclusively large elliptical galaxies, and radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology.
- type
- Astronomical object
- emission mechanism
- Synchrotron radiation
- typical host galaxy
- Large elliptical galaxy
- radio luminosity
- Up to 10^39 W (10 MHz–100 GHz)
- largest size class
- Giant radio galaxies (≥0.7 megaparsecs)
- classification system
- Fanaroff–Riley Class I (FRI) and Class II (FRII)
Lore & Background
The radio emission from radio-loud active galaxies is synchrotron emission, inferred from its smooth, broad-band nature and strong polarization. This implies the radio-emitting plasma contains relativistic electrons (Lorentz factors of ~10^4) and magnetic fields. The plasma must also contain either protons or positrons to remain neutral, though particle content cannot be determined directly from synchrotron observations. A sister process, inverse-Compton scattering, occurs when relativistic electrons interact with ambient photons, producing X-rays and allowing model-dependent estimates of energy densities in particles and magnetic fields. Synchrotron radiation is not confined to radio wavelengths; features may also appear in infrared, optical, ultraviolet, or X-ray if particles are accelerated to high enough energies.
Reader's Guide
Radio galaxies display a wide range of structures, most commonly double lobes on either side of the active nucleus. In 1974, Bernard Fanaroff and Julia Riley divided radio sources into two classes: FRI sources are brightest toward the center and have low luminosity, while FRII sources are brightest at the edges and have high luminosity. FRI objects typically have bright jets in the center, while FRIIs have faint jets but bright hotspots at the ends of the lobes. The FRI/FRII transition depends on host-galaxy environment, appearing at higher luminosity in more massive galaxies. The largest radio galaxies have lobes extending to megaparsec scales, implying growth timescales of tens to hundreds of millions of years. Radio galaxies are important for observational cosmology and for studying effects on the intergalactic medium, particularly in galaxy groups and clusters.
Did You Know?
- The term 'radio galaxy' often refers to the entire jet system, not just the host galaxy.
- Some scientists consider 'black hole jet system' a more accurate and less confusing term.
- Radio galaxies reaching about 0.7 megaparsecs or more are called 'giant radio galaxies'.
- The only unambiguously supersonically expanding system known consists of the inner lobes of Centaurus A.
More in Astronomical radio sources 1-24
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