Hechos y argumentos jurídicos de los recurridos en revisión

The basic model of the generation and evolution of luminous extragalactic radio sources was developed by Blandford & Rees (1974); Scheuer (1974). In this model a cool flow of high collimation and speed was created in the region near or within the AGN. This flow after- wards spreads to the ends of the radio-emitting lobes, where it shocks and stops, changing most of its kinetic energy into relativistic electrons and magnetic fields. This model ex- plains in a simple way the structure of extragalactic radio sources and a large variety of observational evidence strongly supports it.

Recent progress in the theory of relativistic jet production, indicates that such jets may be launched with a spine/sheath flow structure, having a much higher velocity close to the axis and a lower one away from it. Relativistic jets are presumed to obtain much of their energy from the infall of matter into a supermassive black hole. Therefore models of accretion flows are now considering the production of a jet by that flow (Meier, 2003).

In the review of Worrall & Birkinshaw (2006) the twisting of the magnetic field con- nected to the accretion disk is considered to be the most plausible mechanism for the extrac- tion of energy from the jet system. The most propable contents of the jets are considered to be electrons and positrons, or electrons and protons, although electromagnetic (Poynting flux) jets (e.g., Rees 1971) and proton-dominated jets are also proposed (e.g., Mannheim and Biermann 1989). In the resulting magnetohydrodynamic flow much of the momentum would be carried by particles, although Poynting flux may carry a significant fraction of the total energy. An electron-positron pair plasma is a natural consequence of the high energy density near the centres of active galaxies, and hence it might be expected that electrons and positrons would be an important, and perhaps dominant, component of the jet outflow (Worrall & Birkinshaw, 2006).

Radio galaxies can be divided into two subclasses known as Fanaroff and Riley type I (FRI), and type II (FRII) (see fig. 1.1) (Fanaroff & Riley, 1974). Although this classifica- tion is based on morphology, Fanaroff & Riley (1974) found that most sources with radio luminosity at 170 MHz below 2×1025W Hz−1sr−1 are FRI and the rest are FRII.

• Fanaroff-Riley type I (FRI): These sources are brightest near the centre of the active nucleus of the galaxy, as can be seen in the right side of fig 1.1. Until the point where the lobes terminate, these sources become gradually fainter, and their spectra steeper (explained later in spectral ageing). FRI jets show a range of morphologies, from

straight symmetrical two-sided jets to the bent ‘head-tail’ sources. This type is not as powerful as FRII galaxies because their jets are not highly collimated (Rosswog & Bruggen, 2007).

• Fanaroff-Riley type II (FRII): These sources present strong, collimated jets, show- ing quite often hotspots that can be several kpc away (Rosswog & Bruggen, 2007) The jet ends at the beam head (radio hotspot, see fig 1.1) where the fluid of the jet goes through a shock to give rise to a cocoon of radio-emitting plasma. The ambient gas is heated from the bow shock created in front of the jet termination shock, which then fills an area surrounding the lobe of radio-emitting plasma. The observational result on kpc scales is radio emission from jets that power edge brightened lobes. The difference between these two FR types is mainly on the velocity of the jet front. In FRI type, the jets travel subsonically through the ambient medium due to deccelera- tion by entrainment of external material (Ledlow & Owen, 1996) dissipating much of their energy without developing a well-defined beam head, and in FRII the jets travel supersoni- cally, transporting energy efficiently to the ends of the lobes where they stop causing shocks which are thought to create the observed radio hotspots without much deceleration (Bick- nell, 1995).

The structure of a typical radio galaxy includes a core associated with the AGN, oppo- sitely collimated jets coming up from the core, and lobes that rise from the jet terminals. In more detail:

• Core: It is unresolved and corresponds to the nucleus of the galaxy, from which the collimated flow arises (Carilli et al., 1991)

orientated beams- coming directly from the nucleus and going to the lobes. This radio- emitting narrow beam reveals the flow which transmits mass, momentum, energy, and electromagnetic field as it passes through the empty lobes (Carilli et al., 1991) • Lobes: Double, often symmetrical, approximately ellipsoidal structures positioned

on each side of the active nucleus. These radio-emitting structures contain the out- flow from the shock. Sometimes some low-luminosity sources show structures usu- ally known as plumes which are much more ‘stretched’ (Blandford & Rees , 1974; Scheuer, 1974)

• Hotspots: Bright structures that point where the cool flow, coming from the nucleus, is shocked and heated upon colliding with the external medium. The momentum of the jet afterwards moves the hotspots forward into the surrounding medium and the lobes grow in size and luminosity (Blandford & Rees , 1974; Scheuer, 1974)

Figure 1.1 The structure of FRI and FRII galaxies: pseudo-colour images of the large-scale radio structure of the FRII radio galaxy 3C98 on the right and the FRI radio galaxy 3C31 on the left. [http://upload.wikimedia.org/wikipedia/en/4/42/3C31 & 3C98.png].