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The pseudo-colour image is of the large-scale radio structure of the FRII radio galaxy 3C98. Lobes, jet and hotspot are labelled. Credit: .
Another pseudo-colour image is of the large-scale radio structure of the FRI radio galaxy 3C31. Jets and plumes are labelled. Credit: .

"Over the past 30 years, radioastronomy has revealed a rich variety of molecular species in the interstellar medium of our galaxy and even others."[1]

These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. ... Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.

"In 1974, radio sources were divided into two classes Fanaroff and Riley Class I (FRI), and Class II (FRII).[2]

The distinction was originally made based on the morphology of the large-scale radio emission (the type was determined by the distance between the brightest points in the radio emission): FRI sources were brightest towards the centre, while FRII sources were brightest at the edges.

There is a reasonably sharp divide in luminosity between the two classes: FRIs were low-luminosity, FRIIs were high luminosity.[2]

The morphology turns out to reflect the method of energy transport in the radio source. FRI objects typically have bright jets in the centre, while FRIIs have faint jets but bright hotspots at the ends of the lobes. FRIIs appear to be able to transport energy efficiently to the ends of the lobes, while FRI beams are inefficient in the sense that they radiate a significant amount of their energy away as they travel.

The FRI/FRII division depends on host-galaxy environment in the sense that the FRI/FRII transition appears at higher luminosities in more massive galaxies.[3] FRI jets are known to be decelerating in the regions in which their radio emission is brightest,[4]

The hotspots that are usually seen in FRII sources are interpreted as being the visible manifestations of shocks formed when the fast, and therefore supersonic, jet (the speed of sound cannot exceed c/√3) abruptly terminates at the end of the source, and their spectral energy distributions are consistent with this picture.[5]


  1. Dudley Herschbach (March-May 1999). "Chemical physics: Molecular clouds, clusters, and corrals". Reviews of Modern Physics 71 (2): S411-S418. doi:10.1103/RevModPhys.71.S411. Retrieved 2011-12-17. 
  2. 2.0 2.1 Fanaroff, Bernard L., Riley Julia M.; Riley (May 1974). "The morphology of extragalactic radio sources of high and low luminosity". Monthly Notices of the Royal Astronomical Society 167: 31P–36P. 
  3. Owen FN, Ledlow MJ (1994). "The FRI/II Break and the Bivariate Luminosity Function in Abell Clusters of Galaxies". In G.V. Bicknell, M.A. Dopita, and P.J. Quinn, (Eds.) (ed.). The First Stromlo Symposium: The Physics of Active Galaxies. ASP Conference Series,. 54. Astronomical Society of the Pacific Conference Series. p. 319. ISBN 0-937707-73-2.CS1 maint: Multiple names: editors list (link)
  4. Laing RA, Bridle AH (2002). "Relativistic models and the jet velocity field in the radio galaxy 3C31". Monthly Notices of the Royal Astronomical Society 336 (1): 328–57. doi:10.1046/j.1365-8711.2002.05756.x. 
  5. Meisenheimer K, Röser H-J, Hiltner PR, Yates MG, Longair MS, Chini R, Perley RA; Roser; Hiltner; Yates; Longair; Chini; Perley (1989). "The synchrotron spectra of radio hotspots". Astronomy and Astrophysics 219: 63–86.