Galaxies Radio Galaxies
article: Radio Galaxies
Introduction
About 10 percent of Active Galactic Nuclei produce powerful outflows which are launched with relativistic (i.e. close to the speed of light) bulk flow velocities. These outflows contain magnetic fields and relativistic particles which produce emission at radio and sometimes optical and X-ray wavelengths via the synchrotron process. Two oppositely directed outflows or jets seem to be generated along the rotation axis of the accretion disk. The jets push their way out through the [Interstellar Medium] of the host galaxy and propagate out to distances of tens to hundreds of kpc.
Types of Radio Galaxies
A VLA image of the powerful central engine and its relativistic ejection of plasma in the nucleus of Cygnus A.
- Taken at 6 cm, this view reveals the highly ordered structure spanning over 500,000 light-years, fed by ultra-thin jets of energetic particles beamed from the compact radio core between them. The giant lobes are themselves formed when these jets plow into the tenuous gas that exists between galaxies. Despite its great distance from us (over 600 million light-years), it is still by far the closest powerful radio galaxy and one of the brightest radio sources in the sky. The fact that the jets must have been sustained in their tight configuration for over half a million (possibly as long as ten million) years means that a highly stable central object—probably a rapidly spinning supermassive black hole acting like an immovable gyroscope—must be the cause of all this activity.
(Photograph courtesy of Chris Carilli and Rick Perley, NRAO, and AUI.)
Radio Galaxies tend to divide into two main types based on their appearance (morphology) and their radio luminosity as first discussed by Fanaroff and Riley in 1974. The lower luminosity sources (FR type I) tend to have jets which are brightest near the AGN and then expand in width and fade in brightness as they propagate, eventually forming faint, diffuse, plume-like structures. In the more luminous (FR type II) sources, the jets tend to be faint, and are often too faint to be detected, but end in bright compact regions called hot spots. Surrounding the jets are often diffuse cocoons of emission. We believe that the main parameter responsible for this difference is the jet Mach number. In the current paradigm, all radio galaxies are launched with similar properties, but the FRIs start to decelerate possibly due to entrainment of ambient gas on scales of tens of parsecs to a kpc or so. The FRIs then become transonic flows which transition into buoyant plumes. On the other hand, the jets in the FRIIs remain relativistic and supersonic all the way to their end. The supersonic jets end at a strong shock at the location of the working surface between the jet and the ambient medium. The jet material goes through the shock and then travels sideways filling up a large cocoon with shocked jet material.
Preview Image
In this radio image, two jets shoot out of the center of active galaxy Cygnus A. GLAST may solve the mystery of how these jets are produced and what they are made of. (Source: NASA. Credit: NRAO.)
Citation
O'Dea, Christopher, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Radio Galaxies." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 25, 2007].
<http://www.cosmosportal.org/articles/view/137280/>
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